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));
}
}
We can conclude that to use a custom class for a key, it is necessary that hashCode() and equals() are implemented correctly. To put it simply, we have to ensure that the hashCode() method returns: the same value for the object as long as the state doesn't change (Internal Consistency)
Therefore, to use an object as a key in HashMap , HashSet , or Hashtable in Java, we need to override equals and hashcode methods of that object since default implementation of these methods simply check for the instance equality.
Immutabiility is required, in order to prevent changes on fields used to calculate hashCode() because if key object return different hashCode during insertion and retrieval than it won't be possible to get object from HashMap.
If you want to make a mutable object as a key in the hashmap, then you have to make sure that the state change for the key object does not change the hashcode of the object. This can be done by overriding the hashCode() method. But, you must make sure you are honoring the contract with equals() also.
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
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With