'equals ()' returns false, but the object is on the map

Question

I was messing around with some hashCode + equal + map materials and found something ... weird.

The passage is as follows:

class Obj { String n; Obj(String n) {this.n = n;} public int hashCode() {return 0;} public boolean equals(Object o) {return false;} // no instance of this class // equals any other instance } 

Then I did something like this:

  java.util.Map<Obj,String> map = new java.util.HashMap<Obj,String>(); Obj o1 = new Obj("1"); Obj o11 = new Obj("1"); Obj o2 = new Obj("2"); map.put(o1,"val 1"); map.put(o11,"val 2"); map.put(o2,"val 3"); p("size = " + map.size()); // obviously 3 p(map.get(new Obj("1"))); // obviously null p(map.get(o1)); // ... 

The last line is the weird part. The last line returns val 1 . How so? The equals method always returns false . Is this because the == operator is used before equals is called?

Thank you for understanding.

Answer 1

In HashMap.java get method:

 public V get(Object key) { if (key == null) return getForNullKey(); int hash = hash(key.hashCode()); for (Entry<K,V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) return e.value; } return null; } 

The line if (e.hash == hash && ((k = e.key) == key || key.equals(k))) really compares the keys with == before calling equals . That is why your attempt to eliminate equality fails.

Answer 2

The hash map implementation checks for both == and equal key equality. (If the hash code is the same)

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/HashMap.java#HashMap.get%28java.lang.Object%29