为什么这段代码会抛出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.*;

class WhyNullPointerException {
    public static void main( String [] args ){
       // Create a map
        Map<String,Integer> m = new HashMap<String,Integer>();
        // Get the previous value, obviously null.
        Integer a = m.get( "oscar" );
        // If a is null put 1, else increase a
        int p = a == null ? 
            m.put( "oscar", 1) : 
            m.put( "oscar", a++ ); // Stacktrace reports Npe in this line
    }
}

Answer 1

Because m.put returns null (which indicates that there's no "previous" value) while you're trying to assign it to int . Replace int p by Integer p and it will work.

This is specified in JLS 5.1.8 :

5.1.8 Unboxing Conversion

At run time, unboxing conversion proceeds as follows:

• If r is null , unboxing conversion throws a NullPointerException

---

Unrelated to the problem, just a side suggestion with DRY in mind, consider writing it so:

    Integer p = m.put("oscar", a == null ? 1 : a++);

It's a bit more readable :)

Answer 2

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.