CODEWITHSUNDEEP
javanullpointerexceptiontreemap
Doesn't Matter
Doesn't Matter

Reputation: 405

Null Pointer Exception in TreeMap

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());
    }
}

Upvotes: 2

Views: 10927

Answers (4)

Marko Topolnik
Marko Topolnik

Reputation: 200296

There are many obvious deficiencies with your code.

Map is constructed in place as an empty map, but is immediately queried and the get result unboxed:

Map<Integer, Integer> map = new TreeMap<Integer, Integer>();
...
if(map.get(key) == 0)

get returns null for a non-existent map entry, not a zero, and unboxing will fail for a null argument.

Here list is initialized in place, but is then checked for length:

int[] list = {2, 3, 40, 3, 5, 4, 3, 3, 3, 2, 0};
if(list.length > 1)

This is a redundant check, remove it.

for(int i: list)
{
  int key = list[i];

The way you use int i is very probably wrong. The enhanced for loop assigns each array element in turn to i, so what you should in all probability have is just for (int key : list).

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

Calling intValue is redundant -- auto-unboxing will take care of this. You do need to make sure that the value is not null before trying to unbox.

Upvotes: 1

Bharat Sinha
Bharat Sinha

Reputation: 14373

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.

Upvotes: 2

Peter Lawrey
Peter Lawrey

Reputation: 533880

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);
}

Upvotes: 7

Ilya
Ilya

Reputation: 29731

change 

map.get(key) == 0

to 

map.get(key) == null

or 

!map.contains(key)

Upvotes: 1

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