OneTime.java

import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicBoolean;

public class OneTime {
    
    private final Semaphore mSemaphore = new Semaphore(1);
    private final AtomicBoolean mWasCalled = new AtomicBoolean(false);

    public void callbackOption1() {
        try {
            mSemaphore.acquire();
            if (!mWasCalled.get()) {
                doSomethingOnlyOneTime();
            }
            mWasCalled.set(true);
        } catch (InterruptedException e) {
            // propagate error
        } finally {
            mSemaphore.release();
        }
    }

    public void callbackOption2() {
        // must be synchronized on mWasCalled, otherwise another thread could pass the conditional
        // before doSomethingOnlyOneTime() finishes.
        synchronized (mWasCalled) {
            if (!mWasCalled.get()) {
                doSomethingOnlyOneTime();
                mWasCalled.set(true);
            }
        }
    }

    private void doSomethingOnlyOneTime() {

    }
}