bit manipulation - When to use Bitwise Operators during webdevelopment?

Question

Although I grasp the concept of Bitwise Operators, I can't say that I have come across many use cases during the webdevelopment process at which I had to resort to using Bitwise Operators.

• Do you use Bitwise Operators?
• Why do you use them?
• What are some example use cases?

Please remember that this question is specifically intended for use of Bitwise Operators in web languages.

Answer 1

I'm going to be more explicit here because I think bitwise masks are a great tool that should be in any devs belt. I'm going to try to expand on the answers above. First, an example of using an integer to maintain state flags (common usage):

// These are my masks
private static final int MASK_DID_HOMEWORK  = 0x0001;
private static final int MASK_ATE_DINNER    = 0x0002;
private static final int MASK_SLEPT_WELL    = 0x0004; 

// This is my current state
private int m_nCurState;

To set my state, I use the bitwise OR operator:

// Set state for'ate dinner' and 'slept well' to 'on'
m_nCurState = m_nCurState | (MASK_ATE_DINNER | MASK_SLEPT_WELL);

Notice how I 'or' my current state in with the states that I want to turn 'on'. Who knows what my current state is and I don't want to blow it away.

To unset my state, I use the bitwise AND operator with the complement operator:

// Turn off the 'ate dinner' flag
m_nCurState = (m_nCurState & ~MASK_ATE_DINNER);

To check my current state, I use the AND operator:

// Check if I did my homework
if (0 != (m_nCurState & MASK_DID_HOMEWORK)) {
    // yep
} else { 
    // nope...
}

Why do I think this is interesting? Say I'm designing an interface that sets my state. I could write a method that accepts three booleans:

void setState( boolean bDidHomework, boolean bAteDinner, boolean bSleptWell);

Or, I could use a single number to represent all three states and pass a single value:

void setState( int nStateBits);

If you choose the second pattern you'll be very happy when decide to add another state - you won't have to break existing impls of your interface.

My two cents. Thanks.