Java – What is a NullPointerException, and how do I fix it?

What is a NullPointerException (NPE), and how do I fix it?

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

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.

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 and assign to the variable before trying to use the contents of the variable (called dereferencing). So you are pointing to something that does not actually exist.

Dereferencing usually happens when using . to access a method or field, or using [ to index an array.

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.

 

Probably the quickest example code I could come up with to illustrate a NullPointerException would be:

public class Example {

    public static void main(String[] args) {
        Object obj = null;
        obj.hashCode();
    }

}

On the first line inside main, it is explicitly setting the Object reference obj equal to null. This means it is a reference, but it isn’t pointing to any object. After that, it try to treat the reference as though it points to an object by calling a method on it. This results in a NullPointerException because there is no code to execute in the location that the reference is pointing.


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