番石榴的Sets.Intersection方法行爲不端

Question

我有一個場景，我需要從包含在集合中的對象中找到基於特定字段（在這種情況下只有顏色）的兩個交集。

因此，我試圖找到一個十字路口，通過使用谷歌的番石榴減去兩個集合（它使用比較器，使用對象的顏色來決定2個車對象的相等）。但奇怪的是，一個交叉點B不等於B交點A.

請幫我找出它出錯的地方。 爲什麼A交點B不等於B交點A？我只對交叉部分感興趣。

public class Car { 

    public String id; 
    public String color; 
    public String getId() { 
      return id; 
    } 
    public void setId(String id) { 
      this.id = id; 
    } 
    public String getColor() { 
      return color; 
    } 
    public void setColor(String color) { 
      this.color = color; 
    } 


    @Override 
    public int hashCode() { 
      final int prime = 31; 
      int result = 1; 
      result = prime * result + ((color == null) ? 0 : color.hashCode()); 
      result = prime * result + ((id == null) ? 0 : id.hashCode()); 
      return result; 
    } 
    @Override 
    public boolean equals(Object obj) { 
      if (this == obj) 
       return true; 
      if (obj == null) 
       return false; 
      if (getClass() != obj.getClass()) 
       return false; 
      Car other = (Car) obj; 
      if (color == null) { 
       if (other.color != null) 
         return false; 
      } else if (!color.equals(other.color)) 
       return false; 
      if (id == null) { 
       if (other.id != null) 
         return false; 
      } else if (!id.equals(other.id)) 
       return false; 
      return true; 
    } 
    public Car(String id, String color) { 
      super(); 
      this.id = id; 
      this.color = color; 
    } 
    public Car() { 
      super(); 
    } 


    } 

import java.util.Comparator; 
import java.util.Set; 
import java.util.TreeSet; 

import com.google.common.collect.Sets; 


public class Tester { 

    public static void main(String[] args) { 

      final Set<Car> first = new TreeSet<Car>(new Comparator<Car>(){ 
       public int compare(final Car o1, final Car o2){ 
        return comp(o1.getColor(), o2.getColor()); 
       } 
      }); 
      first.add(new Car("1","blue")); 
      first.add(new Car("2","green")); 
      first.add(new Car("3","red")); 

      final Set<Car> second = new TreeSet<Car>(new Comparator<Car>(){ 
       public int compare(final Car o3, final Car o4){ 
        return comp1(o3.getColor(), o4.getColor()); 
       } 
      }); 
      second.add(new Car("4","black")); 
      second.add(new Car("5","green")); 
      second.add(new Car("6","blue")); 
      second.add(new Car("7","red")); 

      final Set<Car> intersection1 = Sets.intersection(first, second); 
      System.out.println("intersection1 size = "+intersection1.size()); 
      for(Car carr : intersection1){ 
       System.out.println("carr.id ="+carr.id+" carr.color ="+carr.color); 
      } 
      System.out.println(); 
      final Set<Car> intersection2 = Sets.intersection(second, first); 
      System.out.println("intersection2 size = "+intersection2.size()); 
      for(Car carr : intersection2){ 
       System.out.println("carr.id ="+carr.id+" carr.color ="+carr.color); 
      } 
      System.out.println(); 

      final Set<Car> Pure1 = Sets.difference(first, second); 
      System.out.println("Pure1 size = "+Pure1.size()); 
      for(Car carr : Pure1){ 
       System.out.println("carr.id ="+carr.id+" carr.color ="+carr.color); 
      } 
      System.out.println(); 

      final Set<Car> Pure2 = Sets.difference(second, first); 
      System.out.println("Pure2 size = "+Pure2.size()); 
      for(Car carr : Pure2){ 
       System.out.println("carr.id ="+carr.id+" carr.color ="+carr.color); 
      } 
      System.out.println(); 

    } 
    static int comp(String a, String b){ 
      if(a.equalsIgnoreCase(b)){ 
       return 0; 
      } 
      else 
       return 1; 
    } 
    static int comp1(String a, String b){ 
      if(a.equalsIgnoreCase(b)){ 
       return 0; 
      } 
      else 
       return 1; 
    } 

} 

輸出：

intersection1 size = 3 
carr.id =1 carr.color =blue 
carr.id =2 carr.color =green 
carr.id =3 carr.color =red 

intersection2 size = 2 
carr.id =5 carr.color =green 
carr.id =7 carr.color =red 

Pure1 size = 0 

Pure2 size = 2 
carr.id =4 carr.color =black 
carr.id =6 carr.color =blue 

Answer 1

你的comp實現是不正確的，你的比較不能滿足他們的合同，這樣你就可以從TreeSet得到不確定和不可預知的行爲。這與Guava的Sets.intersection方法無關。

爲Comparator.compare細則中指出：

比較它的兩個參數的順序。由於第一個參數小於，等於或大於第二個參數，因此返回負整數，零或正整數。

......您的Comparator不適用。

最簡單的修復很可能會使用

new Comparator<Car>(){ 
    public int compare(Car o1, Car o2) { 
     return o1.getColor().compareToIgnoreCase(o2.getColor()); 
    } 
}