设计四个线程,其中两个线程每次对j增加1,另外两个线程对j每次减1,写出程序

/*

* 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。
*/

public class ThreadTest {

private int j;

public static void main(String[] args) {
ThreadTest tt = new ThreadTest();
Inc inc = tt.new Inc();
Dec dec = tt.new Dec();

Thread t1 = new Thread(inc);
Thread t2 = new Thread(dec);
Thread t3 = new Thread(inc);
Thread t4 = new Thread(dec);
t1.start();
t2.start();
t3.start();
t4.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() {
for (int i = 0; i < 10; i++) {
inc();

}
}
}

class Dec extends Thread {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
dec();

}
}

}
}