多线程笔试题

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package offer;

import java.util.Scanner;

/**
 * 多线程
 * 四个线程，A线程打印A，B线程打印B，C线程打印C，D线程打印D，要求按顺序打印ABCD，键盘输入打印的次数
 * 牛客网题目网址：https://www.nowcoder.com/practice/cd99fbc6154d4074b4da0e74224a1582?tpId=37&tqId=21272&tPage=3&rp=&ru=/ta/huawei&qru=/ta/huawei/question-ranking
 * Created by 怪兽 on 2018/3/12.
 */
public class pro5 {
    private static StringBuilder sb ;  //要写入的字符数组
    private int num;    //循环总的次数
    private int numCurrect; //当前循环次数
    private int index;  //线程下标

    public pro5(int num){
        this.num = num;
        index = 1;
        numCurrect = 0;
        sb = new StringBuilder(num * 4);
    }

    public void appandA(){
        synchronized (this){
            while(numCurrect < num){    //小于循环总数
                while (index != 1){ //非当前线程启动,一定要用while，以防止线程A再次获得对象锁----->注意notify唤醒线程是从wait方法之后的地方继续执行，而不是重新执行
                    try {
                        if(numCurrect == num )  //加条件退出，以销毁线程----->注意：并不是循环条件numCurrect < num就说明不会执行该段代码，因为是多线程，所以会存在数据的异步更新
                            break;
                        this.wait();    //释放对象锁
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                if(numCurrect == num )  //同理，加条件退出，以销毁线程
                    break;
                sb.append('A');
                index = 2;  //切换线程
                this.notifyAll();   //唤醒其他所有线程，该操作不会立即释放对象锁
            }
            System.out.println(sb.toString());  //循环次数结束，所有线程被销毁，输出打印sb
        }
    }

    public void appandB(){
        synchronized (this){
            while (numCurrect < num){
                while(index != 2){
                    try {
                        if(numCurrect == num )
                            break;
                        this.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                if(numCurrect == num )
                    break;
                sb.append('B');
                index = 3;
                this.notifyAll();
            }
        }
    }

    public void appandC(){
        synchronized (this){
            while (numCurrect < num){
                while(index != 3){
                    try {
                        if(numCurrect == num )
                            break;
                        this.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                if(numCurrect == num )
                    break;
                index = 4;
                sb.append('C');
                this.notifyAll();
            }
        }
    }

    public void appandD(){
        synchronized (this) {
            while (numCurrect < num) {
                while (index != 4) {
                    try {
                        if(numCurrect == num )
                            break;
                        this.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                if(numCurrect == num ) {
                    break;
                }
                index = 1;
                sb.append('D');
                numCurrect++;
                this.notifyAll();
            }
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNext()){
            int num = scanner.nextInt();
            pro5 pro5 = new pro5(num);

            Thread thread1 = new Thread(new Runnable() {
                @Override
                public void run() { pro5.appandA(); }
            });
            thread1.start();
            Thread thread2 = new Thread(new Runnable() {
                @Override
                public void run() {
                    pro5.appandB();
                }
            });
            thread2.start();
            Thread thread3 = new Thread(new Runnable() {
                @Override
                public void run() {
                    pro5.appandC();
                }
            });
            thread3.start();
            Thread thread4 = new Thread(new Runnable() {
                @Override
                public void run() {
                    pro5.appandD();
                }
            });
            thread4.start();

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

}