SafeAlley implementation

public class SafeAlley extends Alley {

    int up, down, t1, t2;
    Semaphore alleySem, downIncrSem, upIncrSem;

    protected SafeAlley() {
        up = 0;   down = 0;
        alleySem   = new Semaphore(1);
        downIncrSem = new Semaphore(1);
        upIncrSem = new Semaphore(1);
    }

    /* Block until car no. may enter alley */
    public void enter(int no) throws InterruptedException {
        if (no < 5) {
            downIncrSem.P();
            down++;
            if (down == 1) alleySem.P();
            downIncrSem.V();
        } else {
            upIncrSem.P();
            up++;
            if (up == 1) alleySem.P();
            upIncrSem.V();
        }

    }

    /* Register that car no. has left the alley */
    public void leave(int no) {
        try {
            if (no < 5) {
                downIncrSem.P();
                down--;
                if (down == 0) alleySem.V();    // enable up-going cars again
                downIncrSem.V();
            } else {
                upIncrSem.P();
                up--;
                if (up == 0) alleySem.V();    // enable down-going cars again
                upIncrSem.V();
            }
        }
        catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }

}