委托 · 《Java 编程思想》第五版

## [委托](https://lingcoder.gitee.io/onjava8/#/book/08-Reuse?id=%e5%a7%94%e6%89%98) Java不直接支持的第三种重用关系称为委托。这介于继承和组合之间，因为你将一个成员对象放在正在构建的类中(比如组合)，但同时又在新类中公开来自成员对象的所有方法(比如继承)。例如，宇宙飞船需要一个控制模块: ~~~ // 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() {} } ~~~ 建造宇宙飞船的一种方法是使用继承: ~~~ // 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“一种”**SpaceShipControls**，即使你“告诉”**DerivedSpaceShip**调用`forward()`。更准确地说，一艘宇宙飞船包含了**SpaceShipControls \*\*，同时 \*\*SpaceShipControls**中的所有方法都暴露在宇宙飞船中。委托解决了这个难题: ~~~ // 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); } } ~~~ 方法被转发到底层**control**对象，因此接口与继承的接口是相同的。但是，你对委托有更多的控制，因为你可以选择只在成员对象中提供方法的子集。虽然Java语言不支持委托，但是开发工具常常支持。例如，上面的例子是使用 JetBrains Idea IDE 自动生成的。