java.lang.NullPointerException

A `NullPointerException` is a type of `RuntimeException` that occurs when a program attempts to use a reference variable that has a null value. This means that the program is trying to access a method or property of an object that does not exist.

For example, the following code would result in a `NullPointerException`:

String s = null;
int length = s.length();

In this case, the variable `s` has a null value, so when the `length()` method is called, a `NullPointerException` is thrown because the program is trying to access a method of an object that does not exist.

To avoid `NullPointerExceptions`, it is important to check for null values before using reference variables. This can be done using conditional statements, such as `if` statements, to ensure that the program only attempts to use reference variables that have valid values.