//This method getNext must return a unique value nextValue

public class Sequence {
    @GuardedBy("this") private int nextValue;
    public synchronized int getNext() {
        return nextValue++;
    }
}

Thread 1 gets value of nextValue, where nextValue = 5
Thread 2 gets value of nextValue, where nextValue = 5

public class Sequence {
    private int nextValue;
    public synchronized int getNext() {
        return nextValue++;
    }
}