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java – HashMap with byte array key and String value – containsKey() function doesn't work

Posted by: admin June 16, 2020 Leave a comment

Questions:

I’m using a HashMap: byte[] key and String value. But I realize that even I put the same object (same byte array and same string value) by using

myList.put(TheSameByteArray, TheSameStringValue)

into HashMap, the table still inserts a new object with different HashMapEntry. Then function containsKey() cannot work.

Can someone explains this for me? How can I fix this? Thanks. (Android Java)

@Override public boolean containsKey(Object key) {
    if (key == null) {
        return entryForNullKey != null;
    }

    int hash = Collections.secondaryHash(key);
    HashMapEntry<K, V>[] tab = table;
    for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
            e != null; e = e.next) {
        K eKey = e.key;
        if (eKey == key || (e.hash == hash && key.equals(eKey))) {
            return true;
        }
    }
    return false;
}
How to&Answers:

A byte[] (or any array) can’t work properly as a key in a HashMap, since arrays don’t override equals, so two arrays will be considered equal only if they refer to the same object.

You’ll have to wrap your byte[] in some custom class that overrides hashCode and equals, and use that custom class as the key to your HashMap.

Answer:

Adding to Eran’s clear answer,Since byte[] or any array doesnt override hashcode and equals(it uses the default methods of Object class ),you can always wrap around a String Object which takes byte[] as constructor argument.Not only does String form good keys in Map,they are immutable too(the operations in a Hash based map are faster)

http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#String(byte[])

Answer:

NOTE: This is an extremely hack-y way of making an array or a string, a key in a HashMap, without overriding either the equals() or the hashCode() methods.
I’ll include the answer in a generic way, so readers can get the idea and implement as per their requirements.

Say, I have two numbers, n and r. I want a key-value pair with [n,r] as the key, and (n+r) as the value.

Map<List<Integer>, Integer> map = new HashMap<List<Integer>, Integer>();

List<Integer> key = Arrays.asList(n, r);
if( map.containsKey(key) )
    return map.get(key);

What if the map did not contain the key?

map.put(Collections.unmodifiableList(Arrays.asList(n, r)), (n+r));

The unmodifiable part (without going into any further depth), ensures that the key cannot change the hash code.

Now, map.containsKey(key) will be true.

Note: This is not a good way to do it. It is just a workaround.