Correct use of Hashtable with custom class

Question

This piece of code generates unexpected output.

Hashtable<Pair, Integer> results = new Hashtable<Pair, Integer>();
results.put(new Pair(0, 1), 2);
System.out.println("[DBG] " + results.containsKey(new Pair(0, 1)));

The output is [DBG] false . Why did Hashtable fail to register this element? Does it have something to do with the way I try to pass a Pair to the hashtable?

Answer 1

You have to override hashCode() and equals(..) of your Pair class to indicate that two objects having the same numbers are equal. (Better let your IDE generate these 2 methods for you.)

Hashtable uses hashCode() to determine the hash of the object and look it up. When you create a new Pair instance, the default hashing implementation of Object generates a different hash, and hence your Hashtable fails to locate the pair (which is successfully inside)

And finally - use a HashMap instead of Hashtable . It's a newer and better implementation of the concept, and does not have unnecessary synchronization.

Answer 2

it is caused by creating two different object by using new Pair(0,1) . So you have two choises for getting true :

The first one is you should implement hashCode and equals methods of Pair class.

The second one use same object like this:

Hashtable<Pair, Integer> results = new Hashtable<Pair, Integer>();
Pair key=new Pair(0, 1)
results.put(key, 2);
System.out.println("[DBG] " + results.containsKey(key));