Hashmap With Byte Array Key And String Value - Containskey() Function Doesn't Work

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, TheSam

Solution 1:

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.

Solution 2:

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[])

Solution 3:

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.

Solution 4:

You can use ByteBuffer, which is a wrapper for byte[] array with a comparator.

Referring answer from - https://stackoverflow.com/a/14087243/4019660