四个线程，对一个变量进行+1,1。用Thread和Runnable实现

public class Mtest {

    int j;
    
    public synchronized void inc(){
        j++;
        System.out.println("plus j is:"+j+"/t thread is:"+Thread.currentThread().getName());
    }
    
    public synchronized void dec(){
        j--;
        System.out.println("menurs j is:"+j+"/t thread is:"+Thread.currentThread().getName());
    }
    
    class thread1 implements Runnable{
        
        @Override
        public void run() {

            while (true) {

                inc();
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            
        }
        
    }
    
    class thead2 extends Thread{

        @Override
        public void run() {

            while (true) {

                dec();
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
        
    }
    /**
     * @param args
     */
    public static void main(String[] args) {

        Mtest t = new Mtest(); 
        thread1 t1 = t.new thread1();
        for (int i = 0; i < 2; i++) {
            Thread kk = new Thread(t1);
            Thread tt = t.new thead2();
            kk.start();
            tt.start();
        }
    }

}