What is a NullPointerException, and how do I fix it?
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?
When you declare a reference variable (ie 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 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
}
}
Finally, How to pinpoint the exception & cause using Stack Trace
NullPointerException s are exceptions that occur when you try to use a reference that points to no location in memory (null) as though it were referencing an object. Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException . These are the most common, but other ways are listed on the NullPointerException javadoc page.
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 , I'm explicitly setting the Object reference obj equal to null . This means I have a reference, but it isn't pointing to any object. After that, I 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.
(This is a technicality, but I think it bears mentioning: A reference that points to null isn't the same as a C pointer that points to an invalid memory location. A null pointer is literally not pointing anywhere, which is subtly different than pointing to a location that happens to be invalid.)
What is a NullPointerException?
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:
Applications should throw instances of this class to indicate other illegal uses of the null object.
It is also the case that if you attempt to use a null reference with synchronized , that will also throw this exception, per the JLS:
SynchronizedStatement:
synchronized ( Expression ) Block
NullPointerException is thrown. How do I fix it?
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();
}
}
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 to read a stacktrace. 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.
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.
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. If the method was called, make sure to check the order that these methods are called, and the set method isn't called after the print method.
This is enough to give us a solution: add a call to printer.setName() before calling printer.print() .
Other 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();
}
}
See also:
I still can't find the problem
If you tried to debug the problem and still don't have a solution, you can post a question for more help, but make sure to include what you've tried so far. At a minimum, include the stacktrace in the question, and mark the important line numbers in the code. Also, try simplifying the code first (see SSCCE).
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