hashCode和equals在int [] Java中的实现

Question

It looks like the hashCode() and equals() for int[] are poorly implemented, or not implemented at all! (Tested with Android, but I expect it to be true for any Java environment).

In order to get HashSet.contains() working properly, I had to create a wrapper for int[] (plse, don't criticize my coding style, look at the essence):

public class IntArray {
    private int[] value;

    public IntArray(int[] value) {
        this.value = value;
    }

    @Override
    public int hashCode() {
        int sum = 0;
        // Integer overflows are cheerfully welcome.
        for (int elem: value) sum += elem;
        return sum;
    }

    @Override
    public boolean equals(Object o) {
        if (o == null) return (value==null);

        if (value != null) {
            if (o instanceof int[]) 
                return compare((int[])o);

            if (o instanceof IntArray)
                return compare(((IntArray)o).value);
        }

        return false;
    }

    protected boolean compare(int[] other) {
        int len = value.length;
        if (other.length != len) return false;
        for (int i=0; i<len ; i++)
            if (value[i] != other[i]) return false;
        return true;
    }
}

Works OK, but I prefer to avoid a custom wrapper or a third-party library. Is there an option?

Answer 1

Since the standard Java Hashtable does not allow an override of the hash code used for keys, you are out of luck and will need to use a wrapper like you did.

keep in mind that your hashCode implementation is very bad, you can use this (Taken from java.util.Arrays in the standard JDK) to get a better hash distrubtion:

public static int hashCode(int a[]) {
  if (a == null)
    return 0;

  int result = 1;
  for (int element : a)
    result = 31 * result + element;
  return result;
}

An alternative is to use a different Hashtable, which can deal with primitives. one such option is Banana , which is a primitive collections library I created.

Answer 2

After Omry Yadan's message the hashCode function becomes as simple as that!

    @Override
    public int hashCode() {
        return Arrays.hashCode(value);
    }

For a RISC CPU, like ARM, It may be more efficient:

    @Override
    public int hashCode() {
        int code = 0;
        if (value != null) {
            code++;
            for (int elem: value)
                code = (code<<5) - code + elem;
        }
        return code;
    }

May be there is also a standard function for comparing arrays, in which case equals() can be simplified too?