Getting java.lang.NullPointerException while returning an array of integers in java

Question

public class Omit {

    int[] omitnum(int[] a) {
        int[] arr = null;
        for (int i = 0; i <= 7; i++) {
            if (a[i] >= 13 && a[i] <= 19) {
                System.out.print(a[i]);
            } else {
                arr[i] = a[i];
            }
        }
        return arr;
    }
    public static void main(String[] args) {
        Omit c = new Omit();
        int[] a = {
            1,
            2,
            3,
            12,
            113,
            14,
            19,
            20
        };
        int[] b = null;
        b = c.omitnum(a);
        for (int i = 0; i <= 7; i++) {
            System.out.println(b[i]);
        }
    }

}

}

Getting this error while running Exception in thread "main"

java.lang.NullPointerException
    at omit.Omit.omitnum(Omit.java:20)
    at omit.Omit.main(Omit.java:28)

Initialized an array to return some values but it isn't returning any values and giving null error

Answer 1

From the stack trace you posted in your question, line 20 in file Omit.java is throwing NullPointerException . Which line is line 20 in class Omit.java ? Could it be this line:

arr[i] = a[i];

That would mean that either arr is null or a is null. If you debug your program, you will discover which of them is null. (Perhaps both are null?)

Any decent java IDE includes a debugger. Decent java IDE's include Intellij , Eclipse , NetBeans and JDeveloper . I also suggest you readHow to debug small programs

Nonetheless, I see in this line of your code (which is the first line in method omitnum() ) that you are explicitly setting local method variable arr to null:

int[] arr = null;

After that line you never assign a different value to arr and hence, when you get to this line:

arr[i] = a[i];

the variable arr is still null and hence the NullPointerException . Refer to this stackoverflow question: What is a NullPointerException, and how do I fix it?

You need to initialize an array before you can use it. If you Google for the terms java initialize array , you will get several million results, including Initializing Arrays in Java .

Answer 2

You get NullPointerException meaning you try to access some variable that is null .

In java you need to initialize arrays before you can use them, and in your code you just set arr = null so this is why it's not working.

If you want it to work just define arr properly like:

int[] arr = new int[a.length];

Answer 3

That error means you are assigning to a null pointer, you could write arr[a.length] but that would allocate memory for more elements than you actually need, because you need to only store according to a condition, that way you should do it like this:

public class Omit {
  int[] omitnum(int[] a) {
    int maxNum = 0;
    // calculate how much memory needed
    for(int i = 0;i < a.length; i++) {
      if(!(a[i] >= 13 && a[i] <= 19)) {
        maxNum++;
      }
    }
    // --------------------------------
    int[] arr = new int[maxNum];
    for(int i = 0, j = 0; i < a.length; i++) {
      if(a[i] >= 13 && a[i] <= 19) {
        System.out.println("Deleted: " + a[i]);
      }else {
        /* we can't use `i` because that would gives an error
           because arr has less elements than a :) */
        arr[j++] = a[i];
      }
    }
    return arr;
  }

  public static void main(String[] args) {
    Omit c = new Omit();
    int[] a = {1, 2, 3, 12, 113, 14, 19, 20};
    int[] b = c.omitnum(a);
    for(int i = 0; i < b.length; i++) {
      System.out.println(b[i]);
    }
  }
}

Output:

Deleted: 14
Deleted: 19
1
2
3
12
113
20

and array.length gives you flexibility instead of looping to a static position, to avoid changing your code each time you add or remove elements, and from counting of course

Here is another way using ArrayLists

import java.util.ArrayList;

public class Omit {
  ArrayList omitnum(int[] a) {
    ArrayList<Integer> arr = new ArrayList<Integer>();
    for(int i = 0; i < a.length; i++) {
      if(a[i] >= 13 && a[i] <= 19) {
        System.out.println("Deleted: " + a[i]);
      }else {
        arr.add(a[i]);
      }
    }
    return arr;
  }

  public static void main(String[] args) {
    Omit c = new Omit();
    int[] a = {1, 2, 3, 12, 113, 14, 19, 20};
    ArrayList b = c.omitnum(a);
    for(int i = 0, n = b.size(); i < n; i++) {
      System.out.println(b.get(i));
    }
  }
}

Output:

Deleted: 14
Deleted: 19
1
2
3
12
113
20

Here is another way suggested by the user Abra , but I made some changes to preserve the default behaviour of printing

import java.util.Arrays;

public class Omit {
  static boolean printN(int n) {
    System.out.println("Deleted: " + Integer.toString(n));
    return false;
  }

  public static void main(String[] args) {
    int[] a = {1, 2, 3, 12, 113, 14, 19, 20};
    int[] b = Arrays.stream(a).filter(n -> n < 13 || n > 19 ||
      printN(n)).toArray();
    for(int i = 0;i < b.length; i++) {
      System.out.println(b[i]);
    }
  }
}

Output:

Deleted: 14
Deleted: 19
1
2
3
12
113
20

Answer 4

You should initialize an array before you are using it.

Else you can get NullPointerException .

Instead of this:

int[] arr = null;

You can use this:

int[] arr = new int[a.length];