How to make HashMap work with Arrays as key?

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.

It is not possible to do this with arrays, as any two different arrays don't compare equals, even if they have the same elements.

You need to map from container class, for example ArrayList<Boolean> (or simply List<Boolean>. Perhaps BitSet would be even more appropriate.

Map implementations relies on key's equals and hashCode methods. Arrays in java are directly extends from Object, they use default equals and hashCode of Object which only compares identity.

If I were you, I would create a class Key

class Key {    private final boolean flag1;    private final boolean flag2;    public Key(boolean flag1, boolean flag2) {        this.flag1 = flag1;        this.flag2 = flag2;    }    @Override    public boolean equals(Object object) {        if (!(object instanceof Key)) {            return false;        }        Key otherKey = (Key) object;        return this.flag1 == otherKey.flag1 && this.flag2 == otherKey.flag2;    }    @Override    public int hashCode() {        int result = 17; // any prime number        result = 31 * result + Boolean.valueOf(this.flag1).hashCode();        result = 31 * result + Boolean.valueOf(this.flag2).hashCode();        return result;    }}

After that, you can use your key with Map:

Map<Key, Integer> map = new HashMap<>();Key firstKey = new Key(false, false);map.put(firstKey, 1);Key secondKey = new Key(false, false) // same key, different instanceint result = map.get(secondKey); // --> result will be 1

Reference:Java hash code from one field