OS - Tennis Tournament

import java.util.HashSet;
import java.util.concurrent.Semaphore;

public class TennisTournament {

    static int red = 0;
    static int green = 0;
    static Semaphore lock = new Semaphore(1);
    static Semaphore redEnter = new Semaphore(2);
    static Semaphore greenEnter = new Semaphore(2);
    static Semaphore canPlay = new Semaphore(0);
    static boolean flag=false;


    public static class GreenPlayer  extends Thread{


        public void execute() throws InterruptedException {
            System.out.println("Green player ready");
            greenEnter.acquire();
            lock.acquire();
            green++;
            if(red+green==4) {
                if(canPlay.availablePermits()==0) {
                    canPlay.release(4);

                }
            }
            lock.release();
            System.out.println("Green player enters field");
            canPlay.acquire();
            System.out.println("Match started");
            Thread.sleep(200);
            lock.acquire();
            --green;
            if(green == 0 && red == 0 ) {
                System.out.println("Match finished");
                if(redEnter.availablePermits()==0 && greenEnter.availablePermits()==0) {
                    redEnter.release(2);
                    greenEnter.release(2);

                }
            } else {
                System.out.println("Green player finished playing");
            }
            lock.release();
            // TODO: only one player calls the next line per match


        }

        public void run() {
            try {
                execute();

            }catch(InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

    public static class RedPlayer extends Thread{



        public void execute() throws InterruptedException {
            System.out.println("Red player ready");
            redEnter.acquire();
            lock.acquire();
            red++;
            if(red+green==4) {
                if(canPlay.availablePermits()==0) {
                    canPlay.release(4);

                }
            }
            lock.release();
            System.out.println("Red player enters field");
            canPlay.acquire();
            System.out.println("Match started");
            Thread.sleep(200);
            lock.acquire();
            --red;
            if(green == 0 && red == 0 ) {
                System.out.println("Match finished");
                if(redEnter.availablePermits()==0 && greenEnter.availablePermits()==0) {
                    redEnter.release(2);
                    greenEnter.release(2);

                }

            }else {
                System.out.println("Red player finished playing");
            }
            lock.release();
        }

        public void run() {
            try {
                execute();

            }catch(InterruptedException e) {
                e.printStackTrace();
            }
        }

    }


    public static void main(String[] args) throws InterruptedException {
        HashSet<Thread> threads = new HashSet<Thread>();
        for (int i = 0; i < 30; i++) {
            RedPlayer red = new RedPlayer();
            threads.add(red);
            GreenPlayer green = new GreenPlayer();
            threads.add(green);
        }
        // start 30 red and 30 green players in background
        for(Thread t : threads) {
            t.start();
        }
        // after all of them are started, wait each of them to finish for 1_000 ms
        for(Thread t : threads) {
            t.join(1000);
        }
        for(Thread t : threads) {
            if(t.isAlive()) {
                t.interrupt();
                System.err.println("Possible deadlock");
            }
        }
        // after the waiting for each of the players is done, check the one that are not finished and terminate them

    }

}