how to have reference to another class inside the inner java code example

Example 1: inner class vs outer class

1. Can Outer class be static? --> No
2. Can inner class be static? --> Yes
3. Can you create an object from the inner class? --> Only if it's static
4. Can we have main method in inner class ? --> Only if it's static
5. Inner class can only be extended if it's static.

Example 2: use of nested class in java

In Java, it is possible to define a class within another class, such 
classes are known as nested classes. They enable you to logically group 
classes that are only used in one place, thus this increases the use of 
encapsulation, and creates more readable and maintainable code.