POJO Class
POJO stands for “Plain Old Java Object”. A class which contains only private variables and setter and getter methods to use those variables is called POJO class
Static Class
In Java, static is a keyword used to describe how objects are managed within the memory. A static object belongs specifically to the class, instead of instances of that class. The sole purpose of the class is to provide blueprints of its inherited classes. A static class can contain static members only. You cannot create an object for a static class.
Inner class
Inner class means the class which is a member of another class. There are four types of inner classes in java.
1) Nested Inner class
2) Method Local inner classes
3) Anonymous inner classes
4) Static nested classes
Concrete Class
Any normal class which does not have any abstract method or a class having an implementation for all of its methods is basically a concrete class. They cannot have any unimplemented methods. A concrete class can extend its parent class, an abstract class or implement an interface if it implements all their methods. It is a complete class that can be instantiated.
Abstract Class
An abstract class is declared with an abstract keyword and have zero or more abstract methods. These classes are incomplete classes, therefore, to use an abstract class we strictly need to extend the abstract classes to a concrete class. It can have constructors and static methods as well. It can have final methods which will force the subclass to keep the body of the method unhung.
Final Class
Once a variable, method or a class is declared as final, it’s value remains the same throughout. The final keyword in a method declaration indicates that the method cannot be overridden by any subclasses i.e., a class that has been declared final cannot be subclassed. This helps a lot while creating an immutable class like the String class. A class cannot make a class immutable without making it final
