What is a NullPointerException, and how do I fix i

Question

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?

Answer 1

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:

• 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.

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

• Otherwise, if the value of the Expression is null, a 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:

• Avoiding “!= null” statements in Java?

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).

Answer 2

In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.

When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.

Your reference is "pointing" to null, thus "Null -> Pointer".

The object lives in the VM memory space and the only way to access it is using this references. Take this example:

public class Some {
    private int id;
    public int getId(){
        return this.id;
    }
    public setId( int newId ) {
        this.id = newId;
    }
}

And on another place in your code:

Some reference = new Some();    // Point to a new object of type Some()
Some otherReference = null;     // Initiallly this points to NULL

reference.setId( 1 );           // Execute setId method, now private var id is 1

System.out.println( reference.getId() ); // Prints 1 to the console

otherReference = reference      // Now they both point to the only object.

reference = null;               // "reference" now point to null.

// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );

// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...

This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.

Answer 3

A null pointer exception is an indicator that you are using an object without initializing it.

For example, below is a student class which will use it in our code.

public class Student {

    private int id;

    public int getId() {
        return this.id;
    }

    public setId(int newId) {
        this.id = newId;
    }
}

The below code gives you a null pointer exception.

public class School {

    Student obj_Student;

    public School() {
        try {
            obj_Student.getId();
        }
        catch(Exception e) {
            System.out.println("Null Pointer ");
        }
    }
}

Because you are using Obj_Student, but you forgot to initialize it like in the correct code shown below:

public class School {

    Student obj_Student;

    public School() {
        try {
            obj_Student = new Student();
            obj_Student.setId(12);
            obj_Student.getId();
        }
        catch(Exception e) {
            System.out.println("Null Pointer ");
        }
    }
}

Answer 4

It's like you are trying to access an object which is null. Consider below example:

TypeA objA;

At this time you have just declared this object but not initialized or instantiated. And whenever you try to access any property or method in it, it will throw NullPointerException which makes sense.

See this below example as well:

String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown

Answer 5

A lot of explanations are already present to explain how it happens and how to fix it, but you should also follow best practices to avoid NullPointerException at all.

See also: A good list of best practices

I would add, very important, make a good use of the final modifier. Using the "final" modifier whenever applicable in Java

Summary:

1. Use the final modifier to enforce good initialization.
2. Avoid returning null in methods, for example returning empty collections when applicable.
3. Use annotations @NotNull and @Nullable
4. Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn't be null.
5. Use equals with a known object first: if("knownObject".equals(unknownObject)
6. Prefer valueOf() over toString().
7. Use null safe StringUtils methods StringUtils.isEmpty(null).

Answer 6

In Java, everything is in the form of a class.

If you want to use any object then you have two phases:

1. Declare
2. Initialization

Example:

• Declaration: Object a;
• Initialization: a=new Object();

Same for the array concept

• Declaration: Item i[]=new Item[5];
• Initialization: i[0]=new Item();

If you are not giving the initialization section then the NullpointerException arise.