Java Exception Handling

Java Exception Handling – NullPointerException

Today we start the journey through our Java Exception Handling series with a deep dive into the java.lang.NullPointerException. A java.lang.NullPointerException is thrown there’s an attempt to use null anywhere an object is actually required, such as trying to directly modify a null object.

In this article we’ll look at where the java.lang.NullPointerException sits within the Java Exception Hierarchy, plus, we’ll also explore some functional code samples that illustrate how these exceptions might be raised during your own development adventures, so let’s get crackin’!

The Technical Rundown

When Should You Use It?

As we saw in the hierarchical breakdown, the java.lang.NullPointerException inherits from java.lang.RuntimeException, so quite clearly this is an issue that will pop up during execution of an application. Specifically, virtually anytime an attempt is made to directly access a field, method, or the like of a null object a java.lang.NullPointerException is thrown.

To illustrate we’ll start with the full code example below, after which we’ll walk through it step-by-step to see what’s going on:

Probably the most typical case where a java.lang.NullPointerException could be thrown on accident is when calling an instance method or field of an object that has (unknowingly) been set to null. Therefore, we begin with the callInstanceMethodOfNull() method, which creates a new age variable, sets it to null, and then attempts to call the age.toString() method:

As you can probably guess, executing the callInstanceMethodOfNull() method results in a java.lang.NullPointerException coming our way, as we see in the log output:

The obvious fix in the case above would be to confirm that a mutable object, like age, is not null prior to calling a method on it:

We also have defined a simple Book class as part of our example, which we’ll use within the callInstanceFieldOfNull() method:

As you can see, we’re just using Book here to illustrate the creation of a custom class instance object. We then set that new book object to null, then try to access a the field in a log output. As suspected, this also throws a java.lang.NullPointerException at us:

Again, the simple solution is just to check that our book object isn’t null prior to making the field call. In some cases it might be ideal to directly throw a different exception in the event that an object your code is using is null when it shouldn’t be:

Obviously, in the case above we’re just moving from one type of exception to the other, but this is often a good practice if the circumstances are appropriate. Executing this new method shows our new (unexpected) exception was caught by our catch (Throwable exception) block, which is generally a bad practice but helps to illustrate this particular scenario:

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