是否可以扩展字节数组？

Question

I am implementing AES for an assignment, and would like to encapsulate the logic of checking that a key is always an array of bytes whose length is exactly 16, 24 or 32.

I can create a wrapper class that stores the bytes as a byte[] internally, but I wonder if it is possible to extend Array or something similar so that I don't have to reimplement Iterators, toString and other convenience methods that I get from using Java array.

Ie is something like the following possible?

public class Key extends Array<Byte>{

    // Valid length of key in bytes
    public static int[] possibleLengths = new int[]{ 16, 24, 32 };

    // Check if a given byte[] is a valid candidate for a key
    public static boolean isValidLength(byte[] input) {
        for (int length: possibleLengths) {
            if (input.length == length) return true;
        }
        return false;
    }

    // Primary constructor passes bytes directly to internal state
    public Key(byte[] input) throws InvalidKeyLengthException {
        if (!isValidLength(input))
            throw new InvalidKeyLengthException;

        setKey(input);
    }
}

Answer 1

Well...no. You can't extend an array type, because it's dynamically generated upon array instantiation.

It would make far more sense to have this as a field to a wrapper class (namely Key ).

If you're concerned about having to use helper methods and the like, then you could simply delegate the major responsibilities (namely exposing the array and exposing a sensible toString() ).

Answer 2

No, it is not possible to extend the array classes.

I believe it is possible to extend ArrayList, however, which might get you close enough to what you want. You would need to disable ArrayList's ability to arbitrarily expands its capacity, however.

That said, you should probably follow to the guideline "Prefer aggregation over inheritance" in this case and just make your array or ArrayList a field within your class.

Answer 3

Array is a weird notion. They do have some properties, like length although they are not a class, so you can't extend them like you are attempting.

You may convert it to ArrayList :

new ArrayList<Byte>(Arrays.asList(Key))

so, you may create a class extending ArrayList and add your new functionalists to it. Then convert the Byte Array as said above to your custom class and check your requirements with your newly extended methods freely!