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。
 1 package mystudy;
 2 
 3 public class ManyThreads {
 4 
 5     private int j;
 6 
 7     public static void main(String[] args) {
 8         // TODO Auto-generated method stub
 9         ManyThreads many = new ManyThreads();
10         Inc inc = many.new Inc();
11         Dec dec = many.new Dec();
12         for (int i = 0; i < 2; i++) {
13             Thread t = new Thread(inc);
14             t.start();
15             t = new Thread(dec);
16             t.start();
17         }
18     }
19 
20     private synchronized void inc() {
21         j++;
22         System.out.println(Thread.currentThread().getName() + "inc" + j);
23     }
24 
25     private synchronized void dec() {
26         j--;
27         System.out.println(Thread.currentThread().getName() + "dec" + j);
28     }
29 
30     class Inc implements Runnable {
31 
32         @Override
33         public void run() {
34             // TODO Auto-generated method stub
35             for (int i = 0; i < 20; i++) {
36                 inc();
37             }
38         }
39 
40     }
41 
42     class Dec implements Runnable {
43 
44         @Override
45         public void run() {
46             // TODO Auto-generated method stub
47             for (int i = 0; i < 20; i++) {
48                 dec();
49             }
50         }
51 
52     }
53 }
第二种