Why am I getting duplicate keys in Java HashMap?

I seem to be getting duplicate keys in the standard Java HashMap. By "duplicate", I mean the keys are equal by their equals() method. Here is the problematic code:

import java.util.Map;
import java.util.HashMap;

public class User {
    private String userId;
    public User(String userId) { 
        this.userId = userId;
    }
    public boolean equals(User other) {
        return userId.equals(other.getUserId());
    }
    public int hashCode() {
        return userId.hashCode();
    }
    public String toString() {
        return userId;
    }

    public static void main(String[] args) {
        User arvo1 = new User("Arvo-Part");
        User arvo2 = new User("Arvo-Part");
        Map map = new HashMap();
        map.put(arvo1,1);
        map.put(arvo2,2);

        System.out.println("arvo1.equals(arvo2): " + arvo1.equals(arvo2));
        System.out.println("map: " + map.toString());
        System.out.println("arvo1 hash: " + arvo1.hashCode());
        System.out.println("arvo2 hash: " + arvo2.hashCode());
        System.out.println("map.get(arvo1): " + map.get(arvo1));
        System.out.println("map.get(arvo2): " + map.get(arvo2));
        System.out.println("map.get(arvo2): " + map.get(arvo2));
        System.out.println("map.get(arvo1): " + map.get(arvo1));
    }
}

And here is the resulting output:

arvo1.equals(arvo2): true
map: {Arvo-Part=1, Arvo-Part=2}
arvo1 hash: 164585782
arvo2 hash: 164585782
map.get(arvo1): 1
map.get(arvo2): 2
map.get(arvo2): 2
map.get(arvo1): 1

As you can see, the equals() method on the two User objects is returning true and their hash codes are the same, yet they each form a distinct key in map. Furthermore, map continues to distinguish between the two User keys in the last four get() calls.

This directly contradicts the documentation:

More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

Is this a bug? Am I missing something here? I'm running Java version 1.8.0_92, which I installed via Homebrew.

EDIT: This question has been marked as a duplicate of this other question, but I'll leave this question as is because it identifies a seeming inconsistency with equals(), whereas the other question assumes the error lies with hashCode(). Hopefully the presence of this question will make this issue more easily searchable.

Answer

The issue lies in your equals() method. The signature of Object.equals() is equals(OBJECT), but in your case it is equals(USER), so these are two completely different methods and the hashmap is calling the one with Object parameter. You can verify that by putting an @Override annotation over your equals - it will generate a compiler error.

The equals method should be:

  @Override
  public boolean equals(Object other) {
    if(other instanceof User){
        User user = (User) other;
        return userId.equals(user.userId);
    }

    return false;
}

As a best practice you should always put @Override on the methods you override - it can save you a lot of trouble.