All Java

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JAVA

SDK files :

docs JDK documentation in html

bin Java compiler and tools

demo Sample Java programs

include files for native methods

lib library files

src source files for some of java

Class

Think of this a blueprint for an object. There is no object unless you create it. It contains methods and

data.

Methods

Methods are functions that are only defined in classes. Java uses a strong object oriented approach.

Data

Arrays, numbers, other objects,

If you have a class called Molecule, then to create an instance of a Molecule, then you would do:

Molecule oxygen = new Molecule();

now you can do something with the object oxygen

If your class Molecule has data named mass, then you could find the mass of your new molecule

via the dot notation:

oxygen.mass

If your Molecule class has a move method, then you can move your molecule via:

oxygen.move()

The parentheses are a strong reminder that move is a method not data.

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// this is a comment, use lots of these

/* this is a

comment too */

/** this is used for documentation (javadoc) */

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VS.

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import javax.swing.JOptionPane; // this gives us some classes we can use

// for GUI's// javax.swing is a package

// JOptionPane is a class

// read in a number as a string, this method returns a string when it is called

stringnum1 = JOptionPane.showInputDialog(" Enter Pi ");

// convert number in string form to a floating point number

num1 = Double.parseDouble( stringnum1 );

JOptionPane.showMessageDialog( null, Amazing ! You are correct ! ");

JOptionPane.showMessageDialog( null, "Your Pi is wrong by : " + error + " Percent \n" +

"Pi is " + Math.PI );

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Java Data Types

Integers (bytes):

long (8), int (4), short (2), and byte (1)

these DO NOT depend on machine you are using!

Floating point (bytes):

double (8) and float (4)

Roughly 14 and 7 significant digits

float will use less memory and might run faster on 32-bit processors

Characters

char

Unicode (65,536) not ASCII (255)

single quotes are used

Boolean

false or true

Type Conversion

int x = (int)Math.PI;

int y = 5;

double z = (double)y;

int a = 3;

double b = 4.5;

double c;

c = a * b;

Operators

+ - * / ++ -- %

If x=12, then

x++ gives 13

a = 2 * x++ (a will be 24, x will be 13)

b = 2 * ++x (b will be 26, x will be 13)

The operator ++ is where C++ gets its name. Some people

joke that it should be ++C (we dont really want to use the

language until it is improved !). Some people say Java is

really ++C.

Relational: == >=

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Compiling vs. Interpretted

Usually java is run in interpretted mode, ie:

javac MyCode.java

java MyCode

The above is running the .class byte code

But you can also compile java into native machine code,

which should run much faster.

For example, there is the gcj compiler from gnu

Another good Java compiler is jikes from IBM

OTHER JAVA KEYWORDS

instanceof: A Java programming language keyword that takes two object references as

arguments and returns true if the type of its first argument is assignable to its second argument.

If ( apple instanceof Fruit ) {

System.out.println( apple is a fruit.);

Assert

It has two forms:

1st

:

assert Expression1 ;

where Expression1 is a boolean expression. When the system runs the assertion, it

evaluates Expression1 and if it is false throws an AssertionError with no detail

message.

2nd:

The second form of the assertion statement is:

assert Expression1 : Expression2 ;

where:

* Expression1 is a boolean expression.

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* Expression2 is an expression that has a value

transient

transient variables are always skipped when objects are serialized. an object is serialized when

you write all its data to a file. If you have data that would make no sense to have serialized,

then you can denote them as transient

volatile

The volatile keyword is used on variables that may be modified simultaneously by other threads.

This warns the compiler to fetch them fresh each time, rather than caching them in registers.

native

A java keyword that denotes a method implemented on the native system in another language.

Chapman Packages:

1. Io

FileIn: A convenience class to read numeric data from a formatted input file.

FileInLines: A convenience class to read lines of strings from a formatted input file.

FileOut: A class to write formatted numeric data or lines of strings to a formatted output file

StdIn: A class to read numeric data from the standard input stream.

StdinLines: A class to read lines of data from the standard input sream.

Fmt: A class to write formatted data to either standard output stream or a string.

2. Math1

Array: A class to do various array manipulations.

Complex: A class to do complex arithmetics.

Math1: A class that has math methods that are not included in the standard java package.

SigProc: A class to perform various signal processing functions

Statistics: A class to calculate statistics of a data set.

3. Graphics

Hist: A class to create a histogram of a set of data using the awt gui.

Jhist: A class to create a histogram of a set of data using the swing gui.

JCanvas: A class to create a blank swing (canvas) to draw graphics on.

Plot2d: A class to create line and bar plots using the awt gui.

Jplot2d: A class to create line and bar plots using the swing gui.

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Applet (NOT JAVA 2.0)

import java.awt.Graphics;

public class HelloWorld1 extends java.applet.Applet {

public void paint (Graphics g) {g.drawString("Hello World ! This is not Java 2.0", 10, 20);

}

}

Applet (JAVA 2.0)

import javax.swing.*;

import java.awt.Graphics;

public class HelloWorld6 extends JApplet {

public void paint (Graphics g) {

g.drawString("Hello World ! This IS Java 2.0", 10, 20);

}

}

JAVA selection structures:

Single: If

Double: If/else

Multiple: switch

JAVA repetition structures:

While

Do/while

For

NAMED BREAK

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SWITCH

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Arrays

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Multiple Dimension Arrays

int X[]; x = new int [10];int x[][]; x = new int [10][12];

int x[][][]; x = new int [10][20][10];

int b[][] = { { 1, 2, 3}, { 4, 6, 8}, { 9, 12, 10} }; 3 rows with 3 columns

b.length gives number of rows

b[1].length gives columns in 1st

row.

And so on..

Existing methods:

-Application: main,run

-Applets: init,start,stop,paint,destroy

Recursion: Methods can call themselves

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Eg:

public int fact ( int n ) {

if ( n >= 1 ) return n * fact(n-1);

else return 1;

}

Method Overloading:

Write a method twice, the only diff is that each has different variable type.

You can now call the method square and pass it an int or a double and it will use the correct

version of the method. It will not work if you pass it a float.

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Object Oriented Programming (OOP)

OOP encapsulates data (e.g. numbers and

objects) and methods (functions)

Information hiding is a key aspect of OOP

Implementation details are hidden from the

user (we can drive a car without knowing

how it is built)

C is action oriented (verbs)

Java is object oriented (nouns)

C++ can be in between....

OOP

Ket aspects of OOP are:

Encapsulation

Inheritance

Polymorphism

Classes are templates for objects

Benefits of OOP

Modifiability

Readability

Maintainability

Reliability

Reusability

Encapsulation

The grouping together of data and methods

(they are both encapsulated into objects)

This makes code more portable and

maintainable and ...

Also offers data hiding. Some data and

methods can be private

Constructors

Methods with the same name as the class

are constructors

-Constructors cannot return a value

Inheritance

Inheritance permits software reusability

New classes can be created from old classes

The data and methods of the old class are available to the new class

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Differences

Polymorphism

An objects ability to decide what method to apply to itself, depending on where it is in the

inheritance heirarchy, is usually called polymorphism.

OR

(while the message may be the same, objects may respond differently )

Benefits of polymorphism

Polymorphism allows you to avoid very complicated switch or if/else constructs that are

required in other languages to treat special cases

The same method name can be used in many different ways by different objects, this is

especially interesting for inherited objects

Like method overloading

Exception handling

An exception tells you that a problem occurred during your programs execution.

Eg. file not found, out of memory problem, URL not found

Try Catch

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try {

java code....

}

catch ( AnExceptionType except1)

{

exception handling code...

}

finally

{

java code...

}

Thread methods:

interrupt

interrupted (static)

isInterrupted

isAlive

yield

setName

getName

toString

currentThread (static)

join

setPriority ( int )

getPriority

Thread uses:

e.g. graphics, GUIs, robotics

AWT

Abstract windowing toolkit

Two principal types of graphics objects are:

Components and Containers

KEY GUI PIECES

JFrame: a window

JPanel: a piece of a window

JLabel: uneditable text

JTextField: a box that gets text from the keyboard

JButton: an area that triggers on mouse clicks

JCheckBox: an area that is either on or off

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JComboBox: a drop-down list of items

JList: a list of items is shown, the user can select more than one or double click on them.

abstractA Java keyword used in a class definition to specify that a class is not to be instantiated, but rather inheritedby other classes. An abstract class can have abstract methods that are not implemented in the abstractclass, but in subclasses.

booleanRefers to an expression or variable that can have only a true or false value. The Java programming

language provides the boolean type and the literal values true and false.break

A Java keyword used to resume program execution at the statement immediately following the currentstatement. If followed by a label, the program resumes execution at the labeled statement.

byteA sequence of eight bits. Java provides a corresponding byte type.

caseA Java keyword that defines a group of statements to begin executing if a value specified matches the valuedefined by a preceding switch keyword.

catchA Java keyword used to declare a block of statements to be executed in the event that a Java exception, orrun time error, occurs in a preceding try block.

charA Java keyword used to declare a variable of type character.

classIn the Java programming language, a type that defines the implementation of a particular kind of object. Aclass definition defines instance and class variables and methods, as well as specifying the interfaces theclass implements and the immediate superclass of the class. If the superclass is not explicitly specified, thesuperclass will implicitly be Object.

continueA Java keyword used to resume program execution at the end of the current loop. If followed by alabel, continue resumes execution where the label occurs.

defaultA Java keyword optionally used after all case conditions in a switch statement. If all case conditions are

not matched by the value of the switch variable, the default keyword will be executed.

doA Java keyword used to declare a loop that will iterate a block of statements. The loop's exit condition canbe specified with the while keyword.

double

A Java keyword used to define a variable of type double.else

A Java keyword used to execute a block of statements in the case that the test condition with the if keywordevaluates to false.

extendsClass X extends class Y to add functionality, either by adding fields or methods to class Y, or by overridingmethods of class Y. An interface extends another interface by adding methods. Class X is said to be asubclass of class Y. See also derived from.

final

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A Java keyword. You define an entity once and cannot change it or derive from it later. More specifically: afinal class cannot be subclassed, a final method cannot be overridden and a final variable cannot changefrom its initialized value.

finallyA Java keyword that executes a block of statements regardless of whether a Java Exception, or run timeerror, occurred in a block defined previously by the try keyword.

float

A Java keyword used to define a floating point number variable.forA Java keyword used to declare a loop that reiterates statements. The programmer can specify thestatements to be executed, exit conditions, and initialization variables for the loop.

ifA Java keyword used to conduct a conditional test and execute a block of statements if the test evaluates totrue.

implementsA Java keyword included in the class declaration to specify any interfaces that are implemented by thecurrent class.

importA Java keyword used at the beginning of a source file that can specify classes or entire packages to bereferred to later without including their package names in the reference.

instanceofA two-argument Java keyword that tests whether the runtime type of its first argument is assignment

compatible with its second argument.int

A Java keyword used to define a variable of type integer.interface

A Java keyword used to define a collection of method definitions and constant values. It can later beimplemented by classes that define this interface with the "implements" keyword.

longA Java keyword used to define a variable of type long.

nativeA Java keyword that is used in method declarations to specify that the method is not implemented in thesame Java source file, but rather in another language.

newA Java keyword used to create an instance of a class.

nullThe null type has one value, the null reference, represented by the literal null, which is formed from ASCII

characters. A null literal is always of the null type.package

A group of types. Packages are declared with the package keyword.private

A Java keyword used in a method or variable declaration. It signifies that the method or variable can only beaccessed by other elements of its class.

protectedA Java keyword used in a method or variable declaration. It signifies that the method or variable can only beaccessed by elements residing in its class, subclasses, or classes in the same package.

publicA Java keyword used in a method or variable declaration. It signifies that the method or variable can beaccessed by elements residing in other classes.

returnA Java keyword used to finish the execution of a method. It can be followed by a value required by themethod definition.

shortA Java keyword used to define a variable of type short.

staticA Java keyword used to define a variable as a class variable. Classes maintain one copy of class variablesregardless of how many instances exist of that class. static can also be used to define a method as a classmethod. Class methods are invoked by the class instead of a specific instance, and can only operate onclass variables.

superA Java keyword used to access members of a class inherited by the class in which it appears.

switch

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A Java keyword used to evaluate a variable that can later be matched with a value specified bythe case keyword in order to execute a group of statements.

synchronizedA keyword in the Java programming language that, when applied to a method or code block, guaranteesthat at most one thread at a time executes that code.

thisA Java keyword that can be used to represent an instance of the class in which it appears. this can be used

to access class variables and methods.throwA Java keyword that allows the user to throw an exception or any class that implements the "throwable"interface.

throwsA Java keyword used in method declarations that specify which exceptions are not handled within themethod but rather passed to the next higher level of the program.

transientA keyword in the Java programming language that indicates that a field is not part of the serialized form ofan object. When an object is serialized, the values of its transient fields are not included in the serialrepresentation, while the values of its non-transient fields are included.

tryA Java keyword that defines a block of statements that may throw a Java language exception. If anexception is thrown, an optional catch block can handle specific exceptions thrown within the try block. Also,an optional finally block will be executed regardless of whether an exception is thrown or not.

voidA Java keyword used in method declarations to specify that the method does not return any value. void canalso be used as a nonfunctional statement.

volatileA Java keyword used in variable declarations that specifies that the variable is modified asynchronously byconcurrently running threads.

whileA Java keyword used to declare a loop that iterates a block of statements. The loop's exit condition isspecified as part of the while statement.