What is a NullPointerException?

java.lang.NullPointerException is a runtime exception in Java. It occurs when a variable is accessed which is not pointing to any object and refers to nothing hence it is null.

Since the NullPointerException is a runtime exception, it doesn’t need to be caught and handled explicitly in application code.

What are the reasons for getting NullPointerException?

The NullPointerException occurs due to a situation in where an uninitialized object is attempted to be accessed. This means the object reference does not point anywhere and has a null value.

Some of the most common scenarios for a NullPointerException are:

Accessing a null object’s properties

Throwing null from a method that throws an exception

Incorrect configuration for dependency injection frameworks ex:(Spring)

Passing null parameters to a method

Calling methods on a null object

Using synchronized on a null object

Accessing an index element (like in an array) of a null object

Example of a NullPointerException

In this example we are trying to call a printLength methos which accepts a String as a parameter and prints its length. If the value of the String that it is passed as a parameter is null,

a java.lang.NullPointerException will be thrown.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
public class NullPointerExceptionExample {

    private static void printLength(String str) {

        System.out.println(str.length());

    }



    public static void main(String args[]) {

        String nullString = null;

        printLength(nullString);

    }

}





Exception in thread "main" java.lang.NullPointerException

    at NullPointerExceptionExample.printLength(NullPointerExceptionExample.java:3)

    at NullPointerExceptionExample.main(NullPointerExceptionExample.java:8)

How to Avoid getting NullPointerException

The NullPointerException can be avoided using checks and preventive techniques like the following:

Making sure an object is initialized properly by adding a null check before referencing its methods or properties.

Using Apache Commons StringUtils for String operations e.g. using StringUtils.isNotEmpty() for verifying if a string is empty before using it further.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
import org.apache.commons.lang3.StringUtils;



public class NullPointerExceptionExample {

    private static void printLength(String str) {

        if (StringUtils.isNotEmpty(str)) {

            System.out.println(str.length());

        } else {

            System.out.println("This time there is no NullPointerException");

        }

    }



    public static void main(String args[]) {

        String nullString = null;

        printLength(nullString);

    }

}

Using primitives rather than objects where possible, since they cannot have null references e.g. using int instead of Integer and boolean instead of Boolean.

So next time make sure that you double check your code and prevent NullPointerException.