为什么自定义对象不是HashMap的等效键？ [英] Why are custom objects not equivalent keys for a HashMap?

问题描述

  public class ActorId {
 private final int playerId; 
 private final int id; 
 
 ActorId（int playerId，int id）{
 this.playerId = playerId; 
 this.id = id; 
} 
 
 public boolean equals（ActorId other）{
 return this.id == other.id&&& this.playerId == other.playerId; 
} 
 
 public int hashCode（）{
 int hash = 1; 
 hash = hash * 31 + playerId; 
 hash = hash * 31 + id; 
返回哈希; 
} 
 
 public String toString（）{
 return＃+ playerId +。 + id; 
} 
 
 public int getPlayerId（）{
 return playerId; 
} 
} 
  

这是一个失败的JUnit测试

  import static org.junit.Assert。*; 
 import java.util.Map; 
 import org.junit.Test; 
 
 public class ActorIdTest {
 @Test 
 public final void testAsMapKey（）{
 ActorId a = new ActorId（123,345）; 
 ActorId b = new ActorId（123,345）; 
 
 assertTrue（a.equals（b））; 
 assertEquals（a.hashCode（），b.hashCode（））; 
 
 //使用字符串作为键
映射< String，String> map1 = new java.util.HashMap< String，String>（）; 
 
 map1.put（a.toString（），test）; 
 assertEquals（test，map1.get（a.toString（）））; 
 assertEquals（test，map1.get（b.toString（）））; 
 assertEquals（1，map1.size（））; 
 
 //但不是与ActorIds 
映射< ActorId，String> map2 = new java.util.HashMap< ActorId，String>（）; 
 
 map2.put（a，test）; 
 assertEquals（test，map2.get（a））; 
 assertEquals（test，map2.get（b））; // FAILS here 
 assertEquals（1，map2.size（））; 
 
 map2.put（b，test2）; 
 assertEquals（1，map2.size（））; 
 assertEquals（test2，map2.get（a））; 
 assertEquals（test2，map2.get（b））; 
} 
} 
  

解决方案

你需要更改

  public boolean equals（ActorId other）{
 .... 
} 
  

至

  public boolean equals（Object other）{
 .... 
} 
  

< hr>

提示：始终使用 @Override 注释。

如果您使用了 @Override 注释，编译器将会收到错误并说：

类型ActorId的方法equals（ActorId）必须覆盖或实现超类型方法

I'm having trouble using my own class as a key for a HashMap

 public class ActorId {
     private final int playerId;
     private final int id;

     ActorId(int playerId, int id) {
         this.playerId = playerId;
         this.id = id;
     }

     public boolean equals(ActorId other) {
         return this.id == other.id && this.playerId == other.playerId;
     }

     public int hashCode() {
         int hash = 1;
         hash = hash * 31 + playerId;
         hash = hash * 31 + id;
         return hash;
     }

     public String toString() {
         return "#" + playerId + "." + id;
     }

     public int getPlayerId() {
         return playerId;
     }
 }

Here is a failing JUnit test

 import static org.junit.Assert.*;
 import java.util.Map;
 import org.junit.Test;

 public class ActorIdTest {
     @Test
     public final void testAsMapKey() {
         ActorId a = new ActorId(123, 345);
         ActorId b = new ActorId(123, 345);

         assertTrue(a.equals(b));
         assertEquals(a.hashCode(), b.hashCode());

         // Works with strings as keys
         Map<String, String> map1 = new java.util.HashMap<String, String>();

         map1.put(a.toString(), "test");
         assertEquals("test", map1.get(a.toString()));
         assertEquals("test", map1.get(b.toString()));
         assertEquals(1, map1.size()); 

         // But not with ActorIds
         Map<ActorId, String> map2 = new java.util.HashMap<ActorId, String>();

         map2.put(a, "test");
         assertEquals("test", map2.get(a));
         assertEquals("test", map2.get(b)); // FAILS here
         assertEquals(1, map2.size()); 

         map2.put(b, "test2");
         assertEquals(1, map2.size());
         assertEquals("test2", map2.get(a));
         assertEquals("test2", map2.get(b));
     }
 }

解决方案

You need to change

public boolean equals(ActorId other) {
    ....
}

to

public boolean equals(Object other) {
    ....
}

---

Tip of the day: Always use @Override annotation.

If you had used the @Override annotation, the compiler would have caught the error and said:

The method equals(ActorId) of type ActorId must override or implement a supertype method

这篇关于为什么自定义对象不是HashMap的等效键？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！