多线程设值 取值 问题

Question

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class ThreadMap {
private static Map map = new HashMap();

public static void main(String[] args) {
    for (int i = 0; i < 2; i++) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                Integer result = new Random().nextInt();
                map.put(Thread.currentThread(), result);//这里先设置值
                System.out.println(Thread.currentThread() + " has put result = " + result);
                new A().get();//这里取出值，为什么会有打印出null????
                new B().get();//这里取出值，为什么会有打印出null????
            }

        }).start();
    }
}

static class A {
    public Integer get() {
        Integer result = map.get(Thread.currentThread());
        System.out.println(Thread.currentThread() + ", get result = " + result);
        return result;
    }
}

static class B {
    public Integer get() {
        Integer result = map.get(Thread.currentThread());
        System.out.println(Thread.currentThread() + ", get result = " + result);
        return result;
    }
}

}

打印结果：
Thread[Thread-0,5,main] has put result = 2046069101
Thread[Thread-1,5,main] has put result = -1728516627
Thread[Thread-0,5,main], get result = null //这什么这里竟然有null
Thread[Thread-1,5,main], get result = -1728516627
Thread[Thread-0,5,main], get result = null //这什么这里竟然有null
Thread[Thread-1,5,main], get result = -1728516627

Answer 1

random没给取值范围，再说map是不允许重复的