线程程序问题

设计 4 个线程， 其中两个线程每次对 j 增加 1， 另外两个线程对 j 每次减少 1。 写出程序。 

package mystudy;

public class ManyThreads {

    private int j;

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ManyThreads many = new ManyThreads();
        Inc inc = many.new Inc();
        Dec dec = many.new Dec();
        for (int i = 0; i < 2; i++) {
            Thread t = new Thread(inc);
            t.start();
            t = new Thread(dec);
            t.start();
        }
    }

    private synchronized void inc() {
        j++;
        System.out.println(Thread.currentThread().getName() + "inc" + j);
    }

    private synchronized void dec() {
        j--;
        System.out.println(Thread.currentThread().getName() + "dec" + j);
    }

    class Inc implements Runnable {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            for (int i = 0; i < 20; i++) {
                inc();
            }
        }

    }

    class Dec implements Runnable {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            for (int i = 0; i < 20; i++) {
                dec();
            }
        }

    }
}

第二种