## [委托](https://lingcoder.gitee.io/onjava8/#/book/08-Reuse?id=%e5%a7%94%e6%89%98) Java不直接支持的第三種重用關系稱為委托。這介于繼承和組合之間，因為你將一個成員對象放在正在構建的類中(比如組合)，但同時又在新類中公開來自成員對象的所有方法(比如繼承)。例如，宇宙飛船需要一個控制模塊: ~~~ // 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() {} } ~~~ 建造宇宙飛船的一種方法是使用繼承: ~~~ // reuse/DerivedSpaceShip.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 DerivedSpaceShip extends SpaceShipControls { private String name; public DerivedSpaceShip(String name) { this.name = name; } @Override public String toString() { return name; } public static void main(String[] args) { DerivedSpaceShip protector = new DerivedSpaceShip("NSEA Protector"); protector.forward(100); } } ~~~ 然而，**DerivedSpaceShip**并不是真正的“一種”**SpaceShipControls**，即使你“告訴”**DerivedSpaceShip**調用`forward()`。更準確地說，一艘宇宙飛船包含了**SpaceShipControls \*\*，同時 \*\*SpaceShipControls**中的所有方法都暴露在宇宙飛船中。委托解決了這個難題: ~~~ // 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); } } ~~~ 方法被轉發到底層**control**對象，因此接口與繼承的接口是相同的。但是，你對委托有更多的控制，因為你可以選擇只在成員對象中提供方法的子集。 雖然Java語言不支持委托，但是開發工具常常支持。例如，上面的例子是使用 JetBrains Idea IDE 自動生成的。