Introduction to Java for the Server Side
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Transcript Introduction to Java for the Server Side
.opennet Technologies
Introduction to the Java Language and
Object-oriented Concepts
Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana) Time
Geoffrey Fox and Bryan Carpenter
PTLIU Laboratory for Community Grids
Computer Science,
Informatics, Physics
Indiana University
Bloomington IN 47404
[email protected]
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Overview
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What is Java, in a Nutshell?
What is Java?
– A simple, object oriented, distributed, interpreted, robust, safe,
architecture neutral, portable, high performance, multithreaded,
dynamic, programming language.
Java is interesting because
– It is both a general purpose object-oriented language along the
lines of C++, and
– It is particularly designed to interface with Web pages (Java on
client) and to enable distributed applications over the Internet
with Java on Server .
– It has good software engineering properties
The Web is becoming the dominant software
development arena; this is driving Java as perhaps the
best supported, most widely taught language.
– Even outside the Web, e.g. in scientific computing, Java is as
good and in some respects better than other languages.
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Architecture of Java (Client)
Applications
Java applications are
compiled and run on a
machine just like any other
general programming
language such as C/C++.
No web server or network
are required although Java
applications may also use
network connections for
distributed computing.
One can download
dynamically for applets or
servlets
There are also native
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(conventional) Java
Java code
is compiled
to produce
byte code
run by Java
Virtual Machine
(JVM) to produce
results
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Java Applications in a Nutshell
Java programs written in a file with extension “.java”.
Applications are .java files with a main() method. This
is called by the Java system.
Compile and run a Java application (using bytecodes):
– Run the compiler on a .java file:
javac MyProgram.java
producing a file of Java byte code,
MyProgram.class
– Run the interpreter on a .class file:
java MyProgram
which executes the byte code
The tools javac and java are part of JDK.
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The Simplest Java Application: Hello,World!
Since Java is object-oriented, programs are organized
into modules called classes, which may have data in
variables called fields, and subroutines called methods.
Each program is enclosed in a class definition.
main() is the first method that is run.
class HelloWorld {
public static void main (String[] args) {
System.out.println(“Hello World!”);
}
}
The notation class.method or
package.class.method is how to
refer to a public method (with
some exceptions).
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Syntax is similar to C - braces for
blocks, semicolon after each
statement.
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Java vs. JavaScript
Despite the name, JavaScript is a different language from
Java, albeit with some similarities.
A JavaScript program is written directly in the HTML page,
and executed by the JavaScript interpreter, so also allows
dynamic web page content in the browser window.
JavaScript is special purpose - it is an object-based
language that deals directly with browser entities like
windows, text fields, forms, frames and documents.
JavaScript can respond to browser events like mouse
clicks and user-typed text.
JavaScript is fast to write, but not as powerful as Java.
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Multi-tier Architecture
Distributed applications on the web naturally have a multi-tier
architecture.
Java plays a role at all three levels:
– Graphical User Interface and client side analysis systems,
including visualization
– Middle layer servers and software integration, including web
servers, distributed object servers and other application servers.
– Less important for back end client software, which may be legacy
code.
Internet
Client user
interface running
through browser
Middle level
servers
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Internet or
proprietary
network
Backend
computing or
databases
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History of Java Language and Team
Starts in 1991 as Project Green—semi-autonomous task
force in Sun focusing on operating software for
consumer electronic devices, e.g. smart set-top boxes.
Gosling (creator of Sun NeWS) considers C++ too
complex and initiates development of Oak, later
renamed to Java. Similarities to UCSD P-system.
A PDA (Personal Digital Assistant), codename “*7”,
based on Oak/Java ready in 1993. Green Team
becomes FirstPerson, Inc.
*7 proposal to Time-Warner rejected in 1993. 3DO deal
falls through in 1994. FirstPerson, Inc. dissolves.
Small group (~30 people, becomes the Java Team)
continues and decides to adapt Oak as a Web
technology.
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History of Java Language and Team to Dec. 95
Experimental Web browser written in Java, called WebRunner,
later renamed HotJava, ready in 1994.
Alpha release of Java and HotJava April '95.
Netscape licenses Java in May '95 and builds Java into
Netscape 2.0
Beta Java Development Kit (JDK) and first Java books
Summer/Fall '95.
Dec 4, 1995 Business Week cover story: "Software
Revolution—The Web Changes Everything" presents Java as
a breakthrough force in the expanding Web/Internet.
In next week, SGI, IBM, Adobe, Macromedia and finally
Microsoft adopt/license Java. Java is adopted by Web
community.
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Java Features—Simple and Familiar
Familiar as it looks like C++, but simpler to program.
– omits several confusing features of C++ including
operator overloading, multiple inheritance, pointers and
automatic type coercions
Adds automatic garbage collection to make dynamic
memory management much easier than in C or C++.
– No more frees or deletes. No more memory leaks.
Adds Interface construct, similar to Objective C concept, to
compensate for the lack of multiple inheritance.
Small kernel is suitable for Java ports to consumer electronic
devices.
– But need customization J2ME to be effective (remove unnecessary
features and support special capabilities of PDA’s etc.)
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Java Features—Architecture-Neutral
C/C++ programming in a heterogeneous network
environment demands compatibility across several
vendor platforms and their compilers.
Solved in Java by designing platform-independent
binary representation called Java bytecode—
comparable to P-code in UCSD Pascal.
Java compiler reads Java source and generates Java
bytecode, which is shipped to user.
Each client must have a Java Virtual Machine program,
which interprets (“runs”) Java bytecodes.
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Java Features—Robust
Java enforces compile-time type checking and this eliminates
some error prone constructs of C/C++.
Pointer arithmetic is eliminated which allows for, e.g., runtime
checking of array subscripts, and enforces security of the
Java model.
Explicit declarations are always required; argument types of
methods are always checked (unlike C). This allows the Java
complier to perform early error detection.
Java is most secure of popular languages because it is strict
and security was built in
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The Java 2 Platform
Sun are now offering 3 “editions”:
Java 2 platform, Standard Edition (J2SE)
– Refines earlier JDKs
– Available in version 1.4.
Java 2 platform, Enterprise Edition (J2EE)
– Incorporates multiple technologies for server-side and multi-tier
applications.
Java 2 platform, Micro Edition (J2ME)
– Optimized run-time environment for consumer products.
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Installing JDK
Become administrator (for Windows) or root (for Linux, etc.)
– This is recommended though probably not essential.
Go to http://java.sun.com/j2se/1.4
Select “Download Java 2 SDK, standard edition, v1.4.x,” for
Windows or Linux, etc.
Download the software, then read
http://java.sun.com/j2se/1.4/install-windows.html or
http://java.sun.com/j2se/1.4/install-linux.html, etc.
– Pay attention to instructions for setting PATH.
These give a command line interface for the Java compiler
javac and the Java interpreter (JVM driver) java, etc.
– If you are familiar with UNIX environments, but want to use these
commands from Windows, consider installing Cygwin:
www.cygwin.com.
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Java Books—I
Core
Java, by Gary Cornell and Cay S. Horstmann,
offers detailed coverage of the whole language and
packages for advanced programmers, including the
Swing Set. (We will not cover these user interface
issues here) Also Volume 2 gives good coverage of
advanced topics such as JDBC, RMI, JavaBeans
and security.
Java, How to Program, by Deitel and Deitel,
Prentice-Hall, starts with beginning programming
concepts and progresses rapidly through Java
language. It has the most programming exercises
and also has companion teaching multimedia
books. The third edition has Swing Set and also the
advanced API’s.
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Java Books—II
The Java Programming Language, by Ken Arnold and James
Gosling, David Holmes, 3rd Edition, Addison-Wesley, 2000, is
the classic on the language basics for intermediate and
advanced programmers. It covers threads and I/O packages,
but not applets or windowing packages.
Java in a Nutshell, by David Flanagan, is the language
reference book in the familiar O'Reilly series. One of the
better references. Also Java Examples in a Nutshell.
The Java Language Specification, second edition. James
Gosling, Bill Joy, Guy Steele, Gilad Bracha, April 2000. The
ultimate reference for hardened computer scientists/compiler
writers.
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Resources for the Java Programming
Language
http://java.sun.com
web site has plenty of
references including
– Tutorial:
http://web2.java.sun.com/docs/books/tutorial
– Books:
http://web2.java.sun.com/docs/books
Collection of Java Resources:
http://www.gamelan.com
http://www.jroundup.com/
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Java Language Basics
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Obvious similarities to C, C++
Java syntax has many similarities to C, C++.
All variables must be declared
Syntax of expressions and control structures almost
identical to C, C++
C or C++ style comments allowed.
For Fortran programmers, syntax is a bit different (; to end
line { …} to delineate blocks ) for basic material. Fox thinks
“structured programming syntax more natural than Fortran
90.
– Arrays not as good for Science as those in Fortran
– No Array syntax as in Fortran90
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Obvious differences from C, C++
No
low-level pointers or pointer arithmetic.
– Instead have variables and expressions of
reference type.
No
malloc() or free() to allocate more memory for
dynamically created data structures —instead
have a “new” operator for creating objects, plus
automatic garbage collection.
Can
declare variables almost anywhere (like C++).
No
struct, union, enum, typedef—classes and
objects are used uniformly instead.
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Primitive types rationalized
Java
characters use 16-bit Unicode Worldwide
Character Encoding instead of 8-bit ASCII.
Supports all alphabets and languages.
Primitive
types for integers and floats have
machine independent semantics.
Boolean
expressions in Java have value “true” or
“false” (not 0, 1, . . .)
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Three kinds of comments in Java
/* ignore all between stars */
– As for C
// ignore all till the end of this line
– As for C++
/**
this is a documentation comment */
– Should appear immediately before, e.g.,
class or method definition, and describe
intended use.
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Documentation Comments
Used
by documentation-generating tools like
javadoc to produce documentation, typically in
HTML form.
Optionally include formatting tags like @param,
which flags a description of a method parameter:
/** This method does what it feels like.
@param bar This is a pointless
argument. */
void foo (int bar) {. . .}
Other
formatting tags include @returns which flags
a description of a method result value, or @see
name, which creates a hypertext link to name.
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Java Keywords
Java reserves the following keywords:
abstrac
boolean
break
byte
case
catch
char
class
const
continu
default
do
double
else
extends
final
finally
float
for
goto
if
implemen
import
instanceo
int
interface
long
native
new
package
private
protecte
public
return
short
throw
throws
transie
try
void
volatile
while
goto is not allowed in Java, but it’s still reserved (to protect
Fortran programmers)!
null, true, and false are literals with special meaning.
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Java Language—Program Structure
Source
code of a Java program consists of one or
more compilation units, each implemented as a file
with extension “.java”.
Each compilation unit can contain:
– a package statement
– import statements
– class declarations and/or interface declarations.
In
typical Java development environments, exactly
one of the class (or interface) declarations in each
compilation should be marked public.
The file should be named after the public class. e.g. if
the public class is Foo, the file name should be
“Foo.java”.
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Java Types
Each
Java variable or expression has a definite
type, given by a declaration such as
int i;
double x, y, z;
Color c;
There
are two sorts of type:
– Primitive types like ints or booleans are built into
the language.
– Reference types. These include class types like
Color, and array types (and also interface types).
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Primitive Types
There are 4 integer types:
byte short
int long
Sizes are 8, 16, 32 and 64 bits, respectively.
float is 32 bits, double is 64 bits. Floating point arithmetic
and data formats are defined by IEEE 754 standard.
char format is defined by 16 bit Unicode character set.
boolean is either true or false.
One can use casts for arithmetic conversion, as in:
int i ;
float x ;
i = (int) x ;
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Reference Types
These
are the types associated with composite
entities like objects and arrays.
They are called reference types because a variable
or expression in a Java program with reference
type represents a reference (or pointer) to a
composite entity.
– Any variable of reference type may take the
value null.
Reference types can be divided into:
– Class types
– Interface types (discussed later)
– Array types
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Strings—an Example of a Class Type
Java
environments provide predefined classes for
common data types. Every Java environment
provides a String class.
Declaration
of a String variable looks like:
String s ; // variable declaration
The
variable declaration itself doesn’t create any
objects. We can create a new String object by, e.g.:
s = new String(“This is the text”) ; //
object creation
These
may be combined on one line:
String s = new String (“This is the text.”) ;
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A Constructor Function
In the object creation expression:
new String (“This is the text.”)
the term
String (“This is the text.”)
is a constructor invocation.
All classes have special “functions” called constructors. These
functions have the same name as the class. They initialize the
fields of the object.
Constructor functions are only used in object creation
operations—nearly always directly after a new operator.
In this example the constructor has one argument: a string
literal.
– We will see later that in general constructors can have
arbitrary argument lists.
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Some features of Strings.
Strings
are Java objects, but Java provides some
syntax peculiar to strings.
In
fact literal string in double quotes itself refers to a
pre-existing String object—so in practice we may
drop new operation for string constants:
String s = “This is the text.” ;
After
creation, characters of a string object never
change.
– In other words: string objects are immutable.
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Operations on Strings.
Although a String object is immutable, String-valued
variables can be reassigned to refer to new string objects:
String str = “Chicken soup with rice” ;
int n = str.indexOf( ‘w’ ) ;
str = str.substring(0,n) + “is n” +
str.substring(n+6) ;
// Result: “Chicken soup is
nice”.
The operator + is used for concatenation (special syntax for
strings).
indexOf() and substring() are methods of the String class—
and are not a special syntax!
– They illustrate the general
syntax of method invocation33on
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Array Types
As
for objects, declaring an array variable is distinct
from creating on the array:
int states[] ;
// variable
declaration
and:
states = new int[128] ; // array creation
Again,
these can be combined:
int states[] = new int[128] ;
Alternative
(better?) syntax for declaration:
int[] states ;
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Subscripts
With states is declared as above:
int states[] = new int[128] ;
it can be subscripted by integers from 0 to 127.
Subscripts are checked at runtime: states[-1] or states[128]
will immediately generate exceptions.
– This subscript checking is an example of where Java trades
robustness for performance
Array length is given by the length instance variable:
int len = states.length ; // assigns len = 128
One cannot change the length of an Array – the Java vector
class is designed for this
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Arrays of Objects
Arrays
of arbitrary objects can be constructed, e.g.:
Color manycolors[] = new Color[1024];
This creates an array of object references. It does
not create actual objects for individual elements.
Before you use the array elements, you may need to
use object constructors to allocate each object, e.g.:
for (int i = 0 ; i < 1024 ; i++)
manycolors [i] = new Color() ;
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Multidimensional Arrays
Multidimensional arrays are arrays of arrays. In general these
arrays may be “ragged”:
int graph[][] = new int[2][];
graph[0] = new int[4];
// Row 0 has length 4
graph[1] = new int[7];
// Row 1 has length 7
...
graph[1][1] = 9;
Shorthand syntax for creating a rectangular array:
char icon[][] = new char [16][16]; // 16 by 16 array
– Note icon is still logically an arrays of arrays, and nothing in
Java forces it to stay rectangular. E.g. later someone might
do
icon [8] = new char [17] ; // Now ragged!
Does not violate rule that Array lengths constant as icon [8]
points to a new array of length 17.
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Java Language—Expressions
Most Java expressions are similar to C or Fortran. Here
are some examples:
– arithmetic:
2+3
(2 + 3) * i
– auto-increment and decrement:
i++
// equivalent to i = i +1
– Boolean:
((i > 0) && (j > 0)) || (state == –1)
– bit operations:
i << 1
// Shift bit pattern 1 place left
– conditional expression:
False
(i > 0) ? expression1 : expression2
True
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Java Language—More Expressions
Java
has some expressions of its own:
– string concatenation:
“fred” + “jim”
// Value is “fredjim”
– object “instance of” test:
(a instanceof B)
// true iff object a
has type (class) B
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Java Control Flow. I: if Statements
Conditional execution of statements:
if (some Boolean expression) {
statements to be executed if true
}
Optional else clause:
if (some Boolean expression) {
statements to be executed if true
} else {
statements to be executed if false
}
Nested example:
if (some Boolean expression) { . . . }
else if (another Boolean expression) { . . . }
else { . . . }
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Control Flow II: while Loop Constructs
Normal while loop:
while (any Boolean) {
Stuff to do
}
Example:
int i = 0 ;
while(i < a.length) {
a [i] = i * i ;
i++ ;
}
while loop with test at end:
do {
What to do
} while (another Boolean) ;
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Control Flow III: The for Loop Construct
In Java, most often one uses the C++-like variant (cf. DO
loops in Fortran):
for (declaration1 ; booleanExpression ; expressionList2) {
Statements to do
}
The declaration declaration1 is effected at start of loop,
comma-separated expressionList2 is evaluated after every
iteration, and the loop terminates when booleanExpression is
false.
Typical example:
for (int i = 0 ; i < a.length ; i++)
a [i] = i * i ;
The original C-like form (no declaration) also available:
for (expressionList1 ; booleanExpression ; expressionList2) {
Statements to do
}
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Control Flow IV: The switch Construct
Identical to C:
switch (expression) {
case Constant1: // Do following if
expression==Constant1
Bunch of Stuff
break;
case Constant2: // Do following if
expression==Constant2
Bunch of Stuff
break;
default:
// Do the following otherwise
Bunch of Stuff
break;
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Control Flow V: break and continue
Unlabeled break statement immediately exits the enclosing
switch, while, do or for construct:
while (true)
if (++i == a.length || a[i] == v) break ;
Labeled break statement allows to exit an arbitrary enclosing
statement, provided it is labeled:
assign: {
if (i >= a.length) break assign ;
a[i] = v ;
}
(This is not the best way to do this!)
The continue statement skips to the end of the current
iteration of the enclosing while, do or for.
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The Java Object Model:
Classes, Instances and
Methods
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The Java Object Model Overview
Programs are composed of a set of
modules called classes. Each class is a
template specifying a set of behaviors
involving the data of the class.
Each class has variables, or fields, to hold
the data, and methods—akin to functions
or procedures in other languages—to
define the behaviors.
Each object in a program is created as an
instance of a class. Each class instance
has its own copy of the instance variables
defined for the class.
Classes can be used for data
encapsulation, hiding the details of the
data representation from the user of the
class (e.g., by marking variables as
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Instance
Variables
Methods
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Defining a Class
A class
declaration consists of:
– a header giving the class name, modifiers, and possible
superclass and interface structure.
and a class body usually containing:
– declarations of fields (possibly with initializations)—class
variables and instance variables.
– declarations of methods.
– declarations of constructors. These “functions” look like
methods, but have the same name as the class. They do
initialization when objects—class instances—are
created.
– nested class and interface definitions.
– class or (rarely) instance initialization statements.
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Example: a Predefined Class
A (small) part of the Java Date class:
public class Date implements Serializable, Cloneable {
public Date( ) {. . .}
// Constructor
public Date(long msSinceEpoch) {. . .}
// Constructor
public int getTime( ) {. . .}
// Accessor
public void setTime(long msSinceEpoch) {. . .} // Mutator
public boolean after(Date when) {. . .}
public boolean equals(Object obj) {. . .}
...
// Comparision
// Comparision
}
Note: all variables, methods and constructors visible from
“outside” the class—parts of Date that programmers writing
code in other classes are allowed to use—have the public
modifier in their declaration.
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Creating a Class Instance
The Date class represents a particular date and time, with a
resolution of milliseconds.
The first of the two Date constructors (“no-argument
constructor”) constructs an instance of the Date class and sets
its value to the current moment:
new Date()
Constructors (like methods) can be overloaded. Constructors of
same name are distinct if they have distinct argument types. If
ms is a long, the object:
new Date(ms)
represents a moment ms milliseconds after January 1, 1970,
00:00:00 UTC (Coordinated Universal Time).
Java will become obsolete (2^63 – 1) / 1000 seconds after that
(approximately 292 million years AD, UTC). . .
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Example of Using a Class
An example application using a method of the Date class:
import java.util.Date;
public class DateTest {
public static void main (String[ ] args) {
Date early = new Date(1000) ; // very early
seventies!
Date today = new Date() ;
// Now!
if (today.after(early))
System.out.println( "Today is not early!") ;
}
}
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Instance Variables
A very simple class:
public class Complex {
public double real ;
public double imaginary ;
}
Essentially like a C struct. (i.e. just a collection of variables)
Every instance of Complex has its own real and imaginary
variables. These fields are therefore called instance variables.
Use:
Complex z = new Complex() ; // Default constructor
z.real = 0.0 ;
z.imaginary = 1.0 ;
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Class Variables
Besides instance variables, a class may contain “global
variables” that are not associated with any instance.
A class variable (also called a static variable) is flagged by the
static modifier in its declaration:
class Potato {
public String name;
static public int num = 0 ;
// Class variable—number of potatoes.
}
Potato p = new Potato(), q = new Potato() ;
p.name = “one potato” ;
q.name = “two potato” ;
Potato.num += 2 ;
// static field prefix is class name.
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Method Definitions
Subprograms in Java are called methods. In the abstract, the
declaration format is:
methodModifiers returnType methodName
(parameter list) {
declarations and statements
}
The parameter list contains the types and names of all the
parameters.
The declarations and statements are the body of the method.
Parameter names, and variables declared in the body, are local
to it.
Control returns from a method when the body finishes
execution or a return statement is executed. return
statements may return a result value.
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Local variables
Formal parameters of methods, and variables declared
inside the bodies of methods, are local variables.
These are a third kind of variable in Java: they are neither
instance variables or class variables.
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Static and Non-static Methods
Like fields, methods come in two varieties, which are properly
called instance methods and class methods.
The terms non-static methods and static methods are also
commonly used.
In all Java applications illustrated so far, the main() method
had the modifier static—the main method of an application is
required to be a static method.
All other examples of methods illustrated so far were instance
methods.
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Instance Methods
Instance methods operate in the context of a particular class
instance (i.e. a particular object).
The instance variables of the current object can be accessed
without any prefix:
public class Complex {
// Adds z to the current object
public void add(Complex z) {
real
+= z.real ;
imaginary += z.imaginary ;
}
public double real ;
public double imaginary ;
}
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Invoking an Instance method
This example initializes a and b, then increments the value of
a by amount b:
Complex a = new Complex(), b = new Complex() ;
a.real = 0.707 ;
a.imaginary = -0.707 ;
b.real = -1.0 ;
b.imaginary = 0.0 ;
a.add(b) ;
// Method invocation
This is why one would like operator overloading to represent
last as
a = a + b;
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this
Within an instance method or constructor the keyword
this refers to the current instance.
– i.e. the object on which the method was invoked, or which the
constructor is initializing.
Appropriate usage—passing self-reference to some other
method:
public class Complex {
. . . Definition of add(), etc.
public void addTo(Complex accumulator) {
accumulator.add(this) ;
}
}
– The invocation a.addTo(b) adds the value of a to b, i.e. it is
equivalent to b.add(a).
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this as a prefix
this refers to current instance
Some programmers will write the this prefix explicitly on every
access to an instance variable, e.g.:
public void negate() {
this.real
= – this.real ;
this.imaginary = – this.imaginary ;
}
This is legal, but ugly!
One time you must use this as a prefix to an instance variable
is when the field is hidden by declaration of a local variable with
the same name.
– The only common example is in constructor declarations. A
constructor parameter whose value is used to initialize a
field is conventionally given the same name as the field it
initializes. See examples later.
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Static Methods
A static
method does not operate in the context of a
particular instance.
Instance variables of the class cannot be accessed
inside the body of a static method unless an explicit
object prefix is given.
The keyword this cannot be used in the body of a
static method.
To invoke a static method it should be prefixed by
the name of the class (similar rule to accessing
class variables).
– This prefix can be omitted if the method is invoked from
another method, etc, defined in the same class.
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60
Constructors
Constructors
are “functions” (not, strictly speaking,
methods) that have the same name as the class they
belong to.
Any
number of constructors can be defined for a class,
provided they can be distinguished by the number and
type of their parameters (overloading).
If
no constructors are explicitly defined, the compiler
generates a single default constructor with no
arguments.
– Note: the default constructor disappears once any
explicitly-defined constructor is given!
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A Better Potato
class Potato {
public Potato(String name) {
this.name = name ;
// Idiomatic use of this
Onenum++
should add
; as name private
}
Public String getName()
{ // Accessor method
public static int getNum() { // A static method
return
num
;
return
name;
}}
private String name ;
private static int num = 0 ;
// Note: now private
// Also private
}
Potato p = new Potato(“one potato”), q = new Potato(“two potato”) ;
System.out.println(“There are ” + Potato.getNum() + “ potatoes”) ;
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62
Remarks
In the constructor, the unqualified symbol name refers to
the local variable declared in the parameter list.
– Because this declaration hides the declaration of name
as an instance variable, we must prefix with this to
access the latter.
The data fields are now private. This means they can be
accessed only from methods within the class, not from
other classes.
The method getNum() returns a “global” property of the
class—the total number of Potato objects that have been
created.
– Hence it is natural to declare it as a static method—it is
not associated with any individual instance.
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Type Conversions
Java allows implicit type conversions in some contexts.
Generally speaking the conversions allowed implicitly
(without a cast) are what are called widening conversions.
For primitive types, the widening conversions are from any
integer type to any wider integer type, (int to long, etc) or
from a float to a double.
Narrowing conversions, by contrast, would include
conversion from long to int, or from a floating point type to
an integer type.
Narrowing conversions usually have to be specified explicitly
with a cast, e.g.
float x ;
intjava-fall01
i = (int) x ;
64
Overloading
A class can declare several methods with the same name,
providing each declaration has a different number of
arguments, or different argument types.
– We refer to the combination of the method name and its list
of argument types as the signature of the method.
Example:
class Shape {
setColor(Color c) { . . .}
setColor(int rgb) { . . .}
setColor(int r, int g, int b) { . . .}
...
}
– The method setColor() is overloaded with three different
signatures.
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Calling an Overloaded Method
If
the types of the argument expressions in a method
invocation exactly match the types of the parameters
in one particular declaration of the method, the
compiler naturally chooses to call that particular
method implementation.
There
is a complication, though: the Java language
allows implicit type conversion of method arguments.
– The allowed conversions are the widening
conversions.
In
general overload resolution chooses the most
specific method signature matching the actual
arguments.
– If there are several applicable
signatures, and66 no
java-fall01
Examples of overload resolution
•
•
•
•
•
•
•
•
•
•
•
•
•
•
void foo(long p) {. . .}
void foo(int p) {. . .}
void foo(long p, int q) {. . .}
void foo(int p, long q) {. . .}
long l ;
short s ;
int i ;
foo(l) ;
foo(s) ;
use
foo(l, s) ;
by
foo(i, i) ;
// Signature I
// Signature II
// Signature III
// Signature IV
// Exact match—use Signature I.
// Do widening conversion of s to int, and
// Signature II —unique “most specific” case.
// Uses Signature III —only case applicable
// widening conversions.
// Compile time error! Signatures III and IV
java-fall01
67
// are both
applicable but neither is more
Header of Class Definition—Details
In the abstract, the definition format is:
classModifiers class className
[ extends superclass ]
[ implements interfaceList ]
{
body of class
}
The optional extends and implements clauses will
be discussed in detail in later lectures.
Note classmodifiers can be absent – called default mode
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68
Modifiers of Classes
Possible classModifiers are:
members of a class are:
– default—the class may be used by code in current package.
member classes (see later)
– public—the
class may be used freely by code outside the
fields
package.
Methods
– abstract—the class contains abstract methods without
implementation
(abstract classes
will have subclasses that
Nested classes discussed
briefly later
define implementation of methods—see later).
– final—this class cannot have a subclass: see later.
– strictfp—all intermediate results in all float or double
expressions appearing in the class have strict IEEE 754
exponents.
– private—only allowed for a nested class. Meaning as for
other members.
– protected—only allowed for a nested class. Meaning as for
other members.
java-fall01
69
Modifiers of Fields I
In
the abstract, the declaration format is:
–
fieldModifiers type variableDeclaratorList ;
– where a variableDeclarator has the format:
–
fieldName [ dimensionStuff ] [ = expression ]
Examples
are:
– Static int fred = 3;
– int fred = 3;
// Equivalent to
– protected int fred = 3;
Note fieldmodifiers can be absent – called default mode
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70
Modifiers of Fields II
Possible fieldModifiers are:
– public—this field is accessible from any code.
– protected—accessible from code in a subclass (as well as
code in the same package—default accessibility).
– private—only accessible from code in the same class.
– static—this is a class variable: see earlier.
– final—this field cannot be modified after it is initialized.
– transient—the value of this field will not be included in a
serialized representation of an instance.
– volatile—any cached copy of the field maintained by an
individual thread will be reconciled with the master copy
every time the field is accessed.
java-fall01
71
Modifiers of Methods I
In
the abstract, recall, the declaration format is:
methodModifiers returnType
methodName (parameter list) [throws
exceptionList ] {
declarations and statements
}
An example is:
Public String getName() { // Accessor method
return name;
}
Note methodmodifiers can be absent – called default mode
java-fall01
72
Modifiers of Methods II
Possible methodModifiers are:
– public—this method is accessible from any code.
– protected—accessible from code in the same package (default) ,
or a subclass.
– private—only accessible from code in the same class.
Synchronized refers to treatment of concurrency
– abstract—the method has no implementation here—declaration
Which
is implemented
has a semicolon
in placewith
of athreads
body. in Java
anisinteresting
feature
of earlier.
Java, we will
– Although
static—this
a class method:
see
need itmethod
in this class
– Not
final—this
cannot be overriden: see later.
– synchronized—other synchronized methods are locked out while
Strictfp
doesisnot
allow default
Intel 80 bit internal
this method
executing:
see later.
– precision
native—the implementation of this method is given in a platformdependent language. Declaration has a semicolon in place of a
body.
– strictfp—intermediate results in all float or double expressions
appearing in the body have strict IEEE 754 exponents.
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The Java Object Model:
Inheritance and the Class
Hierarchy
java-fall01
74
Some Dependencies between
Classes
Use
– A uses B: the most informal and general relation. A might,
for example, call a method from class B, or have a method
with argument type B or return type B.
Containment
– A has a B: an important special case of use—class A has a
field of type B.
Inheritance
– B is an A: class B has all the properties of class A. The
compiler treats B as a special case of A, and allows an
instance of B to be used in any place where an instance of A
could appear. In general the class B will extend A with some
extra properties of its own.
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Inheritance
The inheritance relation is (unexpectedly?) powerful; it is built
into all fully object-oriented languages.
In Java, if some class A has been defined, we can
subsequently declare a new class, B, and specify that it
extends A.
Class A is called the superclass of B. Class B is a subclass of
A.
The class B is automatically given (inherits) all the fields and
method definitions of A. Further fields and methods can be
added that are specific to B.
In particular, for every method signature in class A, class B will
have a method with identical signature.
Crucially, though, the class B may define a different
implementation for some ofjava-fall01
those methods.
76
Trivial use of Inheritance
class Shape {
void setColor(Color color) {this.color = color
;}
Color color ;
int x, y ;
// position of center, say
}
class Circle extends Shape {
void drawCircle() {. . .}
double radius ;
}
class Rectangle extends Shape {
void drawRectangle() {. . .}
double height, width ;
}
Subclasses automatically inherit color, x, y fields of Shape,
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77
and setColor() method.
A Limited Kind of Polymorphism
void setAllColors(Shape [] shapes, Color color) {
for(int i = 0 ; i < shapes.length ; i++)
shapes [i].setColor(color) ;
}
Shape [] bag = new Shape [N] ;
bag [0] = new Circle() ;
bag [1] = new Rectangle() ;
...
setAllColors(bag, Color.red) ;
...
The function setAllColors works on a collection of shapes, and
works correctly independently of whether each shape is actually
a Circle or a Rectangle.
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78
Class Hierarchies
Class hierarchy diagrams represent inheritance
relations between classes:
Class:
Shape
Class:
Circle
Class:
Rectangle
These diagrams become more complex as subclasses
are further extended. But they are always trees,
because in Java each subclass has a single superclass.
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Inheritance with Overriding
class Shape {
void draw() {}
Color color ;
int x, y ;
}
class Circle extends Shape {
void draw() {. . .}
double radius ;
}
class Rectangle extends Shape {
void draw() {. . .}
double height, width ;
}
Subclasses override the definition of draw() in the superclass.
Bodies of methods contain the actual code for drawing a circle
or rectangle, respectively.
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80
True Polymorphism
void drawAll(Shape [] shapes) {
for(int i = 0 ; i < shapes.length ; i++)
shapes [i].draw() ;
}
Shape [] bag = new Shape [N] ;
bag [0] = new Circle() ;
bag [1] = new Rectangle() ;
...
drawAll(bag) ;
The draw() method invoked is the method defined in the class
of the referenced object (Circle or Rectangle).
– not the implementation defined in the compile-time type of
the variable, namely Shape.
drawAll() correctly draws a mixed bag of shapes whose details
may be unknown when this method is written.
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Runtime Lookup of Methods
class Shape {
void draw() {. . .}
}
class Circle extends Shape {
void draw() {. . .}
}
class Rectangle extends Shape {
void draw() {. . .}
}
Search up the inheritance
tree until find first
class that defines method.
class Square extends Rectangle {
// No declaration of draw()
}
Square s = new Square() ;
s.draw() ;
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Inherited Methods and Overriding
The method associated with the actual class of the instance is
called, even if it invoked from code in the superclass.
Suppose we add a drawInColor() method to Shape:
class Shape {
void draw() {}
void drawInColor(Color color) {
this.color = color ;
draw() ;
}
Color color ;
int x, y ;
}
The implementation of drawInColor() is inherited by the
subclasses. But when it is invoked on one, their own draw()
methods are called! More polymorphism.
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83
Abstract Methods and Classes
In our example, the draw() method in the Shape class did
nothing. It may not be necessary to give a implementation of
this method in the base class at all, because it may be that it is
only ever invoked on instances of subclasses representing
concrete shapes (as here).
In this situation, the superclass and unimplemented methods
can be declared abstract:
abstract class Shape {
//abstract class
abstract void draw() ; // abstract method
Color color ;
int x, y ;
}
– It is not possible to create instances of abstract classes.
One must create a subclass that overrides all abstract
methods of the base class, giving implementations.
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84
Final Methods and Classes
If a method is declared final, it may not be overridden in
subclasses (opposite extreme to abstract, which must be
overridden!)
If we declared draw() in Rectangle to be final, we could never
give a more specialized draw() in a subclass:
class Rectangle extends Shape {
final void draw() {. . .}
double height, width ;
}
class Square extends Rectangle {
void draw() {. . .}
}
// final method
// Compile-time error!!
– In places where the compiler can tell that a final method will be
called, it can produce optimized code to avoid overheads of “late
binding”.
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85
Protected Access
By default (no modifier) a field or method of a class can be
accessed by any code appearing in the same package.
– Packages are discussed later.
The access modifier protected on a field or method means that
this member can also be accessed by any subclass of the class
in which it is declared.
Note this modifier increases accessibility from the default. . .
– . . . because a subclass may be declared outside the
package that contains the superclass.
– Least accessible members are private (visible in declaring
class only), followed by default (declaring package only),
followed by protected (package and subclasses), followed
by public (visible everywhere).
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86
Inner Classes
Java
2 introduced a major addition to the original
language definition---"nested types".
A nested class (or interface) is a class/interface that is
defined “inside” some other class (or interface).
Nested classes and interfaces are sometimes loosely
referred to as "inner classes".
Technically, an inner class is only one of several kinds
of nested types in Java 2 (see next slide).
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87
Nested Types
field
<---->
member class
– includes:
includes:
– static variable
<---->
static member class
– instance variable <---->
inner class
local variable
<---->
local class
includes:
expression
<---->
anonymous class
Anonymous classes used in way reminiscent of lambda
functions in functional languages, (or "blocks" in Smalltalk)
to define pieces of code that are used *very* locally
– typically in the context of a single expression.
– Popular for defining simple event-handlers in GUI
programming, though many other applications as well.
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Critique of Nested Types
Nested types are undoubtedly a powerful and useful addition to
the language.
– Most useful nested types arguably static member classes
(for software engineering) and anonymous classes (for slick
"one-liners").
But the introduction of the whole paraphernalia of nested types
has significantly complicated the Java language definition.
– Many places where, for example, choice between inherited
and enclosing class behaviors could be ambiguous--needs
complicated rules to disambiguate.
– JVM definition was frozen before Java 2 introduced nested
types, so compilers must preprocess nested types to
equivalent outer types--ugly hack.
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89
The Universal Superclass—Object
The
Java language provides a superclass for all other
classes. If no extends clause is given in a class
definition, the class implicitly extends Object.
Array
types are also considered to extend Object.
A variable
of type Object can hold a reference to any
object or array.
– This is useful for generic capabilities which apply to all
Object classes
Strictly
speaking, Object is the root of every
inheritance diagram.
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90
Methods on the Object class
Public class
{
Reflection
is aObject
powerful
capability to allows you to find out
final Class
getClass() { . . . }
// Basis for reflection.
aboutpublic
an Object
at run-time
public
StringatoString()
//A String representation
toString
labels
class { . . . }
equals
testboolean
if Objects
are identical
the case
public
equals(Object
obj) {which
. . . } includes
// Equality test
if twopublic
objects
are identical
int hashcode()
{ . . .references
}
// For use by hash tables
protected Object clone() throws . . . { . . . } // Bit by bit copy
public final void wait() throws . . . { . . . } // Deschedule this thread
public final void wait(long millis) throws . . . { . . . }
public final void wait(long millis, int nanos) throws . . . { . . . }
public final void notify() throws . . . { . . . }
// Reschedule any . . .
public final void notifyAll() throws . . . { . . . } // . . . or all threads.
protected void finalize() throws . . . { . . . }
// invoked by GC.
}
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91
Reference Conversions
Conceptually, we saw, an instance of a subclass “is an”
instance of the superclass.
Hence one can assign a reference to a subclass object to
a variable of a superclass type.
– Shape fred; Circle jim; fred = jim; //allowed
Concretely, this implies a conversion from a subclass
type to a superclass type is regarded as a kind of
widening conversion.
– Recall widening conversions are allowed implicitly in various
contexts.
Narrowing conversions on reference types go the other
way—from a superclass down to some subclass.
– Narrowing conversions require an explicit cast.
– jim = (Circle) fred;
Good programming practice
minimizes use of narrowing
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92
An Aside on Coding Conventions
http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.ht
ml is a good set of “best practice” conventions
Code conventions are important to programmers for a number
of reasons:
80% of the lifetime cost of a piece of software goes to
maintenance.
Hardly any software is maintained for its whole life by the
original author.
Code conventions improve the readability of the software,
allowing engineers to understand new code more quickly and
thoroughly.
If you ship your source code as a product, you need to make
sure it is as well packaged and clean as any other product you
create.
There are another set at
http://g.oswego.edu/dl/html/javaCodingStd.html
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93
Simple Collections
The
package java.util contains a family of collection
classes.
Here we will only mention two of the most widely
used:
– Vector, and
– Hashmap.
Note Vector is supposed eventually to be
superceded by ArrayList.
– Consider using ArrayList in your future programs,
but Vector is so widespread we describe it here.
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94
A Vector is Like an Array
A Vector
can be used essentially like an ordinary
array.
It
has a well-defined current size, returned by the
size() inquiry.
This
can be set with setSize(), but usually a Vector is
grown dynamically using methods on next slide.
Vector
stores all elements as if the have type Object
If
0 < idx < size(), the methods:
void set(int idx, Object obj)
Object get(int idx)
respectively assign and retrieve value of element idx.
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A Vector can Grow and Shrink
Typically
one grows a vector by adding a new element
at the end with:
void addElement(Object obj)
Causes size() to be incremented by 1.
An
arbitrary element can be removed by
Object remove(int idx)
This method causes higher elements to be shifted
down one place, and size() to be decremented by 1.
An
element can be inserted in an arbitrary place by
insertElementAt(Object obj, int idx)
Element at idx and higher are shifted up one place,
and size() is incremented by 1.
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96
Using Vector
void drawAll(Vector shapes) {
for(int i = 0 ; i < shapes.size() ; i++)
( (Shape) shapes.get(i) ).draw() ; // Narrowing
conversion
}
Vector bag = new Vector() ;
bag.addElement(new Circle()) ;
// Widening conversion
bag.addElement(new Rectangle()) ;
...
drawAll(bag) ;
For polymorphism, Vector stores items in Object references.
Hence, get() returns an Object, which usually needs to be cast
back to a more specific type.
If the referenced object is not an instance of the type in the
cast, a run-time ClassCastException occurs.
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A HashMap is an Associative Array
For future reference, we also discuss HashMap here
A HashMap is similar to a vector, but the “index” is an
arbitrary object—very commonly a string.
– Perl has excellent support of this using {} syntax
This index is now called a “key”.
In simple cases you create a HashMap with the no-argument
constructor, then put key-value pairs in it using
Object put(Object key, Object obj)
(returns old value if key was already in the table).
Retrieve the element currently indexed by key by:
Object get(Object key)
Remove the element currently indexed by key by:
Object remove(Object key)
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98
Using HashMap
HashMap table = new HashMap() ; // Create
table.put(“red”, “stop”) ; // Insert hashes
table.put(“green”, “go”) ;
String s = (String) table.get(“red”) ; // returns “stop”
String t = (String) table.remove(“green”) ; // returns
“go”
String u = (String) table.get(“green”) ; // returns null
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99
Widening Conversions on Arrays
There is a widening conversion between two array types if there
is a widening reference conversion between their component
types.
This is useful, but can lead to anomalies if used carelessly:
Circle [] bag = new Circle [N] ;
setAll(bag) ;
// Widening: Circle [] to Shape [].
// OK at compile-time.
void setAll(Shape [] shapes) {
shapes [0] = new Circle() ;
shapes [1] = new Rectangle() ; // Widening: Rectangle to Shape.
// But throws ArrayStoreException
...
// if invoked as above!
}
Effect would be to assign Rectangle to array of Circles.
Requires the compiler to add a new kind of run-time check.
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Overloading with Inheritance
void foo(Object p) {. . .}
void foo(Shape p) {. . .}
void foo(Object p, Shape q) {. . .}
void foo(Shape p, Object q) {. . .}
// Signature I
// Signature II
// Signature III
// Signature IV
Object o ;
Shape s ;
Circle c ;
foo(o) ;
// Exact match—use Signature I.
foo(c) ;
// Do widening conversion of c to Shape, and use
// Signature II—unique “most specific” case.
foo(o, c) ;
// Uses Signature III—only case applicable by
// widening conversions.
foo(s, s) ;
// Compile time error! Signatures III and IV
// are both applicable but neither is more specific
// than the other!
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101
Overload Resolution across Classes
class Shape {
void foo(Circle q) {. . .}
// Signature I
}
class Circle extends Shape {
void foo(Shape q) {. . .} // Signature II
}
Shape s ;
Circle c ;
s.foo(c) ;
// Uses Signature I—exact match.
c.foo(c) ;
// Compile time error! Signatures I and II
// are both applicable but neither is more
specific
// than the other!
In compile-time overload resolution (choice of signature),
the prefix object expression is treated on the same
footing as an extra argument.
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Summary: Overloading vs. Overriding
Resolution of overloading occurs at compile time. The
compiler chooses a unique method signature out of
several different signatures available (or flags a compile
time error if it cannot).
Overriding occurs in the context of a single signature. In
general, if the class hierarchy contains several
definitions with identical method signatures, the
appropriate definition is chosen at run time.
Within the body of a class that overrides a method, the
method from the superclass can be invoked instead by
using the super prefix.
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103
Constructors and Inheritance
Constructors
of subclasses must invoke a constructor
of their superclass, to initialize the fields there.
If
a superclass constructor is not explicitly invoked,
the no-argument constructor of the superclass is
called, implicitly, by the compiler.
– A compile-time error is flagged if no such
constructor exists.
If
any superclass constructor other than the noargument constructor is required, it must be invoked
explicitly.
In
this case the first statement of a subclass
constructor is an explicit constructor invocation using
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the name super.
Superclass Constructor Invocation
class Shape {
public Shape(Color color, int x, int y) {
this.color = color
this.x = x ;
this.y = y ;
}
Color color ;
int x, y ;
}
class Circle extends Shape {
public Circle(Color color, int x, int y, double radius) {
super(color, x, y) ;
// superclass constructor invocation
this.radius = radius ;
}
double radius ;
}
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105
Exceptions
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106
Exceptions are Pervasive
Java
has a concept of exceptions similar to C++.
Unlike
C++, Java exceptions are strictly checked.
Most
classes in the standard Java library throw some
exceptions. We will see, these must be caught or
thrown.
This
means that it is almost impossible to write useful
Java code without some knowledge of the exception
mechanism!
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Exception Objects, and throw
Any kind of exception that can be thrown by Java code is
described by an exception object. It’s class must be a
subclass of Throwable.
If e is a Throwable object, the statement
throw e ;
behaves something like a break statement; it causes the
enclosing block of code to end abruptly.
If the throw statement appears inside a try statement who’s
catch clause matches the class of e, control is passed to the
catch clause.
Otherwise the whole method (or constructor) ends abruptly.
The exception e is thrown again at the point of invocation (in
the calling code).
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108
throw compared with break
try {
...
throw new MyException()
;
...
} catch (MyException e) {
...
}
...
myBlock : {
...
break myBlock ;
...
}
...
Control jumps to start of
matching catch clause
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Control jumps to end of
matching block
109
Methods that throw exceptions
In general, any exception that might be thrown in the body
of a method or constructor, in a place where it is not
enclosed by a matching try-catch construct, must be
declared in a throws clause in the header of the method:
void foo() throws MyException {
...
throw new MyException() ;
// throws clause
// No enclosing
// try-catch(MyException . .
.)
...
}
The compiler will insist invocations of foo() are treated with
the same care as actual throw statements—either
enclosed in matching try-catch constructs, or declared in
turn in the header of the calling method.
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Exception Handling in Nested Calls
void method1() {
try {
method2() ;
} catch (Exception3 e) {
doErrorProcessing(e);
}
}
void method2() throws Exception3 {
method3() ; // method2 just passes exception through
}
void method3 throws Exception3 {
throw new Exception3() ; // create exception
}
java-fall01
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Example using java.io
import java.io.* ;
PrintWriter out ;
try {
out = new PrintWriter(new FileOutputString(“filename”)) ;
// create and open
file
out.write(“stuff put out”) ;
...
out.close() ;
} catch (IOException e) {
// Catches all I/O errors, including read and write stuff,
System.err.println(“IO error: ” + e.getMessage()) ;
System.exit(1) ;
}
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How (not) to Ignore an Exception
Sometimes you can’t think of a good way to recover from
an exception—e.g. an exception thrown by a library
method. But the compiler forces you to do something.
Probably the worst thing you can do is to wrap the method
invocation in a try-catch with an empty catch clause—
– the useless try-catch constructs make the code unreadable, and
– meanwhile, ignoring an error condition and silently carrying on the
program may produce code even less reliable than, say, a typical
C program, where the library error probably at least aborts the
whole program!
Usually it is safer to have your methods throw the
exceptions—all the way up to the main method, if
necessary. Then at least the program will stop.
If you are really lazy you can just declare every method
you ever write with throws Exception. . .
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Part of the Exception Hierarchy
Throwable
Error
Exception
RuntimeException
IOException
...
EOFException
FileNotFoundException
InterruptedIOException
catch(FileNotFoundException e) { . . . } would catch
specific exception whereas
catch(IOException e) { . . . } would catch all
IOexceptions
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Unchecked Exceptions
There are two exceptions (!) to the rule that all
exceptions must be explicitly caught or thrown.
Error classes usually represent problems that might
occur unpredictably in the JVM. For example
OutOfMemoryError (although unusual in practice)
might occur at almost any time.
RuntimeException classes usually represent errors
“built into” the language—not thrown by a throw
statement. There are about 20, including:
– ArithmeticException, ArrayIndexOutOfBoundsException,
NullPointerException, ClassCastException, etc.
Note that exceptions that are thrown but not caught
appear as error message on stderr. For applets they
appear in the “Java console” of the browser.
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Defining you own Exceptions
The Exception class has fields and methods to give information how the
exception occurred. There are two constructors; one includes a custom
message
Can throw an exception of type Exception with a unique message, or
create a subclass:
class MyException extends Exception {
public MyException () {
super ("This is my exception message.") ;
}
public MyException (String gripe) {
super (gripe);
}
}
public static void MyMethod() throws MyException {
...
throw new MyException() ;
...
}
Methods e.getMessage() and e.printStackTrace() can be used on
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exceptions.
Interfaces
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Abstract Classes Revisited
Recall an abstract class is a class that contains some
abstract method declarations, with no implementation.
An abstract class can only be instantiated indirectly, as a
superclass of a class that overrides all the abstract
methods, and gives them an implementation. You
cannot directly create an instance of an abstract class.
– Constructors, static methods, private methods cannot be
abstract.
– A subclass that does not override all abstract methods is still
abstract.
– A method that overrides a superclass method cannot be
abstract
But an abstract class will generally also contain “nonabstract” members—method implementations, instance
variables, etc—and constructors.
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Interfaces
An interface is something like an abstract class where
every method is required to be abstract.
An interface specifies a collection of instance methods
(behaviors) without giving the implementation of their
bodies— akin to giving an API:
public interface Storable {
public abstract void store(Stream s) ;
public abstract void retrieve(Stream s) ;
}
Interfaces cannot include instance variables,
constructors, or static methods.
They can include class variables, but only if they are
declared final—essentially constant definitions.
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Implementing an interface
As for an abstract class, one cannot directly create an
instance of an interface.
Unlike an abstract class, one cannot even extend an
interface to create a class. An interface is not a class,
and it cannot have subclasses.
Instead, a class must implement an interface:
public class Picture implements Storable {
public void store(Stream s) {
// JPEG compress image before storing
...
}
public void retrieve(Stream s) {
// JPEG decompress image after retrieving
...
}
}
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An Interface is a Contract
Any class that implements an interface is guaranteeing
a set of behaviors. The body of the class will give
concrete bodies to the methods in the interface.
– If any methods in the interface are not implemented, the class
must be declared abstract.
Example: a class that defines the behaviour of a new
thread must implement the Runnable interface:
public interface Runnable {
public void run() ;
}
Any interface defines a type, similar to a class type. An
instance of any class that implements a particular
interface can be assigned to a variable with the
associated interface type.
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An Interface Defines a Type
Assume the classes Picture and StudentRecord both
implement the Storable interface:
public class StudentBody {
Stream s;
...
public void register(Picture id_photo, StudentRecord
id_card) {
save(id_photo);
Widening Conversion
save(id_card);
}
public void save(Storable o) {
// o has type Storable
o.store(s); // as Storable must have method store
}
}
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Classes can Implement Several
Interfaces
Interfaces address some of the same requirements as
multiple inheritance in C++ (for example), but avoid
various complexities and ambiguities that come from
inheriting implementations and instance variables from
multiple superclasses.
A class can extend its superclass and implement
several interfaces:
class Picture implements Storable, Paintable {
// Body must now include any methods in Paintable,
// as well as store() and retrieve().
...
}
Instances of the class acquire all the implemented
interface types, in addition to inheriting their superclass
type.
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Interfaces can Extend other Interfaces
e.g.
An interface can extend one or more other interfaces:
interface Material extends Storable, Paintable {
// Additional methods if necessary. . .
...
Magenta
extends Red but implements cyan
}
If non-trivial “lattices” of types are really needed, eg:
NoColor
Red
Blue
Green
Magenta
Yellow
Cyan
AnyColor
they can be implemented using interface types.
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Interfaces can hold Constant Definitions
Interfaces can hold fields, provided they are static and
final.
An interface can be a natural place to define a collection
of related constants, perhaps simulating a C-like
enumeration type:
public interface Direction {
public final static int NORTH = 0 ;
public final static int EAST = 1 ;
public final static int SOUTH = 2 ;
public final static int WEST = 4 ;
}
Use constants by, eg, Direction.NORTH.
Sometimes a class will implement such an interface,
just so it can access the included constants without
using the Direction prefix.
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Interfaces can be used as Markers
The Java environment includes several examples of
empty interfaces that are used only as markers.
By implementing such an interface, the programmer is
typically telling the compiler or runtime system to treat
the class in some special way:
– Cloneable—the Object.clone() method will throw an exception
if invoked on an object from a subclass that does not implement
the empty Cloneable interface.
– Serializable—the ObjectOutputStream.writeObject() method
will not write an object that does not implement the empty
Serializable interface.
– Remote—any class whose methods may be invoked remotely
using the RMI mechanism, must implement the empty Remote
interface.
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Summary
Interfaces play a crucial role in structuring programs that
need to declare multiple sets of behaviors such as
applets and threads.
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Packages
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Packages
One
file can contain several related classes, but only
one of them can be public. If the public class is called
Wheat, then the file must be called Wheat.java.
A set of classes in different files can be grouped
together in a package. Each file must start with a
package declaration, eg:
package mill;
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Packages and Directory Structure (JDK)
In JDK, each of the files in one package must be in the
same directory (which may be in an jar archive file).
For simple package names, the name of the directory
should be the same as the package:
Directory name:
File:
mill
wheat.java:
Package mill ;
Public class Wheat { …}
…
Stone.java:
Package mill ;
Public class Stone { …}
…
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Hierarchical Package Names
Packages can be grouped hierarchically. For example,
the mill package could be nested in a package called
agriculture. Then the name of the package would be
changed to agriculture.mill (full name required).
In JDK, the classes of agriculture.mill should appear in
a directory called:
agriculture/mill
agriculture\mill
(UNIX)
(Windows)
(relative to some directory, which must appear on the
user’s CLASSPATH).
Standard Java libraries are in packages with names like
java.lang, java.util, java.io, etc.
If you need to construct a globally unique name, can
use your Internet domain name, inverted, as a prefix,
eg:
edu.fsu.csit.mpiJava
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Fully Qualified Class Names
A class can always be referred to in Java code by its
fully qualified name which includes the package name
as a prefix, eg:
public class VectorTest {
public static void main (String [] args) {
java.util.Vector bag = new java.util.Vector() ;
bag.addElement(new java.lang.String(“item”)) ;
}
Using fully qualified names is tedious in general.
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Import statements
The import declaration allows you to avoid giving fully
qualified names, eg:
import java.util.Vector ;
// import declaration
public class VectorTest {
public static void main (String [] args) {
Vector bag = new Vector() ;
bag.addElement(new String(“item”)) ;
}
Can also import all classes in, eg, java.util by
import java.util.* ;
(but note wildcard can only appear in last position).
Note classes (like String) in java.lang are automatically
imported.
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CLASSPATH
The import declaration only controls conventions on
naming within a source file. It doesn’t address basic
accessibility of the class files. You can use a class
without importing it.
In JDK (except for classes provided with the Java
language) jar files or root directories of any package
used (or class files for any classes not in any package)
must be in the current directory, or in a directory in the
CLASSPATH environment variable.
This variable is used by both the compiler javac and the
JVM command, java.
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Java System Packages, I
java.lang contains essential Java classes and is by
default imported into every Java file. So import
java.lang.* is unnecessary. For example Thread, Math,
Object and wrapper classes are here.
java.io contains classes to do I/O.
java.util contains various utility classes that didn't make it
to java.lang. Date is here as are Vector, hashtables, etc.
java.net contains classes to do network applications.
Sockets, Internet addresses, URLs etc.
java.applet has the classes needed to support applets
java.awt has the original classes to support windowing—
The Abstract Windows Toolkit.
java.awt.image has image processing classes.
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Java System Packages, II
java.awt.datatransfer contains classes to transfer data from a Java
program to the system clipboard (drag-and-drop).
java.beans contains classes to write reusable software components.
java.lang.reflect enables a program to discover the accessible
variables and methods of a class at run-time.
java.rmi—classes for Remote Method Invocation.
java.security enables a Java program to encrypt data and control
the access privileges provided.
java.sql—Java Database Connectivity (JDBC) enables Java
programs to interact with a database using the SQL language.
java.text are classes that provide internationalization capabilities for
numbers, dates, characters and strings.
java.util.jar combines java .class files and other files into one
compressed file called a Java archive (JAR) file.
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Additional Java 1.2 System Packages
javax.accessibility—contracts
between user interface
components and assistive technology.
javax.swing—additional user interface components
as well as providing standard “look and feel” for old
ones.
– border, colorchooser, event, filechooser, plaf, table,
text, tree, undo
org.omg.CORBA—Provides the mapping of the
Object Management Group CORBA APIs to the Java
programming language, including the class ORB,
which is implemented so that a programmer can use it
as a fully-functional Object Request Broker (ORB).
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Further information
The Java 2 API specification:
http://java.sun.com/products/j2se/1.4/docs
/api
documentation in javadoc format.
The Java Class Libraries, 2nd Edition, Volumes
1 and 2, plus supplements for the Java 2
platform.
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