NullPointerException

[usman ahmad]

hi!
Could you please explain the java.lang.NullPointerException.
Thanks and Regards
Usman

[Muhammad Ali]

A NullPointerException is thrown when you call a method / variable from a reference that doesn't point anywhere.
See the following example class. In the main() method, we create a reference and an object and then set it to null. The reference e doesn't point anywhere when the method show() is called. Thus, JVM generates a NullPointerException.

public class example {
public void show() {
}

public static void main (String args[]) {
example e = new example();
e = null;
e.show();
}
}
Muhammad Ali Shah
Karachi, Pakistan.

[Cindy Glass]

Per the API:

public class NullPointerException extends RuntimeException
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.

[usman ahmad]

Thanks Both of you!!
Regards,
Usman

[Siva Prasad]

I would call this as Software Bug.
All runtime Exceptions are puerly bugs.
It is possible to avoid this exceptions with proper care(i.e. checking the object before doing something with it.)
If you check the JDK documentation, All the subclasses from RuntimeException, clearly shows this.
Cheers
Siva