Java Map hash code

Question

Which is the best way to compute the hash code of a Map, knowing that it may contain entry values of types such as: String, Integer, Object[] ... ?

Map.hashCode() returns a shallow hash code. That means that if you have a String[] in your map, the Map.hashCode() will also use the hash returned by the String[]. Which, unfortunately, is not what I want (the Object.hashCode() implementation). But I want the Arrays.hashCode(String[]) implementation.

So which is the best, generic, approach to handle this ?

I assume your Map is immutable? If your adding/removing entries, your values-based hashCode() is doomed. — Kirk Woll
– Kirk Woll, Commented Dec 10, 2010 at 17:37
I think most people's initial question will be "why?" what are you doing that you need to change the hashcode of the map? calculating a "deep" hashcode could make that operation pretty expensive... — John Gardner
– John Gardner, Commented Dec 10, 2010 at 17:37
@John Gardner: I need to know whether two maps hold the same values, in deep. In a generic manner. — javaq
– javaq, Commented Dec 10, 2010 at 17:42
@javaq, a hashCode will not guarantee that the two maps are equal anyway -- you're barking up the wrong tree. Two objects with different values may return the same hashCode(). If you want a deep equality comparison, you're just going to have to implement that yourself -- your own equals method. — Kirk Woll
– Kirk Woll, Commented Dec 10, 2010 at 17:47

John Gardner · Accepted Answer · 2010-12-10 17:50:51Z

10

If you need to know if two maps contain the same values, you will need to write a deep comparison method. you shouldn't be depending on hashCode.

even with a perfect algorithm, there's no way that every possible collection of every possible object could be uniquely represented by a single signed integer.

Hashcode is just for collision reduction when used in collections, it is not supposed to be used to uniquely identify objects.

answered Dec 10, 2010 at 17:50

John Gardner

25.3k5 gold badges65 silver badges78 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

javaq Over a year ago

Thanks. Do you know any other effective ways of achieving my goal ?

John Gardner Over a year ago

if you need to know that two collections are exactly the same, there's no real other way to do it other than a deep comparison. things that are not the same can fail quickly, but worst case is always comparing every inner object, including inner comparisons of every inner collection...

Laurence Gonsalves · Accepted Answer · 2010-12-10 17:36:56Z

3

The solution to your problem is to not use arrays. Use ArrayLists (or some other form of List, like an ImmutableList from Google's Guava). Lists hash the way you want. Also, arrays don't really play nice with generics (like Maps).

answered Dec 10, 2010 at 17:36

Laurence Gonsalves

144k38 gold badges264 silver badges314 bronze badges

1 Comment

javaq Over a year ago

This is not a solution. I cannot control the input. Eventually I could recursively replace the arrays with array lists, but I would avoid that.

Collectives™ on Stack Overflow

Java Map hash code

2 Answers 2

2 Comments

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Related