NullPointerExceptions 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.
For example,
String s = null;
int len = s.length(); // NullPointerException because s is null
So you should check if the variable is null before calling any method on it, for example:
int len;
if (s == null) {
len = 0;
}
else {
len = s.length(); // safe, s is never null when you get here
}