Inheritance and Composition

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January 12, 2009 at 5:11 AM

Hi friend,

As you progress in an object-oriented design, you will likely encounter objects in the problem domain that contain other objects. In this situation you will be drawn to modeling a similar arrangement in the design of your solution. In an object-oriented design of a Java program, the way in which you model objects that contain other objects is with composition, the act of composing a class out of references to other objects. With composition, references to the constituent objects become fields of the containing object.

For example, it might be useful if the coffee cup object of your program could contain coffee. Coffee itself could be a distinct class, which your program could instantiate. You would award coffee with a type if it exhibits behavior that is important to your solution. Perhaps it will swirl one way or another when stirred, keep track of a temperature that changes over time, or keep track of the proportions of coffee and any additives such as cream and sugar.

To use composition in Java, you use instance variables of one object to hold references to other objects. For the CoffeeCup example, you could create a field for coffee within the definition of class CoffeeCup,

class CoffeeCup {

private Coffee innerCoffee;

public void addCoffee(Coffee newCoffee) {
// no implementation yet
}

public Coffee releaseOneSip(int sipSize) {
// no implementation yet
// (need a return so it will compile)
return null;
}

public Coffee spillEntireContents() {
return null;
}
}

public class Coffee {

private int mlCoffee;

public void add(int amount) {
// No implementation yet
}

public int remove(int amount) {
// No implementation yet
// (return 0 so it will compile)
return 0;
}

public int removeAll() {
// No implementation yet
// (return 0 so it will compile)
return 0;
}
}

January 12, 2009 at 5:13 AM

Inheritence using in java

<HTML>
<HEAD>
<TITLE>demo</TITLE>
<SCRIPT>

function GetCookie(sName){
var aCookie = document.cookie.split("; ");
for (var i=0; i < aCookie.length; i++) {
var aCrumb = aCookie[i].split("=");
if (sName == aCrumb[0])
return unescape(aCrumb[1]);
}
//cookie with the requested name does not exist
return null;
}
</SCRIPT>
<SCRIPT language="javascript" for="oBody" event="onload()">
form1.hiddenelement.value = GetCookie("us")
</SCRIPT>
</HEAD>
<BODY id="oBody">
<FORM name="form1">
<INPUT type="hidden" name="hiddenelement"/>
</FORM>
</BODY>
</HTML>

When a subclass is derived simply from it's parent class then this mechanism is known as simple inheritance. In case of simple inheritance there is only a sub class and it's parent class. It is also called single inheritance or one level inheritance

class TextDemo {
int x;
int y;

int get(int p, int q){
x=p; y=q; return(0);
}
void Show(){
System.out.println(x);
}
}

class Test extends TextDemo{
public static void main(String args[]){
TextDemo a = new TextDemo();
a.get(5,6);
a.Show();
}
void display(){
System.out.println("B");
}
}
----------------------------------------------------------

Visit for more information:

http://www.roseindia.net/java/language/inheritance.shtml

Thanks

Amardeep