why does this code throw a NullPointerException?

Question

Eventually I got the answer, but it puzzled me for a while.

Why does the following code throws NullPointerException when run?

import java.util.*;

c         


        

Answer 1

You are assigning int p to the return value of m.put(). But put() returns null in this situation, and you can't assign an int to null.

From the Javadocs for HashMap.put():

Returns: previous value associated with specified key, or null if there was no mapping for key.