Java arrays, Wat!?

There is a few things you can do with arrays which are surprising.

Is it an array or not?

Serializable array = new Serializable[9];

 Is array an array or a scalar? Well its a scalar which points to an array. Just like 

 Object o = new Object[9];

 You can assign an array to an object because it is also an object.  However, arrays are also Serializable so you can assign them to Serializable.

Where did my [] go?

The [] can appear in surprising places.  This compiles for backward comparability reasons.

public static int method(int[]... args)[] {
    return args[0];
}

And the types here are; args is an int[][] and the return type is int[].  Did you notice the [] after the method declaration!  This is not part of the JLS and the OpenJDK allows this due to backward compatibility reasons.

What is the difference between int[] array and int array[] ?

There is a difference in what comes after it.

int[] array, x[];

and

int array[], y[];

In these cases; x is an int[][] but y is only an int[].

What happens if an array initialization is too large?

Say I initialize an array like this

public static final int[] VALUES = {
    1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,
            /* many, many lines deleted */
    1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,
};

The errors message is;

Error:(6, 31) java: code too large

This seems a little strange.  It doesn't complain the array is too large.  In fact if I have more static fields or use larger constants, it will fail for a smaller array.

This happens because arrays are initialised in byte code.  There byte code creates the array and initialises each value, one at a time.  This results in a lot of code for large arrays which would be such a problem if there wasn't a limit in the size of a method. i.e. 65535 bytes.  The compiler generates one and only one method for a constructor or static initialization so this limits how many enums you can have and how large your initialised arrays can be.

Comments

  1. javin paul1 May 2014 at 06:01

    Cool Puzzles Peter, especially third one :). By the way, I have also shared few thoughts on array at Java Array 101, not puzzles but something useful

    ReplyDelete
  2. Richard1 May 2014 at 13:33

    Very interesting, thanks!

    ReplyDelete
  3. Tech Guy1 May 2014 at 16:28

    super coollllllll....Thanks Peter :)

    ReplyDelete
  4. Unknown16 May 2014 at 08:35

    It makes sense as arrays need to be injected into the heap. It is interesting though that array initialisation data is not stored in runtime constant pool similar to how String literals are stored. Presumably storing individual data values embedded into bytecode is far less efficient than storing an assembled object as data.

    By extension, array initialisation inside a method would be done via code as well and be further restricted by the code already in the method.

    ReplyDelete

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