Dear D A P Peiris

In object-oriented programming (OOP) with Java, accessing a member variable directly and accessing it through a method can have different consequences, depending on the level of encapsulation and the intended design of the class.

When a member variable is directly accessed, the client code has full control over the value of the variable and can change it directly, which can lead to unintended side effects or violate the intended behavior of the class. This is generally discouraged in OOP, as it can make code more difficult to maintain and can break encapsulation, which is one of the core principles of OOP.

On the other hand, when a member variable is accessed through a method, the method can provide a level of abstraction and control over the value of the variable. For example, a method can validate incoming data before setting the value of a member variable, or it can enforce certain constraints or behaviors that cannot be achieved by direct access. In this way, accessing a member variable through a method can help maintain the integrity of the class's behavior and make the code more maintainable and robust.

In general, it is best practice to use methods to access member variables to maintain encapsulation and control the behavior of the class. This is especially true when the member variable is private, as private member variables should not be directly accessed from outside the class.

create interface of the object class and generate contractor with method calling function to access the private and public members of the instances of the class

At the run time it doesn't have much difference, but if you program something to be reused by others, you don't necessarily want to have the variable changed by the user. You might want the variable to only be processed by a function. If the class is only going to be used by you and you know what you are doing and nobody will judge your code, it doesn't change anything.

From the maintenance perspective it's better to use classic immutable POJOs with getters and setters (or records if you're already on Java 17). It would be hard to find (very time-consuming) while debugging where the change is maid to the mutable field. Other thing: when you hiding fields and provide API through methods you would not break customer logic when you need to change internally field type, etc.

Additional advantage - possibility of overriding the methods through reflection API.

Of course, don't forget to separate logic from the data holder objects - from SOLID and GC perspective.

The main difference between direct access and accessing through a method is that direct access can lead to potential problems with data consistency, and makes it difficult to control the access to the variable from outside the class. By using getter and setter methods, we can ensure that the variable is accessed and modified in a controlled manner, while still maintaining the state of the object. Additionally, using getter and setter methods can also allow for additional functionality to be added, such as input validation, error handling, or logging.