Null pointer problem in Java.

Null pointer is a null reference, Java NULL pointer exception is the reference itself is empty, but called the method, this time there will be a null pointer exception. It is understood that member variables and methods belong to the object (except static), the corresponding member variables and methods exist in the object, and then the member variables and methods are called through the object. For a null pointer, it does not point to any object, there is no so-called member variables and methods, this time using it to invoke some properties and methods, of course, there will be a null pointer exception.

Null pointer problem in Java.