現在，我們有一個需求，甲希望從一堆蘋果中選出顏色是綠色的，乙希望從一堆蘋果中選出重量大於150的，程式碼該怎麼實現呢？按照我們的思路一起來：

  public static List<Apple> filterGreenApples(List<Apple> inventory){
        List<Apple> result = new ArrayList<>();
        for (Apple apple: inventory){
            if ("green".equals(apple.getColor())) {
                result.add(apple);
            }
        }
        return result;
    }

    public static List<Apple> filterHeavyApples(List<Apple> inventory){
        List<Apple> result = new ArrayList<>();
        for (Apple apple: inventory){
            if (apple.getWeight() > 150) {
                result.add(apple);
            }
        }
        return result;
    }
 

   那麼，現在丙、丁又分別提出了從蘋果選取產自鄭州、品種為紅富士的需求，怎麼辦呢，繼續寫函式？如果又加了無盡的需求呢？是不是不利於程式碼的擴充套件性呢？那麼以前人們在

遇到這個問題時是怎麼處理的呢？沒錯，用的就是策略模式，即設計一個介面，將介面的不同實現類的例項傳入方法內，而這個介面就稱為斷言。具體看例項吧：

interface ApplePredicate{
		public boolean test(Apple a);
	}

static class AppleWeightPredicate implements ApplePredicate{
   public boolean 
 
test(Apple apple){
      return apple.getWeight() > 150; 
   }
}
static class AppleColorPredicate implements ApplePredicate{
   public boolean test(Apple apple){
      return "green".equals(apple.getColor());
   }
}

public static List<Apple> filter(List<Apple> inventory, ApplePredicate p){
   List<Apple> result = new 
 
ArrayList<>();
   for(Apple apple : inventory){
      if(p.test(apple)){
         result.add(apple);
      }
   }
   return result;
} 

// [Apple{color='green', weight=80}, Apple{color='green', weight=155}]
List<Apple> greenApples2 = filter(inventory, new AppleColorPredicate());
System.out.println(greenApples2);

// [Apple{color='green', weight=155}]
List<Apple> heavyApples = filter(inventory, new AppleWeightPredicate());
System.out.println(heavyApples);