java线程 时而报错时而正常

[Sunking_Yin]

从网上看到一段线程代码，运行时 时而报空指针异常，时而正常，找了半天都没找到什么原因，渴求各路大神不吝赐教
Java代码：

package com.gss.atest;

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

//线程范围内的共享数据
public class ThreadScopeShareData {
	private static int data = 0;
	private static Map<Thread, Integer> threadData = new HashMap<Thread, Integer>();
	public static void main(String[] args) {
		for(int i = 0; i < 2; i++){
			new Thread(new Runnable() {
				@Override
				public void run() {
					int data = new Random().nextInt();
					threadData.put(Thread.currentThread(), data);
					System.out.println(Thread.currentThread().getName()
							+ " has put data: " + data);
					new A().get();
					new B().get();
				}
			}).start();
		}
	}
	
	static class A{
		public void get(){
			data = threadData.get(Thread.currentThread());
			System.out.println("A from " + Thread.currentThread().getName()
					+ " has put data: " + data);
		}
	}
	static class B{
		public void get(){
			data = threadData.get(Thread.currentThread());
			System.out.println("B from " + Thread.currentThread().getName()
					+ " has put data: " + data);
		}
	}
}

异常信息：

Thread-1 has put data: 2115365575
Thread-0 has put data: 1347130149
A from Thread-1 has put data: 2115365575
B from Thread-1 has put data: 2115365575
Exception in thread "Thread-0" java.lang.NullPointerException
	at com.gss.atest.ThreadScopeShareData$A.get(ThreadScopeShareData.java:29)
	at com.gss.atest.ThreadScopeShareData$1.run(ThreadScopeShareData.java:20)
	at java.lang.Thread.run(Thread.java:745)

还请各路线程大神多多赐教。。。。拜托拜托。。。。

[lxyscls]

这人也是闲得蛋疼，既然是线程的私有数据，直接用java.lang.ThreadLocal<T>不就完事了

[Sunking_Yin]

为什么说map是不安全的？hashmap和hashtable有什么区别么？一楼

[qq_26507597]

有，hashmap线程不安全，在线程中使用，容易产生你的那个错误，hashtable和hashmap虽然都是继承map集合，但是hashtable是线程安全的。就避免 了出现这个问题，你网上查一下线程安全和线程不安全的区别

[qq_26507597]

http://bbs.csdn.net/topics/300263474看看这个，

[qq_26507597]

private static Hashtable<Thread, Integer> threadData = new Hashtable<Thread, Integer>();//map集合是不安全的改成Hashtable