What is inner classes in OOP?

There are 4 types of inner classes in Java:

In object-oriented programming (OOP), an inner class or nested class is a class declared entirely within the body of another class or interface . It is distinguished from a subclass.

Non-Static Nested Class (Inner Class) A non-static nested class is a class within another class. It has access to members of the enclosing class (outer class). It is commonly known as inner class .