What is the Purpose of the Pipe Equal (=) Operator in Programming?

What is the Pipe Equal Operator (=)?

Developers who have encountered the pipe equal operator (|=) in open-source library code may wonder about its meaning. This operator, often mistaken for a logic assignment, holds a significant bitwise OR operation.

Understanding Bitwise OR

The pipe equal operator |= works identically to =. In the code below, the |= operator combines the original value of defaults with the constant DEFAULT_SOUND:

notification.defaults |= Notification.DEFAULT_SOUND;

This operation is equivalent to:

notification.defaults = notification.defaults | Notification.DEFAULT_SOUND;

where | denotes the bitwise OR operator.

Bitwise OR in Constant Masks

In the provided example, the constants DEFAULT_SOUND, DEFAULT_VIBRATE, and DEFAULT_LIGHTS are powers of two:

DEFAULT_SOUND = 1
DEFAULT_VIBRATE = 2 (1 << 1)
DEFAULT_LIGHTS = 4 (1 << 2)

This allows the use of bitwise OR to add or remove flags. For instance:

int myFlags = DEFAULT_SOUND | DEFAULT_VIBRATE; // 001 | 010 = 011

Adding another flag:

myFlags |= DEFAULT_LIGHTS;

simply appends a new flag.

Testing Flag Presence

The bitwise AND operator (&) tests for the presence of a flag:

boolean hasVibrate = (DEFAULT_VIBRATE & myFlags) != 0;

If the result is non-zero, the flag is set.

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