What is the difference between AtomicBoolean.set(flag) and AtomicBoolean.compareAndSet(!flag, flag)?

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).

Answer 2

Well the text that you quoted says explicitly what the difference between the two operations is. But to make it clearer, if you ignore the atomicity aspect, the first one is equivalent to:

public void set(boolean newValue) {
    this.value = newValue;
}

and the second one is equivalent to:

public boolean compareAndSet(boolean expected, boolean newValue) {
    if (this.value == expected) {
        this.value = newValue;
        return true;
    } else {
        return false;
    }
}

---

For your example, set(true) sets the state to true, and compareAndset(false, true) sets the state to true iff it is not already true. So, yes, the net effect on the state of the AtomicBoolean is the same.

However, you will notice that the return value differs depending on the initial state of the AtomicBoolean object ... so from that perspective the methods are not equivalent, even with those argument values.