Question

I want to compare two Java Maps by a simple hash.

Each object is on a different computer, so sending a hash over the network will be cheaper that sending the whole object to compare.

For example I have two HashMaps of an ExampleClass

Map<String,ExampleClass> One=new ...;

Map<String,ExampleClass> Other=new ...;

I don’t need to be sure that all elements are equal,
it’s enough for me to trust in a hash.

I was about to iterate at each side and create a “homemade Hash”, then send it to the network to finally compare for example an int or something.

It would be great if this “hash” is calculated every time an object is added or deleted from the Collection, saving me from iterate the whole object. I have to encapsulate every add/delete of the Map. Is there a Java library that does this?

Answer 1

If all your classes implement hashCode() (does not use the “default” memory address hashcode) you can use the map’s hashCode().

The caveat here is that if your ExampleClass does not implement hashCode(), then equal items might have different hashes on the two different machines, which will result in different hashes for the maps.

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To clarify:

Map implements a hashCode() that is defined as the sum of it’s Map.Enytry‘s hashCode()s.

Map.Entry‘s hashCode() is defined to be the xor of the key’s hashCode() and the value’s hashCode().
Your keys are Strings — they have a well defined hashCode() (two equal strings always have the same hashCode()).
Your values are ExampleClass instances — they also need a well-defined hashCode().

In summary, a map that contains { s1 -> ec1, s2 -> ec2 } will have a hashcode equal to:

(s1.hashCode() ^ ec1.hashCode()) + (s2.hashCode() ^ ec2.hashCode())

meaning that it depends on ExampleClass‘s hashCode().

If ExampleClass did implement hashCode() in a way that equal ExampleClasses give equal hashCode()s, everything will work well.
If ExampleClass did not implement hashCode(), it will use Object‘s hashCode(), which will almost always give you different hashCodes().