44

I have a hasmap with a key object, 

HashMap<Key, Object> test;

and make new Key("the same") as key..

so its like..:

test.put(new Key("the same"), someObject);

(without storing that key in a variable)

so.. after a while... i want to access the hashmap, because i do not have the object, i've tried to make new Key("the same") and make it as a key. But it didnt work.

How to make it work? (without saving the first object "key" in a variable)

So meanwhile, for now, im using String object as a key.

HashMap<String, Object>
Keenan Gebze
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    It's hard to know what your question is asking. You might want to consider rewriting it... If you just want all the keys in your hashmap you can use the method `keySet()`... – smessing Feb 25 '12 at 01:47
  • Not duplicate : the other question does not directly deal with maps, but directly with `hashCode` and `equals` which may seem unrelated without the accepted answer. The accepted answer is just enough to understand the issue. – Paul Bombarde Dec 14 '18 at 13:47

3 Answers3

72

You need to implement hashCode and equals on Key. The default implementation of these methods simply checks for instance equality (in other words, two Objects will only be equal if they are in fact the same object).

Further reading

Effective Java - Methods common to all objects

Hernán Eche
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Mark Peters
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40

Your problem is likely that Key did not implement hashCode() and equals() correctly (or at all). In order to be used as a HashMap key the class has to implement these two methods to reflect "equality" of two objects.

If you create two different instances with

Key a = new Key("xyz");
Key b = new Key("xyz");

and expect them to be equal and work in a HashMap, you have to override hashCode() so that it returns the same value in both instances, and equals() returns true when comparing them.

If the object identity is based on the string value, then 

@Override
public int hashCode()
{
    return theStringValue.hashCode();
}

and

@Override
public boolean equals(Object o)
{
    return this.theStringValue.equals(o);
}

should work

Jim Garrison
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    If you are working in Eclipse, just right click and go to Source>Generate hashCode() and equals()… choose the attributes you want to include in these methods and the code will be auto-generated. A time-saver! – ruhong Oct 05 '14 at 17:53
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    thanks @ruhong I generated them easily with eclipse – Enock Lubowa Aug 07 '20 at 10:23
7

When a class does not override the equals() or hashCode() methods, the default implementations found on the Object class are used instead. In particular, equals() simply does a check for reference equality.

That immediately explains why your approach isn't working: the new Key object clearly isn't referring to the old Key object.

If you'd like to be able to specify a new instance with the same property, then you should override the equals() method with an implementation that meets your criteria for key equality. You should override hashCode() as well, to have full control over the key comparison process.

dlev
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