Delegation

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Delegation is midway between inheritance and composition, because you place a member object in the class you’re building (like composition), but at the same time you expose all the methods from the member object in your new class (like inheritance). For example,

// reuse/SpaceShipControls.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

public class SpaceShipControls {
  void up(int velocity) {}

  void down(int velocity) {}

  void left(int velocity) {}

  void right(int velocity) {}

  void forward(int velocity) {}

  void back(int velocity) {}

  void turboBoost() {}
}

// reuse/SpaceShipDelegation.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

public class SpaceShipDelegation {
  private String name;
  private SpaceShipControls controls = new SpaceShipControls();

  public SpaceShipDelegation(String name) {
    this.name = name;
  }
  // Delegated methods:
  public void back(int velocity) {
    controls.back(velocity);
  }

  public void down(int velocity) {
    controls.down(velocity);
  }

  public void forward(int velocity) {
    controls.forward(velocity);
  }

  public void left(int velocity) {
    controls.left(velocity);
  }

  public void right(int velocity) {
    controls.right(velocity);
  }

  public void turboBoost() {
    controls.turboBoost();
  }

  public void up(int velocity) {
    controls.up(velocity);
  }

  public static void main(String[] args) {
    SpaceShipDelegation protector = new SpaceShipDelegation("NSEA Protector");
    protector.forward(100);
  }
}

Although the Java language doesn’t support delegation, development tools often do. The above example, for instance, was automatically generated using the JetBrains Idea IDE.

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/reuse/SpaceShipControls.java

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/reuse/SpaceShipDelegation.java