Abstract class and Interface

• what is Abstraction?

Abstraction is a process of hiding the implementation details and showing only functionality to the user.

Another way, it shows only important things to the user and hides the internal details for example sending sms, you just type the text and send the message. You don’t know the internal processing about the message delivery.

Ways to achieve Abstaction

There are two ways to achieve abstraction in java
1.Abstract class
2.Interface

• Abstract Class

A class which contains the abstract keyword in its declaration is known as abstract class.

  • Abstract classes may or may not contain abstract methods, i.e., methods without body ( public void get(); )
  • But, if a class has at least one abstract method, then the class must be declared abstract.
  • If a class is declared abstract, it cannot be instantiated.
  • To use an abstract class, you have to inherit it from another class, provide implementations to the abstract methods in it.
  • If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

    • Example abstract class

     abstract class A{}

Interface in java

An interface in java is a blueprint of a class. It has static constants and abstract methods.

The interface in java is a mechanism to achieve abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve abstraction and multiple inheritance in Java.

Java Interface also represents IS-A relationship.It cannot be instantiated just like abstract class.

An interface is similar to a class in the following ways −

  • An interface can contain any number of methods.
  • An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
  • The byte code of an interface appears in a .class file.
  • Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

An interface is different from a class in several ways, including −

  • You cannot instantiate an interface.
  • An interface does not contain any constructors.
  • All of the methods in an interface are abstract.
  • An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
  • An interface is not extended by a class; it is implemented by a class.
  • An interface can extend multiple interfaces.

Declaring Interfaces

The interface keyword is used to declare an interface. Here is a simple example to declare an interface −

Example

Following is an example of an interface −

/* File name : NameOfInterface.java */
import java.lang.*;
// Any number of import statements

public interface NameOfInterface {
   // Any number of final, static fields
   // Any number of abstract method declarations\
}

Interfaces have the following properties −

  • An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.
  • Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
  • Methods in an interface are implicitly public.

Example

/* File name : Animal.java */
interface Animal {
   public void eat();
   public void travel();
}

• Java 8 Interface :-

java-8-interface-changes-static-method-default-method

Designing interfaces have always been a tough job because if we want to add additional methods in the interfaces, it will require change in all the implementing classes. As interface grows old, the number of classes implementing it might grow to an extent that it’s not possible to extend interfaces. That’s why when designing an application, most of the frameworks provide a base implementation class and then we extend it and override methods that are applicable for our application.

Let’s look into the default interface methods and static interface methods and the reasoning of their introduction in Java 8 interface changes.

Java Interface Default Method

For creating a default method in java interface, we need to use “default” keyword with the method signature. For example,

package com.journaldev.java8.defaultmethod;

public interface Interface1 {

	void method1(String str);
	
	default void log(String str){
		System.out.println("I1 logging::"+str);
	}
}
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