Alpha value to RGB

Question

int rgb;
int alpha
rgb |= ((alpha & 0xFF) << 24);Shouldn't this append the alpha bits correctly into the RGB value? because it doesnt seem to work.

Answer 1

import java.awt.Color;
public class Test {
    public static void main(String[] args) {
        int rgb = new Color(100, 175, 200, 190).getRGB();
        unpack("1", rgb);
        // Clear alpha bits [24 - 31] in rgb.
        rgb <<= 8;
        unpack("2", rgb);
        rgb >>= 8;
        unpack("3", rgb);
        // Reset alpha.
        int alpha = 175;
        rgb |= ((alpha & 0xff) << 24);
        System.out.printf("alpha = %3d  rgb = %11d  retrieve alpha: %3d%n",
                           alpha, rgb, (rgb >> 24) & 0xff);
        unpack("4", rgb);
        rgb = clearAlpha(rgb);
        alpha = 200;
        rgb |= ((alpha & 0xff) << 24);
        System.out.printf("alpha = %3d  rgb = %11d  retrieve alpha: %3d%n",
                           alpha, rgb, (rgb >> 24) & 0xff);
        unpack("5", rgb);
        rgb = clearByte(8, rgb);
        int g = 225;
        rgb |= ((g & 0xff) << 8);
        System.out.printf("g = %3d  rgb = %11d  retrieve g: %3d%n",
                           g, rgb, (rgb >> 8) & 0xff);
        unpack("6", rgb);
    private static int clearAlpha(int n) {
        n <<= 8;
        n >>= 8;
        return n;
    private static int clearByte(int start, int n) {
        if(start + 8 <= 32) {
            for(int j = start; j < start+8; j++) {
                int bit = (n & (1 << j)) >> j;
                if(bit == 1) {
                    int mask = getMask(j);
                    //System.out.printf("mask  = %s%n",
                    //                   Integer.toBinaryString(mask));
                    n &= mask;
                    //System.out.printf("n[%2d] = %s%n",
                    //                   j, Integer.toBinaryString(n));
        return n;
    private static int getMask(int n) {
        int mask = 0;
        for(int j = 0; j < 32; j++)
            mask |= ((1 << j) | 1);
        mask ^= (1 << n);
        return mask;
    private static void unpack(String s, int n) {
        int a = (n >> 24) & 0xff;
        int r = (n >> 16) & 0xff;
        int g = (n >>  8) & 0xff;
        int b = (n >>  0) & 0xff;
        System.out.printf("%s unpack %11d: a = %3d  r = %3d  g = %3d  b = %3d%n",
                           s, n, a, r, g, b);
}