Transcript Java: the core language

Java: the core language
• What it looks like:
class OnceAgain {
public static void main (String[] args){
int X = 1234;
System.out.println (“we’ve seen this before”);
}
}
• A superficial resemblance to C++
References
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Formal definition: Gosling, Joy & Steele
White paper (early draft): Gosling
Cornell & Horstmann : Core Java
10,000 other books
Java is work in progress: no ISO standard in
sight
white paper buzzwords (Gosling et al)
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Simple
Object-Oriented
Distributed
Robust
Secure
Architecture neutral
• Portable
• Interpreted
• High Performance
• Multithreaded
Core language
• Imperative, procedural language
• Object-oriented: inheritance is central
• Garbage-collected
• conventional control structures
• Exceptions and event-handling
• concurrency
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can’t really be simple will al that...
The type model: primitive types and classes
• Conventional primitive types with value semantics: int,
short, long, byte, float, double, char, boolean
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int Value = 123;
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char BigA = ‘A’;
• Language defines size of numeric types (eg int is 4 bytes)
• char is Unicode (16 bits)
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char badkspace = \u0008;
• Some implicit conversions on numeric operations
Classes
• The basic abstraction mechanism
• A long lineage:
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Simula (algol derivative with concurrency)
Smalltalk (fully dynamic, class is everything)
Eiffel (hybrid)
C++ (Hybrid, multiple inheritance)
CLOS (dynamic, multiple inheritance)
Class encapsulates data and code
Public class counter {
counter (int init) { value = init;}
counter () { value = 0;};
public int val () { return value;};
public void incr () { value = value + 1;);
private int value;
}
Contrast with Ada
• A package contains both type declarations and
subprogram declarations (like a class)
• Related types can appear in the same package
(unlike a class)
• A package is not a type (unlike a class)
• Class unifies (or confuses) the notion of
package and type
OO terminology
• An object is an instance of a class
• A method defines a message that the object can
receive
• A message is received by a particular object at
run-time
• Within a method, the current object is called this
– if X is local data, a reference to X is equivalent
to this.X
Using a class
class TestCounter {
counter c1 = new counter ();
counter c2 = c1; // an alias of c1
c1.incr();
// call with implicit parameter c1)
if (c1.val() > 0) c2.incr(); };
}
Is everything an object?
• Only in Smalltalk (even classes!)
• Often necessary to define data and methods
without any explicit object to which they belong
public class Math {
public static double sqrt (double X);
public static final double PI = 3.14159265;
}
can then write X = Math.sqrt (Y * Math.PI);
Classes are not enough
• Need other tools for large program structuring
• Packages: group of classes, with privileged
visibility
• Mimics directory structure of OS
– package MyGraphics; // locus for current class
– import java.io.*;
// get access to all classes therein
– public class Polygons { … } // in MyGraphics
• Interfaces: abstract classes
Predefined packages
• A large built-in programming environment
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java.lang: Object, String, Math…
java.awt: graphical interface
java.awt.event : interaction with devices
java.applet: the web
java.io: stream and file manipulation
java.util: data and time
java.net: communications
Java is a high-level language
• User does not concern itself with the size of
objects. Storage management is controlled by the
system
• Garbage collection must be safe: user cannot
manipulate addresses directly, language cannot
have pointers.
• No need for destructors or free commands
• Garbage collection is unpredictable: unsuitable for
real-time applications
Can you program without pointers?
• Pointers are implicit: objects and array have
reference semantics
class List {
int value;
List next;
}
primitive types have value semantics
Value/Reference semantics
int x, y;
x = y;
// value copy
y = y + 1;
// x is unaffected
counter c1, c2;
c2 = c2;
// value sharing: denote same object
c2.incr(); // c1 is modified as well
int table1 [], table2 [];
table1 = table2; // reference semantics also
Equality works like assignment (value /reference)
Value/reference choices
• Ada : reference requires explicit access
types. Arrays have value semantics
• C++ : structs have value semantics
• SETL : all types have value semantics
• pure LISP : functional with no side-effects,
no assignment, value semantics with
implicit sharing for efficiency
Arrays and Strings
• Impractical to make them classes: standard
semantics across languages, need efficient
implementation
int table []; // variable definition
table = new int(100); // object allocation
• Indexing from zero. Index always checked
against bounds
Multidimensional arrays and aggregates
• Multidimensional arrays are arrays of arrays
int[][] matrix =
({ 2,4,6}, {3, 6, 9}, 1, 2, 3}};
matrix [2][2] = 0;
matrix [3][3] = 100; // will raise exception
Strings
• Could be arrays (unification or confusion?)
• Instead, special syntax
• Many special-purpose operations
String message = “ what’s new?”;
message = message.substring (0, 6) + “up doc?”;
Note: only operator overloading in Java, no userdefined overloading (reaction to C++)
Strings are constants
• Need separate class for mutable strings:
• class StringBuffer:
public int length (); // also strings and arrays
public int capacity ()
public synchronized char charAt (int index)
public synchronized void setCharAt (int index,
char ch);
Inheritance
• Programming by extension and specialization:
• A class refines an existing class by adding data
members and methods, and redefining methods
• What is not redefined is inherited
class Point { int x = 0; int y = 0; …};
class Pixel extends Point { int R,G,B; …};
Polymorphism
• A variable of a given class can denote an
• instance of any descendant of the class:
class Root…
class Better extends Root …
class Best extends Better…
Root thing = new Root ();
.. Thing = new Best (..); // ok
Dynamic Dispatching
• If class Root has method Mangle, then all its
descendants either inherit or redefine Mangle.
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Thing.Mangle(..); // what gets executed?
• Objects carry run-time identification. The class
denoted by the current value of the object
determines the method to be called
The power of polymorphism
• The functionality of a system can be
extended by adding class extensions. The
code that manipulate existing classes does
not need to be modified.
• Object-oriented programming = inheritance
+ polymorphism + dynamic dispatching
The mother of all classes
• Every class is the extension of another.
• The root of the class hierarchy is Object
• Object has no visible data, and several
generally useful methods:
boolean equals (Object O);
String ToString ();
Mixing classes and primitive types
• We want to write a “generic” matrix that can have
integer or real components.
• Can write Object[][] matrix;
• but can’t put numeric values in matrix because ints
are not objects.
• Solution: wrapper classes
matrix [0][0] = new Integer (5);
Wrapper classes
• Integer, Float, Double, Boolean
• Constructors, conversion operations, etc.
• No overloading:
Integer I1, I2, I3;
• can’t write:
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I1 = I2 + I3;
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I1 = new Integer (I2.value + I3.value);
Class Object and generic programming
Object thing;
• thing can denote an instance of any class: compiler
cannot check legality of any usage, all checks are
at run-time.
Object [] bag;
• no way to define a homogeneous bag.
• The absence of templates/generics is the largest
deficiency in the language
A large built-in collection of classes
• Object
– Font
– Component
– Container
• Panel
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Applet
– Button
– label
• Layout manager
Superclasses and subclasses
Class Root {..};
Class Better extends Root {…};
Root thing1 = new Root ();
Object anything = new Better ();
anything = thing1;
// ok
thing1 = anything;
// illegal
thing1 = (Root)anything; // run-time check
Interfaces
• A package spec with no body
• An abstract class that can have multiple
implementations
• A substitute for genericity
public interface Comparable {
public boolean LessThan (Comparable C);
public boolean Equals (Comparable C);
}
Implementing an interface
• Any class that has operations with the right
signature can be said to implement the interface:
class Point implements Comparable {
int X, Y;
public boolean LessThan (Comparable C) {
return X < (Point (C).X;
}
A class can implement multiple interfaces
Event-Driven programming
• Events happen asynchronously (mouse click, button push)
• Events are objects:
– AWTevent
• ActionEvent
• ComponentEvent
– ContainerEvent, PaintEvent, WindowEvent…..
• Events have a source
• Events can be handled by designated components
Event-Handling
• Delegation-based: source object notifies a
registered listener
• listener must implement the appropriate interface:
Interface MouseListener {
public void MouseClicked (MouseEvent e)
public void MouseEntered (…)
public void MouseReleased (…)
...
A listener
class Ringer extends Frame implements ActionListener {
private Button ring;
public Ringer () { // constructor
ring = new Button (“press here”);
ring.addActionListener (this);
}
public void ActionPerformed (ActionEvent e) {
// play something
}
white paper buzzwords (Gosling et al)
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Simple (no chance)
Object-Oriented (definitely)
Distributed (yes)
Robust (more than C/C++)
Secure (ditto, generic programming less safe)
Architecture neutral (the advantage of the JVM)
Portable (if standard is developed and followed)
Interpreted (performance penalty)
High Performance (hope for now)
Multithreaded (less than Ada)