How to remove a flag in Java

Question

Hi I need to remove a flag in Java. I have the following constants:

public final static int OPTION_A = 0x0001;
public final static int OPTION_B = 0x0002;
public final static int OPTION_C = 0x0004;
public final static int OPTION_D = 0x0008;
public final static int OPTION_E = 0x0010;
public final static int DEFAULT_OPTIONS =
       OPTION_A | OPTION_B | OPTION_C | OPTION_D | OPTION_E;

I want to remove, for example OPTION_E from default options. Why is not the following code correct?

// remove option E from defaul options:
int result = DEFATUL_OPTIONS;
result |= ~OPTION_E;

Answer 1

|= performs a bitwise or , so you're effectively "adding" all the flags other than OPTION_E . You want &= (bitwise and ) to say you want to retain all the flags other than OPTION_E :

result &= ~OPTION_E;

However, a better approach would be to use enums and EnumSet to start with:

EnumSet<Option> options = EnumSet.of(Option.A, Option.B, Option.C,
                                     Option.D, Option.E);
options.remove(Option.E);

Answer 2

You must write

result &= ~OPTION_E;

Longer explanation:

You must think in bits:

~OPTION_E    // 0x0010 -> 0xFFEF
DEFATUL_OPTIONS //     -> 0x001F
0xFFEF | 0x001F //     -> 0xFFFF
0XFFEF & 0x001F //     -> 0x000F

The OR will never clear 1 bits, it will at most set some more. AND on the other hand will clear bits.

Answer 3

You should use the and operator instead of or :

result &= ~OPTION_E;

One way to think about it is that |= sets bits whereas &= clears bits:

result |= 1;  // set the least-significant bit
result &= ~1; // clear the least-significant bit