Interface vs Abstract Class, Abstract method

Interface

• Description

  • Used when common functionality is needed without being bound by inheritance relationships between parent and child.

    • Abstract Methods are defined,

    • and the implementing classes Override each function to implement it in various forms, so it is related to polymorphism

  • An Interface exists to enforce the same methods and behavior on the classes that implement it

• Advantages

  • It can compensate for the limitations of Abstract Classes caused by Java's inability to support multiple inheritance

• Disadvantages

  • If all classes use an Interface, there is the inconvenience of having to Override and redefine commonly needed functionality in all implementing classes

• Summary

  • Interfaces are convenient to use when specifying common functionality for classes that inherit from different Abstract Classes

Abstract method

• Description

  • It refers to a method that must be overridden in a child class

  • A method is divided into a declaration part () and an implementation part {}, and an abstract method is one where only the declaration is written without the implementation

  • Abstract methods are used because the method content varies depending on which class inherits it

  • The implementation part {} must be written in the sub-class that inherits the class

    • If the implementation is not written, an error occurs!

  • Comments should describe what functionality the method performs

  • abstract is not written on overriding methods

Abstract Class

• Description

  • A class that has even one Abstract method is called an Abstract class

    • That is, a regular class cannot have abstract methods!

  • A child class that inherits an Abstract class must override the abstract methods

    • The abstract class forces redefinition of abstract methods on child classes

    • abstract class <-> final class

      • It is the opposite of a final class which prohibits inheritance to restrict overriding!

  • Abstract classes are intended for program design

    • On the other hand, regular classes are intended for creating multiple objects and storing data

  • Abstract classes cannot be created as objects

    • The child class must inherit the abstract class, override the abstract methods to complete the implementation, and then use the child class to create objects

  • In a parent-child relationship, when inheriting (extends) the Abstract Class, among child classes that inherit the same parent Class (here, the Abstract Class):

    • Each implements common functionality,

    • extends it,

    • and it is related to inheritance

      • Inheritance is used to utilize and extend the functionality of the SuperClass!

  • Inheriting Abstract Classes enables distinction between classes

• Advantages

  • Once design is completed in the abstract class, it is convenient to extend functionality by inheriting in child classes

  • Since implementation of abstract methods is enforced in child classes, the degree of standardization is increased

  • Since common aspects of classes can be managed in one place, development and maintenance become convenient

• Disadvantages

  • Since Java does not support multiple inheritance, there are limitations in enforcing Abstract Methods that must be implemented using only Abstract Classes

  • What if multiple inheritance were possible?

    class Vehicle extends Car, Motorcycle {
        @Override
        public void run(){
            super.drive();
        }
    }

    • If a drive() method is defined in both Car and Motorcycle, it becomes ambiguous which one is being inherited and Overridden

    • This is the ambiguity of multiple inheritance, and this is why Java has blocked multiple inheritance

Abstract Class vs Interface

• Abstract Class represents IS - A "is a",

  • while Interface represents HAS - A "is capable of"

• Abstract Class is about inheritance,

  • while Interface is about polymorphism