What is the difference between Composition and Association relationship?

Question

In OOP, what is the difference between composition (denoted by filled diamond in UML) and association (denoted by empty diamond in UML) relationship between classes. I\'m a

Answer 1

I believe that a code-based example can help to illustrate the concepts given by the above responses.

import java.util.ArrayList;

public final class AssoCia
{
    public static void main( String args[] )
    {
        B b = new B();
        ArrayList cs = new ArrayList();

        A a = new A( b, cs );

        a.addC( new C() );
        a.addC( new C() );
        a.addC( new C() );
        a.listC();
    }
}

class A
{
    // Association -
    // this instance has a object of other class
    // as a member of the class.
    private B b;

    // Association/Aggregation -
    // this instance has a collection of objects
    // of other class and this collection is a
    // member of this class
    private ArrayList cs;

    private D d;

    public A(B b, ArrayList cs)
    {
        // Association
        this.b = b;

        // Association/Aggregation
        this.cs = cs;

        // Association/Composition -
        // this instance is responsible for creating
        // the instance of the object of the
        // other class. Therefore, when this instance 
        // is liberated from the memory, the object of
        // the other class is liberated, too.
        this.d = new D();
    }

    // Dependency -
    // only this method needs the object
    // of the other class.
    public void addC( C c )
    {
        cs.add( c );
    }

    public void listC()
    {
        for ( C c : cs )
        {
            System.out.println( c );
        }
    }

}

class B {}

class C {}

class D {}