Untitled

public class SafeAlley extends Alley {
    private int up, down;
    private final Semaphore alleySem, downSem, upSem;

    protected SafeAlley() {
        up = 0;
        down = 0;
        alleySem = new Semaphore(1);
        downSem = new Semaphore(1);
        upSem = new Semaphore(1);
    }

    /* Block until car no. may enter alley */
    public void enter(int no) throws InterruptedException {
        if (no < 5) {  // down-going cars
            downSem.P();
            
            if (down == 0) {
                alleySem.P();  // First down-going car acquires alley
            }
            down++;
            
            downSem.V();
        } else {  // up-going cars
            upSem.P();
            
            if (up == 0) {
                alleySem.P();  // First up-going car acquires alley
            }
            up++;
            
            upSem.V();
        }
    }

    /* Register that car no. has left the alley */
    public void leave(int no) throws InterruptedException {
        if (no < 5) {  // down-going cars
            downSem.P();
            
            down--;
            if (down == 0) {
                alleySem.V();  // Last down-going car releases alley
            }
            
            downSem.V();
        } else {  // up-going cars
            upSem.P();
            
            up--;
            if (up == 0) {
                alleySem.V();  // Last up-going car releases alley
            }
            
            upSem.V();
        }
    }
}