Null Pointer Exception in TreeMap

Question

I have tried to implement a simple treemap to count the occurances of integers, but it is giving me a NullPointerException and I don't know how to fix it.

Exception in thread "main" java.lang.NullPointerException
    at exercises.CountOccurances_20_07.main(CountOccurances_20_07.java:21)

Here is the code:

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class CountOccurances_20_07 
{
    public static void main(String[] args) 
    {
        int[] list = {2, 3, 40, 3, 5, 4, 3, 3, 3, 2, 0};
        Map<Integer, Integer> map = new TreeMap<Integer, Integer>();
        for(int i: list)
        {
            int key = list[i];
            if(list.length > 1)
            {
                if(map.get(key) == 0)
                {
                    map.put(key, 1);
                }
                else
                {
                    int value = map.get(key).intValue(); // line 21
                    value ++;
                    map.put(key, value);
                }
            }
        }
        //get all entries into set
        Set<Map.Entry<Integer, Integer>> entrySet = map.entrySet();
        //get key and value from entry set
        for(Map.Entry<Integer, Integer> entry: entrySet)
            System.out.println(entry.getValue() + "\t" + entry.getKey());
    }
}

Answer 1

In your case map.get(key) is returning null and will never be 0. Also you are using the key, to lookup up itself which doesn't sound right.

for(int key: list) {
    Integer count = map.get(key);
    if (count == null) count = 0;
    map.put(key, count+1);
}

Answer 2

The NullPointerException at line 21

int value = map.get(key).intValue(); // line 21

is due to the fact that map.get(key) would return null if the key is not present in the map.

You should use

if(!map.containsKey(key)){
}

instead of

if(map.get(key) == 0) {
}

as it evaluates to

if(null == 0){
}

and your condition is false and then the control goes to line 21.