Delegation

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

  public void methodA() {
    b.methodB();
  }
}

When clients of A call methodA, class A delegates the call to B‘s methodB.

Rationale. Class A exposes behaviours that belong elsewhere. This can happen in single-inheritance languages where class A inherits from one class, but its clients need behaviours that are implemented in a different class. Further study.

Hybrid Delegation

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

  public void methodA() {
    b.methodB( this );
  }
}

The difference between delegation that involves simple forwarding and delegation that acts as a substitute for inheritance is that the callee must accept a parameter of the caller, exemplified as:

    b.methodB( this );

Rationale. Allows class B instances to use functionality available from class A, just as class B would if it inherited from class A–but without inheritance. Further study.

Composition

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

  public A() {
  }
}

Once no more references to a particular instance of class A exist, its instance of class B is destroyed.

Rationale. Allows classes to define behaviours and attributes in a modular fashion. Further study.

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.

Demonstration Without References

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