Why are custom objects not equivalent keys for a H

问题:

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));
     }
 }

回答1:

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

回答2:

Your code is correct, but you also need to override the equals method inherited from Object.

Add this to your ActorId class:

@Override
 public boolean equals(Object other) {
    if(other == null || other.getClass() != getClass())
        return false;
    return equals((ActorId)other);
 }

回答3:

You definitely must override the method equals(Object), and for certain implementation of a Map (HashMap) it is also necesary that you overrdide the method hashCode().

I had the same problem, and without the custom hashCode implementation the equals method of the class "ActorId" was never called.

回答4:

By default Java invokes boolean equals(Object obj); So, you login is correct but if you want to OVERRIDE equals() use Object as a parameter and and check the class by instanceOf or getClass() and do a class casting.

if (obj instanceOf ActorId) {
    ActorId other = (ActorId)obj;
    ... compare fields
}