Phil Campbell London South Bank University Java 1 First Steps

Phil Campbell London South Bank University

Java 1

First Steps


Java

Is an Object Oriented Language which supports the creation of Object models discussed in the UML section.

The next few slides show how to define a class in Java.

We then discover how to use that class in a simple Java program


•Java is an Object Oriented Language

•Java is a compiled language

•Java is portable (Multi-platform)

but . . . .

You are not here to learn Java . . . .

You are here to learn Software Development


SimpleCounter

The class called SimpleCounter holds a single value in an attribute called value

When a new instance of the class is created, a value has to be supplied to give the counter an initial value

The only methods available to the user of the class are:

click() - which adds one to the value in the object and

getValue() - which returns the number in the value attribute


A Class


// filename : SimpleCounter.class//// Demonstration of a **very** simple and not very// useful class// Phil Campbell ver v0.1 September 03

public class SimpleCounter extends Object{


public class SimpleCounter extends Object{ private int value;


// the constructor public SimpleCounter (int startValue){ value = startValue; } // End SimpleCounter()


The constructor

...places a newly created instance into a well defined initial state.


// a simple method public void click( ){ value++; } // End click()


public int getValue(){ return value; } // End getValue()

return type


public class SimpleCounter extends Object{ private int value;

// the constructor public SimpleCounter (int startValue){ value = startValue; } // End SimpleCounter()

// a simple action public void click (){ value++; } // End click()

// a 'getter' public int getValue(){ return value; } // End getValue()} // End class SimpleCounter


Demonstrating SimpleCounter

count = new SimpleCounter( 5);

count

5

public class SimpleCounterDemonstration extends Object{

public static void main( String[] args){ SimpleCounter count = null;

reference variableobject :an instance of the class SimpleCounter

count: SimpleCounter

value = 5


An object is an instance of a class that has state

and behaviour.



count

5


public static void main( String[] args){ SimpleCounter count = null; count = new SimpleCounter( 5);

count.click( );

public void click ( ){ value++; }

6



count

5

public static void main( String[] args){ SimpleCounter count = null; count = new SimpleCounter( 5);

count.click( );

public void click ( ){ value++;}

6

System.out.println( count.getValue( ))

public int getValue(){ return value;}

6



public static void main (String[] args){

SimpleCounter count = null; count = new SimpleCounter(5);

System.out.print( "First value : "); System.out.println( count.getValue()); count.click(); System.out.print( "Second value : "); System.out.println( count.getValue()); count.click(); count.click(); System.out.print( "Third value : "); System.out.println( count.getValue()); } // End of main } // End of SimpleCounterDemonstration

count

5

output

First value : 5Second value: 6Third value : 8

678


The state of an object is the aggregate value of its

attributes.

Each time the value of an attribute is changed in an object,

the object changes state.




SimpleCounter count = null; count = new SimpleCounter(5); SimpleCounter count2 = null; count2 = new SimpleCounter(10); count.click(); System.out.print( "First value : "); System.out.println( count.getValue()); count2.click(); count.click(); count2.click(); System.out.print( "Second value : "); System.out.println( count2.getValue()); } // End of main } // End of SimpleCounterDemonstration

count

5

output

First value : 6Second value: 12

67

count2

101112




SimpleCounter count = null; count = new SimpleCounter(5); SimpleCounter count2 = null; count = new SimpleCounter(10); count.click(); System.out.print( "First value : "); System.out.println( count.getValue()); count2.click(); count.click(); count2.click(); System.out.print( "Second value : "); System.out.println( count2.getValue()); } // End of main } // End of SimpleCounterDemonstration

count

5

output

First value : 11

count2

10

<----ERROR

11


Words used so farclass

reference variable

method

state

constructor

inquiry method

object

instance

Describes the contents and behaviour of a set of instances (objects) that share the same attributes, methods and relationships

class



reference variable

method

state

constructor

inquiry method

object

instance

Used as the name of an instance. Can only refer to instances of a certain kind. Is not the instance itself.

reference variable



reference variable

method

state

constructor

inquiry method

object

instance

Describes the behaviour of an object.

Provides a way to interact with the data held in the object.

method



reference variable

method

state

constructor

inquiry method

object

instance

The state of an object is the aggregate value of all of its attributes

state set of all values in



reference variable

method

state

constructor

inquiry method

object

instance

Places a newly created instance into a well defined initial stateconstructor



reference variable

method

state

constructor

query method

object

instance

A method that returns a value indicating some aspect of the state of an instanceinquiry method



reference variable

method

state

constructor

inquiry method

object

instance

An object is an instance of a class that has state, behaviour and identity

object



reference variable

method

state

constructor

inquiry method

object

instance

Instantiation is the process of creating a physical example of a class... an object.

instance


The IDE Integrated Development Environment

Editor pane

File Chooser

Interaction pane


The IDE



Exercise 1

Colour

hue : String

first : Colour

hue : "red"

first second third

"red"

"blue"

"green"

getHue():StringsetHue(String theHue)

Write the class Colour in Java


public class Colour extends Object{

} // End class Colour

Colour

hue : String


Object

public String getHue(){ return hue;} // end getHue()

private String hue;

public Colour (String theHue){ hue = theHue;} // End constructor()

public void setHue( String theHue){ hue = theHue;} // End setHue()


public class Colour extends Object{ private String hue;

public Colour (String theHue){ hue = theHue; } // End constructor()

public String getHue(){ return hue; } // end getHue()

public void setHue( String theHue){ hue = theHue; } // End setHue()} // End class Colour

Colour

hue : String


Object

Write a demonstration program for the Colour Class

AttributeConstructorModifier MethodInquiry Method


} // end main()} // end class ColourDemo

public class ColourDemo extends Object{

public static void main ( String[] argv){

first = new Colour("Red");

System.out.println("First has colour " + first.getHue());

first.setHue( "Blue" );System.out.println(" First now has " + first.getHue());

System.out.println("Changing the colour ");

Colour first = null;

first

"Red""Blue"


Exercise 2Rectangle

height : doublewidth : doubleRectangle (h : double, w : double)area(): doubleperimeter(): doublegetHeight(): doublegetWidth(): doublesetHeight( aHeight: double)setWidth( aWidth: double)

Write the Rectangle class in Java


public class Rectangle extends Object{ private double height; private double width;

// the constructor public Rectangle (double height, double width){ super(); this.height = height; this.width = width; } // End Rectangle()

public double getHeight (){ return height; } // End getHeight()

public double getWidth (){ return width; } // End getWidth() public void setHeight ( double arg){ height = arg; } // End setHeight()


public void setWidth ( double arg){ width = arg; } // End getWidth() public double area(){ return height * width; } // End area() public double perimeter(){ return 2 * ( height + width); } public String toString(){ return "(" + height +"," + width + ") area= " + area() + " perimeter= " + perimeter(); } // End toString()} // End class Rectangle

Documents

Phil Campbell London South Bank University Java 1 First Steps