裝飾器模式允許使用者向現有物件新增新功能而不改變其結構。 這種型別的設計模式屬於結構模式，因為此模式充當現有類的包裝器。此模式建立一個裝飾器類，它包裝原始類並提供附加功能，保持類方法簽名完整。我們通過以下示例展示裝飾器模式的使用，其中我們將用一些顏色裝飾形狀而不改變形狀類。
實現例項
在這個例項中，將建立一個Shape介面和實現Shape介面的具體類。然後再建立一個抽象裝飾器類-ShapeDecorator，實現Shape介面並使用Shape物件作為其例項變數。這裡的RedShapeDecorator是實現ShapeDecorator的具體類。DecoratorPatternDemo這是一個演示類，將使用RedShapeDecorator來裝飾Shape物件。裝飾模式示例的結構如下所示 -

public interface Shape {
   void draw();
}

第2步建立兩個實現相同介面的具體類。
Rectangle.java

public class Rectangle implements Shape {

   @Override
   public void draw() {
      System.out.println("Shape: Rectangle");
   }
}

Circle.java

public class Circle implements Shape {

   @Override
   public void draw() {
      System.out.println("Shape: Circle");
   }
}
 

第3步建立實現Shape介面的抽象裝飾器類。
ShapeDecorator.java

public abstract class ShapeDecorator implements Shape {
   protected Shape decoratedShape;

   public ShapeDecorator(Shape decoratedShape){
      this.decoratedShape = decoratedShape;
   }

   public void draw(){
      decoratedShape.draw();
   }
}

第4步

public class RedShapeDecorator extends ShapeDecorator {

   public RedShapeDecorator(Shape decoratedShape) {
      super(decoratedShape);        
   }

   @Override
   public void draw() {
      decoratedShape.draw();           
      setRedBorder(decoratedShape);
   }

   private void setRedBorder(Shape decoratedShape){
      System.out.println("Border Color: Red");
   }
}

第5步使用RedShapeDecorator裝飾Shape物件。DecoratorPatternDemo.java

public class DecoratorPatternDemo {
   public static void main(String[] args) {

      Shape circle = new Circle();

      Shape redCircle = new RedShapeDecorator(new Circle());

      Shape redRectangle = new RedShapeDecorator(new Rectangle());
      System.out.println("Circle with normal border");
      circle.draw();

      System.out.println("\nCircle of red border");
      redCircle.draw();

      System.out.println("\nRectangle of red border");
      redRectangle.draw();
   }
}

第6步驗證輸出，執行上面的程式碼得到以下結果 -

Circle with normal border
Shape: Circle

Circle of red border
Shape: Circle
Border Color: Red

Rectangle of red border
Shape: Rectangle
Border Color: Red