Using String(byte[]) as key to map

Question

I have a program that gets ids from a database returned as a byte[]. I want to use those ids (byte arrays) as keys to a map. By default this does not work because what is compared is "object equality" not "content equality".

The next thing I tried which seemed to work is instantiating a String with the new String(byte[]) constructor. While this seems to work I know I'm doing some potentially dodgy things. I am not specifying an encoding which relies on whatever the system default is. I also don't know if all byte sequences will have representation in every encoding.

Is "converting" byte[] to String for the purpose of doing a map.put() and map.get() safe? Are there edge cases I'm not considering where this approach would inadvertently create a collision (where 2 different byte[]'s with different content could become the same String).

Note: The map is stored in memory and only lives for a single application run. Simple wrapper class I wrote to make my life easier if this approach is viable:

public class Cache { Map<String, Set<String>> cache = new HashMap<String, Set<String>>(); public Cache() {} public void put(byte[] key, Set<String> value) { cache.put(new String(key), value); } public Set<String> get(byte[] key) { return cache.get(new String(key)); } }

This seems related and may be of some help: stackoverflow.com/questions/16839182/… — Chop
– Chop, Commented Jul 29, 2015 at 12:53

OldCurmudgeon · Accepted Answer · 2015-07-29 12:50:54Z

6

You should probably use Biginteger. It has a BigInteger(byte[]) constructor and has an effective comparison system.

answered Jul 29, 2015 at 12:50

OldCurmudgeon

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1 Comment

Russ Over a year ago

This seems pretty straightforward. I'm going to go with this unless there is a blatant reason not to. The only thing I could think being an issue is a byte array with contents with "leading" zeros but this shouldn't be the case for me where that is a reality. Thanks for the answer!

Lyndon Armitage · Accepted Answer · 2015-07-29 13:15:44Z

Could you not build a simple wrapper around your byte[] array instead?

Something akin to:

public class CacheKey { private final byte[] key; public CacheKey(byte[] key) { this.key = key; // You may want to do a defensive copy here } @Override public boolean equals(Object o) { if (this == o) { return true; } if (o == null || getClass() != o.getClass()) { return false; } CacheKey cacheKey = (CacheKey) o; return Arrays.equals(key, cacheKey.key); } @Override public int hashCode() { return key != null ? Arrays.hashCode(key) : 0; } }

And use that as the map key? It would be a little more lightweight than using the built in String object and makes the key type actually clear.

Hey this might work too. Does Arrays.hashCode(prim[]) produce a hash based on content of the array?
That hashcode and equals method were actually generated by my IDE. Arrays.hashcode() guarantees a call to Arrays.equals() between two arrays with the same hashcode will evaluate to true, so it must do.

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