Question: What causes a NullPointerException (NPE)?

As you should know, Java types are divided into primitive types ( boolean , int , etc.) and reference types. Reference types in Java allow you to use the special value null which is the Java way of saying "no object".

A NullPointerException is thrown at runtime whenever your program attempts to use a null as if it was a real reference. For example, if you write this:

public class Test { public static void main(String[] args) { String foo = null; int length = foo.length(); // HERE } }

the statement labeled "HERE" is going to attempt to run the length() method on a null reference, and this will throw a NullPointerException .

There are many ways that you could use a null value that will result in a NullPointerException . In fact, the only things that you can do with a null without causing an NPE are:

assign it to a reference variable or read it from a reference variable,

assign it to an array element or read it from an array element (provided that array reference itself is non-null!),

pass it as a parameter or return it as a result, or

test it using the == or != operators, or instanceof .

Question: How do I read the NPE stacktrace?

Suppose that I compile and run the program above:

$ javac Test.java $ java Test Exception in thread "main" java.lang.NullPointerException at Test.main(Test.java:4) $

First observation: the compilation succeeds! The problem in the program is NOT a compilation error. It is a runtime error. (Some IDEs may warn your program will always throw an exception ... but the standard javac compiler doesn't.)

Second observation: when I run the program, it outputs two lines of "gobbledy-gook". WRONG!! That's not gobbledy-gook. It is a stacktrace ... and it provides vital information that will help you track down the error in your code if you take the time to read it carefully.

So let's look at what it says:

Exception in thread "main" java.lang.NullPointerException

The first line of the stack trace tells you a number of things:

It tells you the name of the Java thread in which the exception was thrown. For a simple program with one thread (like this one), it will be "main". Let's move on ...

It tells you the full name of the exception that was thrown; i.e. java.lang.NullPointerException .

. If the exception has an associated error message, that will be output after the exception name. NullPointerException is unusual in this respect, because it rarely has an error message.

The second line is the most important one in diagnosing an NPE.

at Test.main(Test.java:4)

This tells us a number of things:

"at Test.main" says that we were in the main method of the Test class.

method of the class. "Test.java:4" gives the source filename of the class, AND it tells us that the statement where this occurred is in line 4 of the file.

If you count the lines in the file above, line 4 is the one that I labeled with the "HERE" comment.

Note that in a more complicated example, there will be lots of lines in the NPE stack trace. But you can be sure that the second line (the first "at" line) will tell you where the NPE was thrown1.

In short, the stack trace will tell us unambiguously which statement of the program has thrown the NPE.

1 - Not quite true. There are things called nested exceptions...

Question: How do I track down the cause of the NPE exception in my code?

This is the hard part. The short answer is to apply logical inference to the evidence provided by the stack trace, the source code, and the relevant API documentation.

Let's illustrate with the simple example (above) first. We start by looking at the line that the stack trace has told us is where the NPE happened:

int length = foo.length(); // HERE

How can that throw an NPE?

In fact, there is only one way: it can only happen if foo has the value null . We then try to run the length() method on null and... BANG!

But (I hear you say) what if the NPE was thrown inside the length() method call?

Well, if that happened, the stack trace would look different. The first "at" line would say that the exception was thrown in some line in the java.lang.String class and line 4 of Test.java would be the second "at" line.

So where did that null come from? In this case, it is obvious, and it is obvious what we need to do to fix it. (Assign a non-null value to foo .)

OK, so let's try a slightly more tricky example. This will require some logical deduction.

public class Test { private static String[] foo = new String[2]; private static int test(String[] bar, int pos) { return bar[pos].length(); } public static void main(String[] args) { int length = test(foo, 1); } } $ javac Test.java $ java Test Exception in thread "main" java.lang.NullPointerException at Test.test(Test.java:6) at Test.main(Test.java:10) $

So now we have two "at" lines. The first one is for this line:

return args[pos].length();

and the second one is for this line:

int length = test(foo, 1);

Looking at the first line, how could that throw an NPE? There are two ways:

If the value of bar is null then bar[pos] will throw an NPE.

is then will throw an NPE. If the value of bar[pos] is null then calling length() on it will throw an NPE.

Next, we need to figure out which of those scenarios explains what is actually happening. We will start by exploring the first one:

Where does bar come from? It is a parameter to the test method call, and if we look at how test was called, we can see that it comes from the foo static variable. In addition, we can see clearly that we initialized foo to a non-null value. That is sufficient to tentatively dismiss this explanation. (In theory, something else could change foo to null ... but that is not happening here.)

So what about our second scenario? Well, we can see that pos is 1 , so that means that foo[1] must be null . Is this possible?

Indeed it is! And that is the problem. When we initialize like this:

private static String[] foo = new String[2];