NullPointerException when adding to HashMap

Question

I am getting a NullPointerException when adding data to a HashMap. I am writing a class to count the given frequencies of certain Objects. Here is my code (stripped of any unnecessary details):

public class FrequencyCounter {

    private Map<Object, Integer> freq;

    public FrequencyCounter() {
        freq = new HashMap<Object, Integer>();
    }

    public int add(Object key) {        
        System.out.println("Map is null: " + (freq == null));
        System.out.println("Key is null: " + (key == null));
        if (freq.containsKey(key)) {
            return freq.put(key, freq.get(key) + 1);
        }
        return freq.put(key, 1);
    }

    public static void main(String[] args) {
        FrequencyCounter fc = new FrequencyCounter();
        fc.add(new Object());
    }
}

The NPE is occuring on the line return freq.put(key, 1); Both println statements print false.

Do any of you know what I may be doing wrong?

Answer 1

This is because HashMap.put(key, value) will return the previous value associated with key, or null. In your code, you cannot return freq.put(key,1) as int because it is null.

Answer 2

add method returns int. int is primitive data type , which can not be null. freq.put(key,1)returns null so it throws exception. change return type of add method to Integer or add null check.