Java Set of Maps hashCode incorrect?

Question

I am getting unexpected results from a Set of Maps (Set<Map<String,String>>), where s.contains(s.iterator().next()) is false.

The set in question contains only one map that is [{=262.666666666666667}] (the empty String mapped to the String 262.666666666666667).

I have been unable to put together a minimal working example that replicates the problem as the following outputs true:

Set s = new HashSet<Map<String,String>>();
Map<String,String> m = new HashMap<>();
m.put("", "262.666666666666667");
s.add(m);
System.out.println(s.contains(s.iterator().next()));

HashMap does not override hashCode but Abstract map does (see below) so I don't see the problem of putting a HashMap in a HashSet.

public int hashCode()
{
 int h = 0;
 Iterator<Entry<K,V>> i = entrySet().iterator();
 while (i.hasNext())
 h += i.next().hashCode();
 return h;
}

What is the reason for this behaviour and how can I fix it?

Edit: Thanks at doublep and morgano, I indeed modified the map after adding it and resolved the issue by adding after the modification instead of before.

• Jon Skeet: Multiple threads? Until we've got something which reproduces the problem, it's hard to help...
• Mena: Your self-contained example returns true when executed on a single thread.
• Crazyjavahacking: What do you mean by unexpected results?
• Smutje: Perhaps some rounding issue?
• user319799: If you modify a member map after adding it to the set, and thus changing map's hashcode, set will become broken. That's the reason Python, for example, requires that dictionary keys are immutable.

Answer 1

If you want to reproduce the error, modify your code in this way:

 Set s = new HashSet<Map<String,String>>();
 Map<String,String> m = new HashMap<>();
 s.add(m);
 m.put("", "262.666666666666667");
 System.out.println(s.contains(s.iterator().next()));

That is, add the map to the set and then put a new key/value in the map.

The problem is that, as you said, Abstract map overrides equals() to depend on the key/values that the map holds at that moment. Set uses internally a Map, being the key the value of equals() at the moment your Map is being added to the set. When you add a new key/value to your Map then the value returned by equals() also changes and doesn't correspond to the original value, this way you get false for System.out.println(s.contains(s.iterator().next()));