哈希图中的线程问题

Question

I am using two threads to put values in the same hashmap. Thread 1 will read the values what it was stored and thread 2 will read only the values what it was stored in the map.

In that i am getting NullPointerException . Is that due to threading issue??

HashMap map = new HashMap();

    Thread1:

           map.put(1,1);
           map.put(2,2);
           map.get(1).toString(); // here null pointer i am getting.. Is it possible due to threading issue..

    Thread 2:
       map.put(4,4);
       map.put(5,5)

Answer 1

There are 2 solutions

1) Use java.util.ConcurrentHashMap, which is designed to operate in a threaded enviornment.

2) Synchronize the hashmap. Java allows you to synchronize on objects, so you can ensure that only one thread touches the object at at time.

ex.

synchronized(map) {
    map.put("important", "stuff");
}

Method #2 is NOT the preferred solution for multithreading (CuncurrentHashMap is), but is your only option if you cannot change the type of the variable (like on a large project).

Answer 2

@Dylan is correct, short answer is you should never use HashMaps in a concurrent environment. Luckily, java.util.concurrent.ConcurrentHashMap to the rescue.