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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.

Reference: Java arrays, Wat!? from our JCG partner Peter Lawrey at the Vanilla Java blog.

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