What is difference between Abstract and Interface, give me some example of your project in which you have used Abstract class and Interface.

Question

Accepted Answer

Abstract class and Interface are both used for abstraction, but with some differences.

Abstract class can have both abstract and non-abstract methods, while Interface can only have abstract methods.
A class can implement multiple interfaces, but can only inherit from one abstract class.
Abstract class can have instance variables, while Interface cannot.
Abstract class provides partial implementation, while Interface provides full abstraction.
Example of using Abstract class: Creating a base class for different types of vehicles with common properties and methods.
Example of using Interface: Implementing a Comparable interface to enable sorting of objects based on a specific criteria.

Accepted Answer

There are many differences in an Abstract class and an Interface like a class can extend only one Abstract service class but multiple interfaces can be implemented the same class. But the most important difference I think was you can write concrete methods in Abstract class and not in interface. Though, this difference has been diluted by introduction of default keyword in Java 8.

In our code we have one abstract class with a default implementation of basic database operations we need to perform on an entity for example save. We set some generalised but mandatory information like created time, updated time, created by, updated by and session id before saving entity to the database.

Every entity service class is extended with this abstract class. We use the existing implementation in abstract class to save the entity generally. But, when we want some other operations to execute on entity we override the existing method. Execute the operations and then pass entity to parent save method using super keyword.

What is difference between Abstract and Interface, give me some example of your project in which you have used Abstract class and Interface.

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