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Introduction to java
3101-003
Class 1
Fall 2003
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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.
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