面试题 - 代码与编程题 - 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

public class ThreadTest1 {
  private int j;

  public static void main(String args[]) {
    ThreadTest1 tt = new ThreadTest1();
    Inc inc = tt.new Inc();
    Dec dec = tt.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 {
    public void run() {
      for (int i = 0; i < 100; i++) {
        inc();
      }
    }
  }

  class Dec implements Runnable {
    public void run() {
      for (int i = 0; i < 100; i++) {
        dec();
      }
    }
  }
}