Question

I wonder if there is any difference(or possible side effects) between calling:

AtomicBoolean.set(true)

and

AtomicBoolean.compareAndset(false, true)

The JavaDoc of AtomicBoolean#set states:

Unconditionally sets to the given value.

While AtomicBoolean#compareAndSet states:

Atomically sets the value to the given updated value if the current value == the expected value.

In both cases the value will be set to true. So what is the difference?

Answer 1

compareAndset(false, true) will return false if the value is already true.
It’s actually equivalent to !getAndSet(true).