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Difference Between Abstract Class and Interface in Java

What Is Abstract Class in Java

A class is said to be abstract if the word "abstract" appears in the class declaration. Abstract classes in Java are intended to be subclassed by other classes and cannot be instantiated by themselves. The function of an abstract class is to specify a common set of methods and attributes that its subclasses must follow or override. By using abstract classes, concrete class blueprints can be created. The inheriting class must, however, implement the abstract method.

Abstract classes can have both regular methods with implementations and abstract methods that are declared but have no implementation in the abstract class itself. Subclasses of an abstract class are required to provide concrete implementations for these abstract methods. Typically, a hierarchy of related classes with shared behaviours or attributes is created using abstract classes.

Table Of Content

Definition
Comparison Chart
Key Differences

Key Benefits of Using Abstract Classes

For the subclasses, abstract classes provide default functionality.
abstract classes designs a template for upcoming targeted classes
Abstract classes assists you in defining a common interface for its subclasses.
The abstract class enables code reuse.

Abstract Class example

 // Abstract Class  example

abstract class nano{
	abstract void Speed();
	  public void Weight() {
		   System.out.println("250 Kg");
	  }
}
class Vehicles extends nano{    
	@Override
	 void Speed() {
		System.out.println("90 kmph");	
	 }  
}
class Main{
	public static void main(String[] args) {
		Vehicles vehicles=new Vehicles();
		vehicles.Speed();
		vehicles.Weight();
	}
}

Output:

90 kmph

250 Kg

Abstract Method in Java

An abstract method is one that is declared with abstract keyword but does not have implementation.

Syntax

 abstract void Exam();  // does not have implementation

What is Interface in java

In Java, an interface is a reference type that details a collection of abstract methods or a contract that a class must adhere to. Without specifying how those methods should be implemented, it lists the methods that a class must offer. In other words, an interface specifies what a class that implements it should do, but not how.

Using interfaces to achieve abstraction, you can distinguish between how a behaviour is defined and how it is actually implemented. You can write more flexible and maintainable code by clearly separating the implementation from the interface (what should be done and how it should be done).

Key Benefits of Using Interface

Abstraction is accomplished using interfaces.
Designed to support run-time dynamic method resolution
You can achieve loose coupling with the use of an interface.
Interface enables you to distinguish between how a method is defined and its inheritance structure.

Java Interface example

 // Java method overridden example

interface Displayable {
	void display();
}
class Test implements Displayable {
	public void display(){
	System.out.println("Hello");
	}
}
class Main{
	public static void main(String[] args){
		Test test = new Test();
		test.display();
	}
}

Output:

Hello

Comparison Chart

Difference Between Interface and Abstract class are mention below

S.N	Aspect	Interface	Abstract class
1.	Methods	Interface can have only abstract methods.	An abstract class can have abstract and non-abstract methods. Java 8 allows for the addition of default and static methods. It can also have private concrete methods as of Java 9.
2.	Variables	By default, variables declared in a Java interface are final,public and static.	Abstract class can have only Non-final variables.
3.	Access	The interface's definitions are all assumed to be public by default.	A Java abstract class may have class members such as private, protected, and so on.
4.	Implementation	Interface use "implements" keyword for implementation	The keyword "extends" can be used to extend an abstract class.
5.	Inheritance	Interface supports multiple inheritance	Multiple inheritance is not supported by the abstract class.
6.	Code	Since an interface is abstract, it does not provide any code implementation	An abstract class can provide Default code with optional overrides

Interface: Embracing Pure Abstraction

Methods: Interfaces consist solely of abstract methods, enforcing a contract.
Variables: Interface variables are final, public, and static by default.
Access: All components are inherently public.
Implementation: Implemented using the "implements" keyword.
Inheritance: Supports multiple inheritance.
Coding Possibilities: No concrete code provided, meant for implementation.

Abstract Class: Abstraction

Methods: Combines abstract and concrete methods.
Variables: Non-final variables can be utilized.
Access: Provides flexibility in access modifiers.
Implementation: Extended using the "extends" keyword.
Inheritance: Allows single inheritance.
Coding Possibilities: Provides default concrete methods.

The Most Significant Differences Between Abstract class and Interface

A class can implement multiple interfaces in Interface, but it can only inherit one Abstract Class.
No Access modifiers are within an Interface. Every element within an Interface is inherently regarded as having a public modifier. On the other hand, an Abstract Class retains the possibility of possessing an access modifier.
Data fields are not permitted in the Interface, but they are permitted in the abstract class.
Overloading achieves early binding by resolving which overloaded function will be run during build time. Overriding achieves late binding by resolving which overridden function will be invoked during runtime.

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