Count bits

Count.java

import java.util.Date;

public class Count {
    static int[] _counts = {
        0,1,1,2,
        1,2,2,3,
        1,2,2,3,
        2,3,3,1
    };
    static int[] counts;

    private static void init() {
        counts = new int[(1 << 16)];
        for (int i = 0; i <= 0xFFFF; i++) { // Need unsigned shorts!
            counts[i] = _counts[(i & 0xF)] +
                        _counts[((i >> 4) & 0xF)] +
                        _counts[((i >> 8) & 0xF)] +
                        _counts[((i >> 12) & 0xF)];
        }
    }

    public static int countBits(int x) {
        if (counts == null) init();
        int t;
        for (t = 0; x != 0; x >>>= 16) t += counts[(x & 0xFFFF)];
        return t;
    }

    public static int countBitsNaive(int x) {
        int t;
        for (t = 0; x != 0; x >>>= 1) t += (x & 1);
        return t;
    }

    public static int countBitsK(int x) {
        int t;
        for (t = 0; x != 0; t++) x &= x - 1;
        return t;
    }

    public static void main(String[] args) {
        // init();
        int min = 0 - (1 << 24);
        int max = 1 << 24;
        Date a; Date b;
        a = new Date();
        for (int i = min; i <= max; i++) countBitsNaive(i);
        b = new Date();
        System.out.println("Naïve: " + (b.getTime() - a.getTime()));
        a = new Date();
        for (int i = min ; i <= max; i++) countBits(i);
        b = new Date();
        System.out.println("Table: " + (b.getTime() - a.getTime()));
        a = new Date();
        for (int i = min ; i <= max; i++) countBitsK(i);
        b = new Date();
        System.out.println("Kernighan: " + (b.getTime() - a.getTime()));
    }
}