What is a NullPointerException, and how do I fix i

Question

What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?

What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?

Answer 1

When you declare a reference variable (i.e. an object) you are really creating a pointer to an object. Consider the following code where you declare a variable of primitive type int:

int x;
x = 10;

In this example, the variable x is an int and Java will initialize it to 0 for you. When you assign it the value of 10 on the second line, your value of 10 is written into the memory location referred to by x.

But, when you try to declare a reference type, something different happens. Take the following code:

Integer num;
num = new Integer(10);

The first line declares a variable named num, but it does not actually contain a primitive value yet. Instead, it contains a pointer (because the type is Integer which is a reference type). Since you have not yet said what to point to, Java sets it to null, which means "I am pointing to nothing".

In the second line, the new keyword is used to instantiate (or create) an object of type Integer and the pointer variable num is assigned to that Integer object.

The NullPointerException occurs when you declare a variable but did not create an object. So you are pointing to something that does not actually exist.

If you attempt to dereference num BEFORE creating the object you get a NullPointerException. In the most trivial cases, the compiler will catch the problem and let you know that "num may not have been initialized," but sometimes you may write code that does not directly create the object.

For instance, you may have a method as follows:

public void doSomething(SomeObject obj) {
   //do something to obj
}

In which case, you are not creating the object obj, but rather assuming that it was created before the doSomething() method was called. Note, it is possible to call the method like this:

doSomething(null);

In which case, obj is null. If the method is intended to do something to the passed-in object, it is appropriate to throw the NullPointerException because it's a programmer error and the programmer will need that information for debugging purposes.

Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a null parameter and behave differently. You should also explain this in the documentation. For example, doSomething() could be written as:

/**
  * @param obj An optional foo for ____. May be null, in which case 
  *  the result will be ____.
  */
public void doSomething(SomeObject obj) {
    if(obj != null) {
       //do something
    } else {
       //do something else
    }
}

Finally, How to pinpoint the exception & cause using Stack Trace

Answer 2

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 labelled "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.
• "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.
• 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 that possible?

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

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

we allocate a String[] with two elements that are initialized to null. After that, we have not changed the contents of foo ... so foo[1] will still be null.

Answer 3

A lot of explanations are already present to explain how it happens and how to fix it, but you should also follow best practices to avoid NullPointerExceptions at all.

See also: A good list of best practices

I would add, very important, make a good use of the final modifier. Using the "final" modifier whenever applicable in Java

Summary:

1. Use the final modifier to enforce good initialization.
2. Avoid returning null in methods, for example returning empty collections when applicable.
3. Use annotations @NotNull and @Nullable
4. Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn't be null.
5. Use equals with a known object first: if("knownObject".equals(unknownObject)
6. Prefer valueOf() over toString().
7. Use null safe StringUtils methods StringUtils.isEmpty(null).

Answer 4

A null pointer exception is thrown when an application attempts to use null in a case where an object is required. These include:

1. Calling the instance method of a null object.
2. Accessing or modifying the field of a null object.
3. Taking the length of null as if it were an array.
4. Accessing or modifying the slots of null as if it were an array.
5. 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.

Reference: http://docs.oracle.com/javase/8/docs/api/java/lang/NullPointerException.html

Answer 5

In Java, everything (excluding primitive types) is in the form of a class.

If you want to use any object then you have two phases:

1. Declare
2. Initialization

Example:

• Declaration: Object object;
• Initialization: object = new Object();

Same for the array concept:

• Declaration: Item item[] = new Item[5];
• Initialization: item[0] = new Item();

If you are not giving the initialization section then the NullPointerException arise.

Answer 6

It's like you are trying to access an object which is null. Consider below example:

TypeA objA;

At this time you have just declared this object but not initialized or instantiated. And whenever you try to access any property or method in it, it will throw NullPointerException which makes sense.

See this below example as well:

String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown