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Introduction to Programming in Java

AP Computer Science A

Dr. Persin

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Contents

• 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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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.*/

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

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

}//end class

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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”;

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.• For more practice go to http://www.javabat.com