converting 'int' to 'long' or accessing too long array with 'long'

问题

Let's say I have an array that is long enough to access any of its index with int, is there any way to access the index of such an array with long? And how the Java handle this kind of array? Example:

int[] a = new int[]{1,5,2,4........9,2,1}

Assume in the above array that 9,2,1 are at indices that are beyond the range of int (2³¹). How would I access these elements?

回答1:

You wouldn't - array indexes are always int values in Java. It doesn't allow for an array with more than Integer.MAX_VALUE elements.

The length of an array is represented by the length field, which is of type int. Therefore it is impossible to create an array with a length greater than Integer.MAX_VALUE.

The spec doesn't explicitly call this out, but you can infer it from the types involved.

回答2:

You can't have a that long array. But this idea has been proposed for the project coin.

Arrays must be indexed by int values; short, byte, or char values may also be used as index values because they are subjected to unary numeric promotion (§5.6.1) and become int values. An attempt to access an array component with a long index value results in a compile-time error.

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Resources :

• JLS - Array Access
• JLS - Array Creation Expressions
• Project coin - Large array V1 - V2

回答3:

As others mentioned, the length and index values must be ints. If you really need this, there are workarounds though. For instance, you could have an array of very big int arrays. You could then do some modulo arithmetic on a long to determine which array you want and which index in that array is needed.

回答4:

You'd need a custom data structure, try this:

/**
* Because java uses signed primitives only the least significant 31 bits of an int are used to index arrays,
* therefore only the least significant 62 bits of a long are used to index a LongArray
* 
* @author aaron
*/
public class LongArray<Element> {

    //inclusive
    public static final long maximumSize = (~0)>>>2;//0x 00 FF FF FF  FF FF FF FF
    public static final long minimumSize = 0;

    //Generic arrays are forbidden! Yay dogma!
    private Object[][] backingArray;

    private static int[] split(long L) {
        int[] rtn = new int[2];
        rtn[1] = Integer.MAX_VALUE & (int)(L>>7);
        rtn[0] = Integer.MAX_VALUE & (int)L;
        return rtn;
    }
    private static long join(int[] ia) {
        long rtn = 0;
        rtn |= ia[0];
        rtn <<= 7;
        rtn |= ia[1];
        return rtn;
    }

    private static boolean isValidSize(long L) {
        return L<=maximumSize && L>=minimumSize;
    }

    public LongArray(long size){
        if (!isValidSize(size)) throw new IllegalArgumentException("Size requested was invalid, too big or negative");

        //This initialises the arrays to be only the size we need them to be
        int[] sizes = split(size);
        backingArray = new Object[sizes[0]][];
        for (int index = 0; index<backingArray.length-1; index+=1) {
            backingArray[index] = new Object[Integer.MAX_VALUE];
        }
        backingArray[backingArray.length-1] = new Object[sizes[1]];
    }

    public Element get(long index) {
        int[] ia = split(index);
        return (Element)backingArray[ia[0]][ia[1]];
    }
    public void set(long index, Element element) {
        int[] ia = split(index);
        backingArray[ia[0]][ia[1]] = element;
    }

}

来源：https://stackoverflow.com/questions/3943585/converting-int-to-long-or-accessing-too-long-array-with-long