Transcript lecture1 - University at Albany

MSI 692: Special Topics in
Information Technology
Sanjay Goel
University at Albany, SUNY
Fall 2004
Sanjay Goel, School of Business, University at Albany, SUNY
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Outline for the Class
Part 1
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Introduction
What is Java?
How is Java different from other languages?
Tools for Programming in Java
Java Language – Lexical elements
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Keywords
Identifiers
Literals
Operators
Punctuation
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Outline for the Class
Part 2
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Data Types
Variables and initializations
Methods
Numbers
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Types
Ranges
Expressions
Conversions
• Operators
– Assignment and Increment
– Precedence and Associativity
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Programming
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Programming
Why do we need to write programs?
• Humans talk in a different language than computers do
and we need to translate human talk into computer talk
or instructions.
– You write the program in a language you are comfortable with
– The compiler takes your program and smooshes it, digests it,
and creates a stream of zeros and ones for the computer to
understand.
• Normally, different computers have different languages
that they understand similar to the various human
languages (i.e. French, English, Sanskrit & German)
• Thus, we have different compilers for different
computers.
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Programming
What are the advantages of Java?
• Eliminates pitfalls of C/C++ by eliminating pointer
arithmetic and explicit memory management.
• Is object-oriented which helps enforce better
programming techniques and to better visualize the
code
• Interpreted environment which improves speed of
development
– Reduces compile-link-load cycle
– Is portable across multiple platforms
• Runs more than one parallel activity via threads
• Enables security by checking code modules that are
loaded
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Programming
How is Java different from other languages?
• Uses a Java Virtual Machine (JVM)
– JVM is a byte-code interpreter unique for different hardware
– Takes burden of compling on multiple hardware away from user
• C/C++: Source  Compiler  Machine Code
• Java: Source  Java Compiler  Byte Code  Java Virtual Machine  Machine Code
• Garbage Collection
– No memory management
• Code Security
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Programming
What are the advantages?
• Expands your programming horizons across the
enterprise
– You are able to write programs which span across the internet
– You can have a database sitting in one location on a compute
server in another location and a web server in a third place
which will communicate
– Java makes it easier to program across the network
– You can develop user interfaces
• All these facilities are available as extensions of the basic
core Java language.
– We are going to focus on the core language in this class
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Programming
IDE Options
• Requirements for Development of Programs
– Editor (Emacs/Wordpad)
– Compiler (Java SDK for Windows)
– Execution Environment (Unix Shell/DOS Prompt)
• Several Different Development Environments exist
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BlueJ
JBuilder
Forte from Java
JDeveloper from Oracle
Java Visual Café
• If any one wants to install java on their personal computers a link to the
instructions is located on the syllabus.
– Please interact with Tony Manes or D.J. Anderson with questions about this
installation.
– Come to me if they are unavailable or unable to help.
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Language Constructs
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Language Constructs
Introduction
• Like any language a programming language has some
symbols and some rules to combine these symbols to
make sense.
– A compiler breaks individual characters the program into
tokens. These tokens are then interpreted with respect to the
context they exist in and processed.
• Most of the previous languages were based on the 127
ASCII characters
– Adequate representation to the English language keyboard.
• Java is based on Unicode that has 64000 characters
– Allows representation of multiple languages other than
English for programming.
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Language Constructs
Types of Tokens
• There are five types of tokens which are processed:
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Keywords
Identifiers
Literals
Operators
Punctuation
• There are two types of tokens that are not processed
– White Space
– Comments
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Language Constructs
White Space
• Uses
– Used to separate tokens in program that are not separated by
separators
– Make the program more readable
• There are three types of white spaces
– space (when you hit the space bar)
– tab (when you hit the tab key)
– newline (when you hit the newline key)
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Language Constructs
Comments
• Comments are used for documenting the code.
• There are 3 types of comments
– Single line comments
// This is a single line comment
– Multi-line comment
/*
This is a multi-line comment
So write multiple lines here or only one if you choose to
*/
– javadoc comments
/** This is a javadoc comment to generate documentation at the end
of the code
@author Sanjay Goel
@deprecated
*/
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Keywords
Reserved words with special meaning
• These are reserved words in the language and have a
special meaning to them.
– There are 47 key words in Java language.
– The keyword is separated from other keywords or identifiers
by white space, comment or punctuation.
• const and goto have no meaning in java but had
meaning in C & C++.
– They have been included to facilitate error reporting when
users use these words in java
– null, true and false are not keywords but are literals.
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Keywords
Keywords in Java
• The keywords in the Java language are as follows:
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abstract double int strictfp
boolean else interface super
break extends long switch
byte final native synchronized
case finally new this
catch float package throw
char for private throws
class goto protected transient
const if public try
continue implements return void
default import short volatile
do instanceof static while
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Identifiers
Naming the Java elements
• These are the names that used to specify elements of
the Java program, such as class variable or method i.e.
– These are the names that you assign to different constructs of
the program.
• An identifier is any sequence of Java letters and digits.
– The first of that must be a Java letter.
– A key word can not be an identifier and the literals null, true
and false can not be identifiers.
• Java letters include the upper and lower case letters of
the English alphabet
– i.e. a-z, A-Z, $, and underscore "_"
• Java digits are 0-9
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Literals
Value representations for primitive numeric types
• Java has built in types for numbers, characters and Booleans.
– It also has a built in class for String.
• Literals are the value representations for the primitive numeric
types, the Boolean and character as well as the standard class type
of String.
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String
String
char
char
Boolean
Boolean
double
int
int
"Carrot“
"Emily“
'a‘
'b‘
true
false
3.21
12
234234
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Operators
Mathematical
• Operators
– /, *, +, -, =
• Precedence
– Hierarchy that determines the order in which operators are
evaluated
• Associativity
– Order in which operators of equal precedence are evaluated
(Left to Right or Right to Left)
• BODMAS
– Binary operation of Division, Multiplication, Addition,
Subtraction
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Operators
Relational
• They determine the relationship between two operands (values).
• They are used in a condition expression in order to test whether
a particular kind of relationship exists between two operands.
• All relational operators return a Boolean Value, which evaluates
if the relationship between the operands is true or false.
• Java utilizes a conditional expression, which uses one or more
relational operators to discover if the statement is true or false.
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Operators
Relational
• Relational Operators:
Equal to
==
Not equal to
!=
Greater than
>
Less than
<
Greater than or equal to
>=
Less than or equal to
<=
• Example: The if..else statement. If the expression is true, execute
the if statement, and if it is false, execute the else statement.
b < = a
• If b is less than or equal to a, then the expression is true, and the if
statement is executed. If b is greater than a, then the expression is
false, and the else statement is executed.
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Operators
Logical
• In order for Java to assess two relational expressions, they must
be linked together using a logical operator.
• Logical Operators:
And
&&
OR
ll
And
&
OR
l
Ternary if-then-else
?:
• The AND Logical Operator (&&)- For the statement to be
true, both relational expressions have to be true. If one or both
of the expressions is false, then the AND logical operator
returns a Boolean false.
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Operators
Logical
• The OR Logical Operator (ll)- For the statement to be true, only
one of the relational expressions has to be true. If both of the
expressions are false, then the OR logical operator returns a Boolean
false.
• The Single AND and OR Logical Operators (&,l)- When using
the && and ll Operators, only the first expressions are evaluated. If
the first expression is false in the && Operator, then the second does
not need to be evaluated. If the first expression is true in the ll
Operator, then the second does not need to be evaluated. However,
when using the single AND and OR Operators, both expressions need to be
evaluated regardless of if they return a Boolean true or false.
• The Ternary Operator (?:)- Uses a relational expression and two
values. If the relational expression is true, the ternary operator uses
the first value. If it is false, it uses the second value.
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Operators
Bitwise
• Bitwise Operators help the programmer change the value of a bit
(binary digit) from zero to one, or one to zero.
• Bitwise AND Operator (&)- It compares two bits. If both of the
bits are 1’s, then the AND operator returns a 1. If only one bit is 1 or
both are 0, then it returns a 0.
Example
00001111
& 00001010
00001010
• Bitwise Inclusive OR Operator (l)- If one or both bits are a binary
1, then a binary 1 is returned. If both bits are a binary 0, then a binary
0 is returned.
Example
00001111
l 00001010
00001111
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Operators
Bitwise
• Bitwise Exclusive OR Operator (^)- If one bit is a binary 1, then a
binary 1 is returned. If both bits are a binary 1, then a binary 0 is
returned. If both bits are a binary 0, then a binary 0 is returned.
Example- 00001111
^ 00001010
00000101
• Bitwise Left Shift Operator (<<)- This operator moves bits one
position to the left. Then leftmost bit then drops, and the rightmost bit
is replaced with a 0.
Example- 00001111

00011110
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Operators
Bitwise
• Bitwise Signed Right Shift Operator (>>)- This operator
moves bits one position to the right. The empty position on the
left is filled with a value that represents the same sign as the
rightmost bit that was shifted. (positive bit = 0, negative bit = 1)
Example- 10001111

10000111
• Bitwise Unsigned Right Shift Operator (>>>)- This operator
moves bits one position to the right. The empty position on the
left is always filled with a binary 0 (the sign of the bit is not
significant).
• Bitwise Complement Operator (~)- This operator reverses the
values of the bits. Binary 1’s become Binary 0’s, and vice versa.
Example- 00001111

11110000
• “Two’s Complement”- This term refers to reversing all of the
bits, and then adding 1 to the result.
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Punctuation
Separators
• These are the separators used in expressions i.e.
– ";"
– ","
– "{ }”
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Data Types and
Variable Declarations
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Data Types and Variable Declarations
Introduction
• Data type determines how data is represented in computer
memory. Each variable has to have a defined type
• There are 8 primitive types.
– Numeric - byte, short, int, long, float, double
– Character – char
– Logic - boolean (can have value of true or false)
• In addition there are built in classes like Button, Frame, Point,
String.
– There are more than a few thousand classes and new classes are added
continuously.
– There is a SUN community process that determines what new classes to
add.
• The data values of classes are called objects.
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Data Types and Variable Declarations
Variables
• These are identifiers that are used to refer to values
stored in the computers memory.
– e.g. int i;
– int j, k;
• Variables can be initialized also
– e.g. int j = 10;
– boolean flag = true, iflag = false;
– String sentence = "Who am I";
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Number Types
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Number Types
Types of Numbers
• Two types
– Integer representations
– Floating point representations
• A binary digit (bit) is the smallest unit of information that can be
stored on the computer and can have a binary value i.e. 0 or 1.
• A combinations of bits is used to represent larger range of
values.
• For integers bits are interpreted with the binary number system.
– Integers can be represented as a combination of binary digits or bits.
– Different precisions are used to store different numbers
• Negative numbers use two’s complement representation
– Allows use of binary arithmetic operations on signed integers
– Positive 2's complement numbers are represented as simple binary.
– Negative 2's complement numbers are represented as the binary number
that when added to a positive number of the same magnitude equals zero.
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Number Types
Negative Numbers and Floating Point Types
• Integral Types
byte
8
-128 to 127
short
16
-32768 to 32767
char
16
0 to 65536
int
32
-2147483648 to –2147483647
long
64
-9223372036854775808 to 9223372036854775807
• Floating point types
Float
32
Precision: 7 digits
Double
64
Precision: 15 digits
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Number Types
Arithmetic Expressions
• There are 5 basic arithmetic operators
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Addition
Subtraction
Multiplication
Division
Modulus
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Number Types
Conversion
• Any primitive numeric types can be used with these arithmetic
operators.
• If the operators are of different types the types are converted so
that they are of the same type.
• This conversion scheme is called the binary numeric promotion.
– if either operand is double the other operand is converted to double
– else if either operand is float other operand is converted to float
– else if either operand is long other operand is converted to long
– else both operands are converted to int.
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Number Types
Explicit Type Conversions
• There are two type of conversions
• Widening primitive conversions
– These go from a lower resolution to a higher resolution type
and thus there is no loss of information.
– These conversions are sometimes done via binary numeric
promotion
– Example:
int x = 1, y = 2;
float z = x + y;
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Number Types
Explicit Type Conversions cont’d
• Narrowing primitive conversions
– These go from a higher resolution type to a lower resolution type and can
lead to loss of resolution.
– An explicit conversion is required via a cast operator to achieve this.
– This is dangerous and should be done carefully
int i = 127, j = 128;
byte iAsByte = (byte)i;
byte jAsByte = (byte)j;
System.out.println(i);
System.out.println(j);
output:
127
-128
– Since the max positive number is 127 the number representation gets
screwed up in memory.)
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Number Types
Operators
• Assignment operator : "=“
– assigns the rhs to the lhs
– i.e. i = 10;
+= -= *= %= >>= <<= &= ^= !=
• Increment and decrement operators
++i --i i++ i--
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Number Types
Associativity and precedence
• Precedence - hierarchy of evaluation
• Associativity - If level is the same what is the order of
evaluation
• You can make the expression more explicit by using
parentheses.
– e.g. a + b * c vs. (a + b) * c
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Building a Program
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Building a Program
Sample Program
• Let us examine a simple program
• Prior to dissecting this let us look at the various structures in
the program.
• Just like in writing a book you construct the basic characters
into words, words into statements, statements into paragraph
and paragraphs into chapters and chapters into books.
• Similarly we can arrange the program constructs into a
hierarchy.
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Building a Program
Methods
• Methods are group of instructions with a name.
• You can write very simple programs in a single monolithic block
but programs can get large and as large as a few million lines.
– You need to separate pieces out for ease of development and
maintenance.
• The way to separate them is writing methods which are a group
of instructions with a name and signature.
– Depending on the size of the program there can be many methods.
– Some methods are already defined in the language.
• main method
– The main method is a special method. The program starts execution from
the main method.
public static void main(String args[]) {
}
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Building a Program
Writing to console
• System.out.print(ln)
– System.out.print("I want to test without carriage return");
– System.out.println("I want to test with carriage return");
Hello World program
/** Hello world program for class lecture
* @author Sanjay Goel
* @created August 26, 2001
*
*/
import java.io.*;
public class HelloWorld {
public static void main(int args[]) {
String string1 = "Hello ";
String string2 = "World";
System.out.println(string1 + string2);
}
}
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/** Program to sum two numbers
* @author Sanjay Goel
* @created August 26, 2001
*
*/
import java.io.*;
public class Sum{
int a = 10;
int b = 30;
int sum = a + b;
System.out.println("a = " + a + " b = " + b + "
Sum = " + sum);
}
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Review
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Recap
Keywords and Identifiers
• Keywords
– Reserved words which have special purpose and can’t be used
for any thing else
– 47 in total – new words may be added as language expands
• Identifiers
– Also called variables are names of the data elements which
you have stored in memory
– Formed by sequence of Java letters and digits
– All keywords and three literals (true, false, null) can not be
used as identifiers
– Note: Java letters are A-Z, a-z, underscore & $
– Java numbers 0-9
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Recap
Literals
• Literals
– These are the values you assign to the variables
– The variables have to be of specific types
• Types of Literals
– Numeric
• Whole Numbers – integer type numerics
• Fractional Numbers – floating point type numerics
– Char
• single character, digit, symbol
– Boolean logic
– Strings
– Text
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Recap
Numeric Types
• The numeric types are:
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Byte
Short
Int
Char
Long
Float
Double
8 bits
16 bits
32 bits
32 bits
64 bits
32 bit
64 bits
• Why do we have so many different types?
– To economize on use of memory
– We use more memory for larger whole numbers or for fractional numbers
with higher precision.
• The ranges were discussed in the previous class.
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Recap
Operators, Punctuation, White Space & Comments
• Operators
– These are the symbols used for arithmetic operations on numbers
– Unary operators operate only on one number ++, -– Binary operators operate on two numbers +, -, /, %
• Punctuation
– These are the elements that act as separators
– ; {} [] ,
• White Space
– Spaces, tabs, newline
• Comments
– Three types: Single line, multiline, Javadoc comment
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Recap
Initialization, Methods, and I/O
• Variable initialization
– Specify type, value, and name
– Terminated by a semicolon
• Methods
– User Defined
– System Defined
• Simple I/O
– System.out.println(“This is a test”);
– Console.in.readInt();
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Recap
Arithmetic Expressions and Conversion
• Arithmetic Expressions
– Syntax
– Mixed mode arithmetic
– Binary numeric promotion
• Type conversion
• Widening primitive conversions
– Goes from lower precision to higher precision
– Can be automatic or an explicit cast
• Narrowing primitive conversions
– Has to be explicit
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Recap
Arithmetic Operators, Associativity and Precedence
• Arithmetic Operators
– Variable = expression (a = b + c)
– Assignment (=, +=, -=, *=, /=, %=)
– Increment and Decrement Operators (++, –-)
• Precedence
– order in which operators are evaluated
• Associativity
– Order in which operators of same precedence are evaluated
(L->R, R->L)
• You can make the expression more explicit by using
parentheses.
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