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。

方法一：利用内部类实现，两个实现加减的类实现Runnable接口，然后再实现4个具体线程。

package com.jn.test;

public class ManyThreads {
	private int j;
	
	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<5;i++){
				inc();
			}
		}
	}
	class Dec implements Runnable{
		@Override
		public void run(){
			for(int i=0;i<5;i++){
				dec();
			}			
		}
	}
	public static void main(String[] args){
		ManyThreads threads = new ManyThreads();
		Inc inc = threads.new Inc();
		Dec dec = threads.new Dec();
		for(int i=0;i<2;i++){
			Thread t = new Thread(inc);
			t.start();
			t = new Thread(dec);
			t.start();
		}
	}
}

第二种方式：具体加减操作写在一个类的方法里，没有内部类，用另外一个类去调用。

package com.jn.test;

public class ManyThreads2 {
	private int j;
	
	public  synchronized void inc(){
		j++;
		System.out.println(Thread.currentThread().getName()+"-inc:"+j);
	}
	
	public synchronized void dec(){
		j--;
		System.out.println(Thread.currentThread().getName()+"-dec:"+j);
	}
}

调用类：

package com.jn.test;

public  class ThreadTest {
	 private ManyThreads2 many = new ManyThreads2();

	  public void test() {
	        for (int i = 0; i < 2; i++) {
	            new Thread(new Runnable() {

	                @Override
	                public void run() {
	                    // TODO Auto-generated method stub
	                    for (int i = 0; i < 5; i++) {
	                        many.inc();
	                    }
	                }
	            }).start();
	            new Thread(new Runnable() {

	                @Override
	                public void run() {
	                    // TODO Auto-generated method stub
	                    for (int i = 0; i < 5; i++) {
	                        many.dec();
	                    }
	                }
	            }).start();
	        }
	    }
	 
	    public static void main(String[] args) {
	        // TODO Auto-generated method stub
	    	ThreadTest tTest = new ThreadTest();
	        tTest.test();
	    }	  
}