What is a Null Pointer Exception, and how do I fix it?

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 that you asked about occurs when you declare a variable but did not create an object. 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 sometime you write code that does not directly create the object.

For instance you may have a method as follows:

public void doSomething(Integer num){
   //do something to num
}

in which case you are not creating the object num, rather assuming that is was created before the doSomething method was called. Unfortunately it is possible to call the method like this:

doSomething(null);

In which case num is null. The best way to avoid this type of exception is to always check for null when you did not create the object yourself. So doSomething should be re-written as:

public void doSomething(Integer num){
    if(num != null){
       //do something to num
    }
}