java - 为什么自定义对象不是 HashMap 的等效键？

Question

我在使用自己的类作为 HashMap 的键时遇到问题

 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;
     }
 }

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());

         // 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));
     }
 }

Answer 1

你需要改变

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

到

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

---

每日提示:始终使用@Override 注释。

如果您使用了 @Override 注释，编译器会捕获错误并说:

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