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Introduction to java

3101-003

Class 1

Fall 2003

Shlomo Hershkop

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Welcome

• History• First Java Application• Data types• Variables• Strings• Assignments• Math, Boolean expressions• Relational operations• If statements• System.exit

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Very Brief History

• Started in 1991 by SUN Microsystems

• Targeted at consumer electronics. Wanted reliable programming language.

• Integrated into browsers

• Evolved into write once run anywhere, integrates into Netscape

• General purpose libraries released

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Course Information

• Textbook: Java in a Nutshell

Reading class 1chapters 1,2

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Basic Definitions

• Java is an object oriented language.– Object

– Method

– Class

– Applications

– Applets

– Native classes

– Threads

– Exceptions

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First Application

/***Hello World, first application, only output.*/import java.io.*;

public class hello{ public static void main (String [] args) {

System.out.println(“Hello World\n”);} //end main

}//end class

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How to get it running

• Text in hello.java file– Why?

• To compile:– javac hello.java

• To run:– java hello

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Notice:

• Java is CASE SENSITIVE!!

• Whitespace is ignored by compiler

• Whitespace makes things easier to read…hence it affects your grade

• File name has to be the same as class name in file.

• Need to import necessary class definitions

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Variables

• Variables:– Name– Type– Value

• Naming:– May contain numbers,underscore,dollar sign, or letters– Can not start with number– Can be any length– Reserved keywords– Case sensitive

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Primitive data typesByte 8 -27 27-1

Short 16 -215 215-1

Int 32 -231 231-1

Long 64

Float 32

Double 64

Boolean 1 0 1

Char 16

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Assignment

• =

• Example:

int n;

n = 10;

or

int n = 10; //same

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Strings

• Not a primitive class, its actually something called a wrapper class

• To find a built in class’s method use API documentation.• String is a group of char’s• A character has single quotes

– char c = ‘h’;• A String has double quotes

– String s = “Hello World”;• Method length

– int n = s.length;

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Using Strings

public class hello{

public static void main (String [] args) {

String s = “Hello World\n”;

System.out.println(s); //output simple string

} //end main

}//end class hello

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Math

• Unaryint x = -9;

• Regular math (+,-,*,/)int y = 3+x;

• % modulo operator

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Incrementing

• Increment and Decrement

• i++ equivalent to i = i + 1;

• Can also do ++i, which uses i before incrementing it.

• Decrementing: i--;

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Casting

int n = 40;

Wrong : byte b = n;

why??

Right: byte b = (byte) n;

Type casting converts to target type

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Casting II

• Type char is stored as a number. The ASCII value of the character.

• A declaration of :– char c = ‘B’;

stores the value 66 in location ccan use its value by casting to inthow??

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Assignment

• +=

• -=

• *=

• /=

• %=

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Boolean Expressions

• boolean bb will be either true (1) or false (0)

• Logical operations: !(not), && (and) || (or)• boolean a,b;

a = true;b = false;System.out.println (“a && b is “ + (a && b));

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Relational Operators

• == equality

• != inequality

• > greater than

• < less than

• >= greater than or equal to

• <= less than or equal to

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The if - branching statement

• if ( x < y) {x = y;

}

• if ( x < y ) {x = y;

}else { x = 88;}

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If/Else

• if (logic condition) {something}else if (logic condition) { something}else {something else}

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Nested IF

if ( x < 0 ) {System.out.println( “ x is negative “ );}

else {if ( x > 0 ) {

System.out.println ( “x is positive” );}//end if x > 0else {System.out.println ( “x is zero “ );}

} //end else x >=0

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Switch/Case

• Switch(variable){case(1): something;

break;

case(23): something;

break;

default: something;

}

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Exceptions

• Java exception object.

• java.io.Exception most general one.Some exception like in Throwable class define methods to get the message.

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try….catch blocks.

• Try {…….} catch ( IOException v) {……….}

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System.out.println

• println is a method in the Printstream class.• Defined:

– public void println(String x)

can be any type of string or combination string using addition to join parts.Example: println(“hello “ + “world “ + x);

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System.exit()

• One method in java.lang.System• Defined:

public static void exit ( int status)• Terminates currently running Java VM• Status is status code, non zero will usually mean

something abnormal.• Used at end to indicate success, or in middle to

signal problems.