Threads interruption

Hello.
I have 4 threads, they're writing on System.out in while(true) loop one
letter: A, B, C or D.
A parent thread read from System.in and according to input have to stop
some of threads. But it doesn't work - threads still printing letters.
I don't know how to stop these threads. can you help me?

here is my parent thread class:

class Klawiatura extends Thread
{
    int n;

    static void ustaw(int t)
    {
        switch (t)
        {
        case 0: Th.end[0] = false;
        case 1: Th.end[1] = false;
        case 2: {Th.end[0] = false;Th.end[1] = false;}

        }
    }

    public void run()
    {
        BufferedReader i = new BufferedReader(new
InputStreamReader(System.in));

        Th Th1 = new Th(1);
        Th Th2 = new Th(2);
        Th Th3 = new Th(3);
        Th Th4 = new Th(4);

        Th1.start();
        Th2.start();
        Th3.start();
        Th4.start();

        while(true)
        {
            try
            {
                n = i.read();
                switch (n)
                {
                case 49: Klawiatura.ustaw(0);
                case 50: [1] = false;
                case 51: Th1.end[0] = false;
                }
                System.out.println("N: "+n);
            }
            catch(IOException e)
            {}
        }
    }
}