NullPointerException explained

How many times has a NullPointerException in your code caused the code to break? In this blog post, I will be explaining what a NullPointerException is and what you can do to prevent it.

 

What is a NullPointerException?

A NullPointerException is a RuntimeException. It is an unchecked exception and so does not need to be handled by the code. It occurs when you try to manipulate an object that has a null value.

 

When does it occur ?

A NullPointerException can occur in several scenarios.

Scenario 1 – Invoking a method on an object that has a null value.

The most common scenarios where a NullPointerException occurs is when you invoke a method on an object that has a null value. Consider the following code snippet:

 

String str = "Hello World";System.out.println("Length of the String is "+str.length());str = null;System.out.println("Length of the String is "+str.length());

 

A variable called “str” is initialized to the value “Hello World“. Its length is then printed using the String.length method. Then the variable str is set to null . The str.length method is invoked again. When you run this code, it will print the following output:

 

Length of the String is 11Exception in thread "main" java.lang.NullPointerExceptionat learnjava.exceptions.NullPointerExceptionDemo.main(NullPointerExceptionDemo.java:9)

 

This error occurs since the str.length method is invoked after the variable str is initialized to null. So a method is invoked on a null object.

 

Scenario 2 – Accessing fields of an object that has a null value

A NullPointerException  can also occur when you invoke the fields of an object with null value. Consider the following code snippet:

public class Rectangle {int length;int width;}public class RectangleDemo {public static void main(String[] args) {Rectangle r = null;int length = r.length;System.out.println("Length of the Rectangle is "+length);}}

 

In this code, there is a class called Rectangle with fields for length and width. There is another class called RectangleDemo. Here a Rectangle object called r is defined and initialized to null. Then, the length field of the Retangle r is accessed via the dot operator. Since the variable “r” has a null value, a  NullPointerException will occur. When you run this code, it will print the following output:

Exception in thread "main" java.lang.NullPointerExceptionat learnjava.exceptions.nullpointerdemo.RectangleDemo.main(RectangleDemo.java:7)
Scenario 3 – Accessing fields of an array with null value

A NullPointerException  can also occur when you access the fields of an array that has a null value. Consider the following code snippet:

public static void main(String[] args) {int[] arr = null;System.out.println(arr[2]);

In the code above, an array called arr is defined. It is set to null. Then the 3rd element in the array is accessed using arr[2]. Since the array has a null value, this will cause a NullPointerException  exception to occur. When you run this code, it will print the following output:

Exception in thread "main" java.lang.NullPointerExceptionat learnjava.exceptions.nullpointerdemo.NullPointerArrayDemo.main(NullPointerArrayDemo.java:7)

How can you prevent it?

NullPointerException is an unchecked exception and so does not need to be handled by the code. You can prevent your code from breaking due to a  NullPointerException by adding appropriate null checks.

So in Scenario 1 above, the error can be avoided using the following code snippet:

String str = "Hello World";if(str != null)System.out.println("Length of the String is "+str.length());str = null;if(str != null)System.out.println("Length of the String is "+str.length());

 

So now, a null check is added for the str object. If the str object has a null value, the str.length method will not be invoked. When this code is executed, it will print the following output:

Length of the String is 11

 

 

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