package multithread;public class MyThread {    //j变量私有    private int j;    //同步的+1方法    private synchronized void add(){        j++;        System.out.println(Thread.currentThread().getName()+"----------> "+j);    }    //同步的-1方法    private synchronized void subtract(){        j--;        System.out.println(Thread.currentThread().getName()+"----------> "+j);    }    //实现Runnable接口的内部加类    class Add implements Runnable{        @Override        public void run() {            for (int i = 0; i < 100; i++) {                add();            }        }    }    //实现Runnable接口的内部减类    class Subtract implements Runnable{                @Override        public void run() {            for (int i = 0; i < 100; i++) {                subtract();            }        }    }        public static void main(String[] args) {        //创建外部类和内部类的实例        MyThread mt = new MyThread();        Add add = mt.new Add();        Subtract subtract = mt.new Subtract();        //循环启动4个线程        for (int i = 0; i < 2; i++) {            Thread t = new Thread(add);            t.start();            t = new Thread(subtract);            t.start();        }    }}