Java面试题——多线程

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 减少 1。

借鉴了生产者消费者案例。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadTest2 {
	public static void main(String[] args) {
		Resource r=new Resource();
		System.out.println(r.j+"");
		Add a=new Add(r);
		Subtract s=new Subtract(r);
		Thread t0=new Thread(a);
		Thread t1=new Thread(a);
		Thread t2=new Thread(s);
		Thread t3=new Thread(s);
		t0.start();
		t1.start();
		t2.start();
		t3.start();		
	}	
}

class Resource{
	int j;
	boolean flag=false;
	Lock lock = new ReentrantLock();
	Condition con=lock.newCondition();
}

class Add implements Runnable{
	Resource r;
	
	public Add(Resource r){
		this.r=r;
	}
	public void run(){
		while(true){
			try{
				r.lock.lock();
				while(r.flag){
					try{							
						r.con.await();
					}catch(InterruptedException e){ }
				}
				r.j++;
				System.out.println(Thread.currentThread().getName()+"："+r.j);
				r.flag=true;
				r.con.signalAll();
			}finally{
				r.lock.unlock();
			}
		}
	}
}
class Subtract implements Runnable{
	Resource r;
	
	public Subtract(Resource r){
		this.r=r;
	}
	public void run(){
		while(true){
			try{
				r.lock.lock();
				while(!r.flag){
					try{	
						r.con.await();
					}catch(Exception e){ }
				}
				r.j--;
				System.out.println(Thread.currentThread().getName()+"："+r.j);
				r.flag=false;
				r.con.signalAll();
			}finally{
				r.lock.unlock();
			}
		}
	}
}