Question

I have run into an interesting problem which I’m pretty sure is the fault of HashMap. Consider the following debug code (AMap is a HashMap, key is a value passed to this method)

System.out.println("getBValues - Given: " + key);
System.out.println("getBValues - Contains Key: " + AMap.containsKey(key));
System.out.println("getBValues - Value: " + AMap.get(key));
for(Map.Entry<A,HashSet<B>> entry : AMap.entrySet()) {
    System.out.println("getBValues(key) - Equal: " + (key.equals(entry.getKey())));
    System.out.println("getBValues(key) - HashCode Equal: "+(key.hashCode() == entry.getKey().hashCode()));
    System.out.println("getBValues(key) - Key: " + entry.getKey());
    System.out.println("getBValues(key) - Value: " + entry.getValue());
}

Now in this Map I insert a single key (Channel) and value. Later I try and get the value back with get() and run this debug code which in my case gives this output:

getBValues - Given: Channel(...)
getBValues - Contains Key: false <--- Doesnt contain key?!
getBValues - Value: null   <--- Null (bad)
getBValues(key) - Equal: true <--- Given key and AMap key is equal
getBValues(key) - HashCode Equal: true
getBValues(key) - Key: Channel(Same...)
getBValues(key) - Value: []    <--- Not null (This is the expected result)

As you can see, fetching the key from the HashMap directly doesn’t work but looping through I get the exact same key, meaning its there it just can’t be found with get(). My question is what would cause this? How can get() not find a key that exists?

I would provide an some example code of this but I can’t seem to reproduce this independently.

Any suggestions on what might be causing this?

Answer 1

From what I can see, we still haven’t ruled out if it has to do with immutability.
If you do:

aMap.put(key, value);
key.setFieldIncludedInHashCodeAndEquals(25);

then you would get the result from above.

To rule this out, either show us more of your code or, in the for loop in your example above, add

System.out.println(aMap.get(entry.getKey()));

Also, use a debugger. That way, you can see if your object is in the correct bucket.