An Executor only runs one task at a time.

SequentialExecutor.java

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.concurrent.Executor;

/**
 * An Executor only runs one task at a time.
 */
public class SequentialExecutor implements Executor {

    private final Executor delegate;
    private final Deque<Runnable> jobs = new ArrayDeque<>();
    private boolean executing = false;
    private Object lock = new Object();

    public SequentialExecutor(Executor delegate) {
        this.delegate = delegate;
    }

    @Override
    public void execute(Runnable command) {
        synchronized (lock) {
            jobs.addLast(command);
            if (!executing) {
                poll();
            }
        }
    }

    /**
     * poll should always be guarded by synchronization on lock
     */
    private void poll() {
        if (jobs.isEmpty()) {
            return;
        }
        Runnable runnable = jobs.removeFirst();
        executing = true;
        delegate.execute(() -> run(runnable));
    }

    private void run(Runnable runnable) {
        try {
            runnable.run();
        } finally {
            synchronized (lock) {
                executing = false;
                poll();
            }
        }
    }

}