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

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18    5 months ago

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?

Answers { 1 }
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5 months ago

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
    }
}