CodeseniorSoftware Tutorials

Delegation

public class A {
  private B b = new B();

  public void method() {
    b.method();
  }
}

When clients of A call method, class A delegates the method call to B.
Rationale. Class A can inherit from one class, but expose behaviours that belong elsewhere.
Further Study. http://beust.com/java-delegation.html

Composition

public class A {
  private B b = new B();

  public A() {
  }
}

Once there are no more references to a particular instance of class A, its instance of class B is destroyed.
Rationale. Allows classes to define behaviours and attributes in a modular fashion.
Further Study. http://www.artima.com/designtechniques/compoinh.html

Aggregation

public class A {
  private B b;

  public A( B b ) {
    this.b = b;
  }
}

public class C {
  private B b = new B();

  public C() {
    A a = new A( this.b );
  }
}

Once there are no more references to a particular instance of class A, its instance of class B will not be destroyed. In this example, both A and C must be garbage collected before B will be destroyed.
Rationale. Allows instances to reuse objects.
Further Study. http://faq.javaranch.com/java/AssociationVsAggregationVsComposition

Demonstration Without References

The names given to these simple patterns are defined by their referential relationships.