Decorator pattern in java design pattern series

What is the Decorator pattern?
Dynamicly add some additional responsibilities to an object. In terms of adding functionality, the Decorator pattern is more flexible than generating subclasses.
1. Structure

Component: Define an object interface that can dynamically add responsibilities to these objects.

interface Component {
  public void operation();
}
ConcreteComponent : 实现 Component 定义的接口。
class ConcreteComponent implements Component {
  @Override
  public void operation() {
    System.out.println("初始行为");
  }
}

Decorator: Decorative abstract class, which inherits Component, extends the functions of the Component class from external classes, but for Component, there is no need to know the existence of Decorator.

class Decorator implements Component {
  // 持有一个 Component 对象，和 Component 形成聚合关系
  protected Component component;
   
  // 传入要进一步修饰的对象
  public Decorator(Component component) {
    this.component = component;
  }
   
  @Override
  // 调用要修饰对象的原方法
  public void operation() {
    component.operation();
  }
}

ConcreteDecorator: A specific decoration object that has the function of adding responsibilities to Component.

class ConcreteDecoratorA extends Decorator {
  private String addedState = "新属性1";
   
  public ConcreteDecoratorA(Component component) {
    super(component);
  }
   
  public void operation() {
    super.operation();
    System.out.println("添加属性: " + addedState);
  }
}
 
class ConcreteDecoratorB extends Decorator {
  public ConcreteDecoratorB(Component component) {
    super(component);
  }
 
  public void operation() {
    super.operation();
    AddedBehavior();
  }
   
  public void AddedBehavior() {
    System.out.println("添加行为");
  }
}

Test code

public class DecoratorPattern {
  public static void main(String[] args) {
    Component component = new ConcreteComponent();
    component.operation();
     
    System.out.println("======================================");
    Decorator decoratorA = new ConcreteDecoratorA(component);
    decoratorA.operation();
     
    System.out.println("======================================");
    Decorator decoratorB = new ConcreteDecoratorB(decoratorA);
    decoratorB.operation();
  }
}

Run result

初始行为
======================================
初始行为
添加属性: 新属性1
======================================
初始行为
添加属性: 新属性1
添加行为

2. Application scenarios

1. It is necessary to add responsibilities to an object dynamically and transparently, that is, without affecting other objects.
2. Functions need to be added to an object dynamically, and these functions can be dynamically revoked.
3. A very large number of functions generated by the permutation and combination of some basic functions need to be added, making the inheritance relationship unrealistic.
4. When the method of generating subclasses cannot be used for expansion. In one case, there may be a large number of independent extensions, and a large number of subclasses will be generated to support each combination, causing the number of subclasses to explode. Another case could be because the class definition is hidden, or the class definition cannot be used to generate subclasses.

3. Key points

1. Decorated objects and real objects have the same interface. This way the client object can interact with the decorated object in the same way as a real object.

2. The decoration object contains a reference to the real object.

3. The decoration object accepts all requests from the client. It forwards these requests to the real objects.

4. Decoration objects can add some additional functions before or after forwarding these requests. This ensures that additional functionality can be added externally at runtime without modifying the structure of a given object. In object-oriented design, functional extension of a given class is usually implemented through inheritance.

The above is an introduction to the Java decorator pattern. I hope it will be helpful to everyone's learning.

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