How to modify the alpha of an ARGB hex value using integer values in Java

Question

I'm working on a project that allows a user to adjust the alpha of a color using a custom slider. The slider returns an integer from 0-255 that defines the new alpha value the user wishes to use.

The problem is that the colors have to be in hexadecimal, and I don't know how to convert the 0-255 integer into a hexadecimal integer that can modify the original ARGB hexadecimal. I have done a little research (Like " How would one change the alpha of a predefined hexadecimal color? "), but nothing I found could fix my problem. I thought about using the Color class from java AWT, however, it doesn't have a getRGBA() method.

What I want to happen:

    /** 
     * Original ARGB hexadecimal
     * Alpha: 255, Red: 238, Blue: 102, Green: 0 
    */
    int originalColor = 0xFFEE6600;

    /**
     * Creates a new hexadecimal ARGB color from origColor with its alpha
     * replaced with the user's input (0-255)
     * EX: If userInputedAlpha = 145 than the new color would be 0x91EE6600
    */
    int newColor = changeAlpha(origColor, userInputedAlpha);

All I need is the changeAlpha method, which modifies the color parameter's alpha (which is a hexadecimal integer) with the user inputed alpha (which is an integer from 0-255)

Answer 1

You know that the alpha value is stored in the bits 24 to 31, so what you can do is to apply first a mask to drop the previous alpha value and then shift the one the user inputted to apply it to the color.

int changeAlpha(int origColor, int userInputedAlpha) {
    origColor = origColor & 0x00ffffff; //drop the previous alpha value
    return (userInputedAlpha << 24) | origColor; //add the one the user inputted
}

Which can be easily reduced to a one liner:

return (origColor & 0x00ffffff) | (userInputedAlpha << 24);

It seems that you were perturbed about the fact that the values are in hexadecimal or not. An integer is an integer, hexadecimal is just a notation. After all there's only 0 and 1 in our computers.