What is a NullPointerException in Java?

A good place to start is the JavaDocs. They have covered the following.

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 was an array.
• Accessing or modifying the slots of null as if it were an array.
• Throwing null as if it was a Throwable value.
• If the value of the expression in synchronized block is null.

Applications should throw instances of this class to indicate other illegal uses of the null object.

Examples

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 to 10 in the second line your value 10 is written into the memory location pointed 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 contain a primitive value. Instead it contains a pointer (because the type is Integer which is a reference type). Since you did not say as yet what to point to Java sets it to null, meaning “I am pointing at 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 this object. You can now reference the object using the dereferencing operator . (a dot).

The Exception occurs when you declare a variable but did not create an object. If you attempt to dereference num BEFORE creating the object you get NullPointerException. In the most trivial cases compiler will catch the problem and let you know that “num may not have been initialized” but sometimes you 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, rather assuming that it was created before the doSomething method was called. Unfortunately, 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
    }
}

How to fix NullPointerException in Java?

So you have a NullPointerException. How do you fix it? Let’s take a simple example which throws a NullPointerException:

public class Printer {
    private String name;

    public void setName(String name) {
        this.name = name;
    }

    public void print() {
        printString(name);
    }

    private void printString(String s) {
        System.out.println(s + " (" + s.length() + ")");
    }

    public static void main(String[] args) {
        Printer printer = new Printer();
        printer.print();
    }
}

Step1 – identify the null values

The first step is identifying exactly which values are causing the exception. For this, we need to do some debugging. It’s important to learn how to read a stack trace. This will show you where the exception was thrown:

Exception in thread "main" java.lang.NullPointerException
    at Printer.printString(Printer.java:13)
    at Printer.print(Printer.java:9)
    at Printer.main(Printer.java:19)

Here we see that the exception is thrown on line 13 (in the printString method). Look at the line and check which values are null by adding logging statements or using a debugger. We find out that s is null and calling the length method on it throws the exception. We can see that the program stops throwing the exception when s.length() is removed from the method.

Step 2 – trace where these values come from

Next check where this value comes from. By following the callers of the method we see that s is passed in with printString(name) in the print() method and this.name is null.

Step 3 – trace where these values should be set

Where is this.name set? In the setName(String) method. With some more debugging we can see that this method isn’t called at all.

This is enough to give us a solution: add a call to printer.setName() before calling printer.print().

Alternative fixes

The variable can have a default value (and setName can prevent it being set to null):

private String name = "";

Either the print or printString method can check for null, for example:

printString((name == null) ? "" : name);

Or you can design the class so that name always has a non-null value:

public class Printer {
    private final String name;

    public Printer(String name) {
        this.name = Objects.requireNonNull(name);
    }

    public void print() {
        printString(name);
    }

    private void printString(String s) {
        System.out.println(s + " (" + s.length() + ")");
    }

    public static void main(String[] args) {
        Printer printer = new Printer("123");
        printer.print();
    }
}