Question

I’m attempting to use java’s Bitwise & operator and I think I’m misusing it. Using the below example, with messageAddress of 7 both condition 1 and condition 3 are satisfied. Shouldn’t a messageAddress of 7 just meet the 3rd condition and not the first? Any ideas on how to change the below logic so that messageAddress of 7 would only meet the last condition?

    public static final int SLOW = 1;
    public static final int SMEDIUM = 2;
    public static final int SHIGH = 3;
        String messageAddressHex="7";
        int messageAddress = Integer.parseInt(messageAddressHex, 16);
        if ((messageAddress & SLOW) == SLOW) {
                   //condition 1 met logic
                } else if ((messageAddress & SMEDIUM) == SMEDIUM) {
                    //condition 2 met logic
                } else if ((messageAddress & SHIGH) == SHIGH) {
                    //condition 3 met logic
                }

Answer 1

As it is, condition 1 is all the odd numbers, condition 2 is all the numbers with their second least significant bit turned on, and condition 3 is a combination of 1 and 2.

I assume that what you really want is to check the two least significant bits as a number between 0 and 3. If that’s the case, you should use:

if ((messageAddress & SHIGH) == SLOW) {
    //condition 1 met logic
} else if ((messageAddress & SHIGH) == SMEDIUM) {
    //condition 2 met logic
} else if ((messageAddress & SHIGH) == SHIGH) {
    //condition 3 met logic
}

EDIT:
It’s a much better design to declare another constant as the bit mask:

public static final int MASK=3;

And then we can write:

if ((messageAddress & MASK) == SLOW) {
    //condition 1 met logic
} else if ((messageAddress & MASK) == SMEDIUM) {
    //condition 2 met logic
} else if ((messageAddress & MASK) == SHIGH) {
    //condition 3 met logic
}

The number 3 has two roles – MASK, which is used as the bitmask to filter only the two last bits, and SHIGH – which is one of the possible values for the last two bits that you want to check.

BTW, now you can use a switch-case statement instead of the if-else-if chain:

switch (messageAddress & MASK) {
    case SLOW:
        //condition 1 met logic
        break;
    case SMEDIUM:
        //condition 2 met logic
        break;
    case SHIGH:
        //condition 3 met logic
        break;
}