Very common question asked by people:
“I am getting NullPointerException in automation script. I am correctly using selenium concepts but unable to resolve it.”
So, in this post, we will see what is reason behind above exception and how to solve it.
What is NullPointerException?
When we declare a reference variable, internally a pointer is created to reference variable.
Suppose Demo is a class and we create an object of class Demo as below:
Demo d= new Demo();
See the below image:
Now consider below lines of code:
Above, I just declared a reference variable/object of Class Demo. I am not initialing it. When a reference variable is not initialized, it points to NULL. It means reference variable is pointing to nothing.
And when we want to perform any action on such reference variable, we get NullPointerException.
We will see an example below:
We perfectly declared and initialized np1 and called print method on it. It printed output successfully. And we just declared np2 and did not initialized it. So np2 is pointing to null. When we call print method on it, we got NullPointerException.
As per JAVADOC:
NullPointerException is thrown when an application attempts to use null in a case where an object is required. These include:
- Calling the instance method of a null object.
- Accessing or modifying the field of a null object.
- Taking the length of null as if it were an array.
- Accessing or modifying the slots of null as if it were an array.
- Throwing null as if it were a Throwable value.
Applications should throw instances of this class to indicate other illegal uses of the null object.
I can list out some more reasons of NullPointerException from here:
1) Invoking methods on an object which is not initialized
2) Parameters passed in a method are null
3) Calling toString() method on object which is null
4) Comparing object properties in if block without checking null equality
5) Incorrect configuration for frameworks like spring which works on dependency injection
6) Using synchronized on an object which is null
7) Chained statements i.e. multiple method calls in a single statement
So , NullPointerException is not selenium/Java problem. It is programmer’s fault. It is because of wrong programming practice.
How to avoid NullPointerException ?
- NullPointerException is an unchecked exception which extends RuntimeException. Since it is unchecked exception, it can not be caught by java compiler during compilation.
- So it is programmer responsibility to avoid this exception.
- To avoid the NullPointerException, we must ensure that all the objects are initialized properly, before you use them. When we declare a reference variable, we must verify that object is not null, before we request a method or a field from the objects.
- To check if any reference variable is null or not, we can use isNull() method of Objects class. This method returns
trueif the provided reference is
- You can learn about more ways of avoiding NullPointerException here.
What to do if NullPointerException occurs?
You have taken so many precautions to avoid NullPointerException but still you got sometimes. Don’t worry. You can easily track reference variable which is reason behind exception.
Using stack trace printed in console:
Eclipse is a very smart IDE and it gives you line number where NullPointerException occurred. You can just backtrack from there.
This is the best way to find culprit reference variable. You just need to run program in debugging mode with debugging toggle points and you can check how variables are declared/initialized/modified.
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