How to make HashMap work with Arrays as key?

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You cannot do it this way. Both t and a will have different hashCode() values because the the java.lang.Array.hashCode() method is inherited from Object, which uses the reference to compute the hash-code (default implementation). Hence the hash code for arrays is reference-dependent, which means that you will get a different hash-code value for t and a. Furthermore, equals will not work for the two arrays because that is also based on the reference.

The only way you can do this is to create a custom class that keeps the boolean array as an internal member. Then you need to override equals and hashCode in such a way that ensures that instances that contain arrays with identical values are equal and also have the same hash-code.

An easier option might be to use List<Boolean> as the key. Per the documentation the hashCode() implementation for List is defined as:

int hashCode = 1;
Iterator<E> i = list.iterator();
while (i.hasNext()) {
    E obj = i.next();
    hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
}

As you can see, it depends on the values inside your list and not the reference, and so this should work for you.

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