Java TreeMap包含一个键，但containsKey调用返回false（即使键是完全相同的未更改对象）

Question

Why is it possible to loop the keySet of a TreeMap and getting a .containsKey == false ?

for (Object thisObject : map.keySet()) {
    if (!map.containsKey(thisObject)) {
        System.out.println("This line should be never reached.");
    }
}

After a lot, lot of different iterations and calls this line gets hit. A map.get(thisObject) would return null . But debug shows that the key ( same reference, value and hash ) and an actual value is in the map. The map is a small (25 elements) TreeMap<Long, Double>

UPDATE:

As guessed by @rgettman theres a custom sort Comparator used at the construction of the TreeMap (Didn't see it because it was constructed from another class). This comparator was just (I guess) copy pasted from here

Changing the Comparator :

  public int compare(Object a, Object b) {

    if((Double)base.get(a) > (Double)base.get(b)) {
      return 1;
    } else if((Double)base.get(a) == (Double)base.get(b)) {
      return 0;
    } else {
      return -1;
    }
  }

to

...
    } else if(base.get(a).equals(base.get(b))) {
      return 0;
...

fixes the problem. The reason why this problem was showing up just after millions of operation was that there were no cases where the map had two similar values for two different keys, as this is very unlikely in the context.

So at:

25151l, 1.7583805400614032
24827l, 1.7583805400614032

it fails.

Thanks for your help!

Answer 1

I just executed the code, this case did return true to me.

          TreeMap<Long,Double> otm = new TreeMap<Long, Double>();
          otm.put(1L, 1.0);
          otm.put(2L, 2.0);

        for (Object thisObject : otm.keySet()) {
                System.out.println(otm.containsKey(thisObject));
        }     

Can you please give us the data that you input in the TreeMap. Thanks

This is the containKey(Object key) implementation from JavaDocs

containsKey

boolean containsKey(Object key)

Returns true if this map contains a mapping for the specified key.

More formally, returns true if and only if this map contains a mapping for a key k such that, (key==null ? k==null : key.equals(k)) . (There can be at most one such mapping.)

Parameters:

 key - key whose presence in this map is to be tested Returns: true if this map contains a mapping for the specified key Throws: ClassCastException - if the key is of an inappropriate type for this map (optional) NullPointerException - if the specified key is null and this map does not permit null keys (optional 

Hope that helps.

Answer 2

您必须对支持entrySet()/Map.Entry进行更改，从而更改键顺序，从而使搜索失败containsKey Key。

Answer 3

Most of these implementation classes rely on both hashCode() and equals() being supported and correct.

If you truly have same hashcode from objects, try match equality. The answer suggested by me is that they do not match.

Otherwise, the scenario ought to be small enough for you to publish the objects and/or their respective hashcode and equals method.