Difference between Abstract Class and Interface in Java
As we know that abstraction refers to hiding the internal implementation of the feature and only showing the functionality to the users. i.e. what it works (showing), how it works (hiding). Both abstract class and interface are used for abstraction, henceforth Interface and Abstract Class are required prerequisites.
Abstract class vs Interface
- Type of methods: Interface can have only abstract methods. An abstract class can have abstract and non-abstract methods. From Java 8, it can have default and static methods also. From Java 9, it can have private concrete methods as well.
- Final Variables: Variables declared in a Java interface are by default final. An abstract class may contain non-final variables.
- Type of variables: Abstract class can have final, non-final, static and non-static variables. The interface has only static and final variables.
- Implementation: Abstract class can provide the implementation of the interface. Interface can’t provide the implementation of an abstract class.
- Inheritance vs Abstraction: A Java interface can be implemented using the keyword “implements” and an abstract class can be extended using the keyword “extends”.
- Multiple implementations: An interface can extend one or more Java interfaces; an abstract class can extend another Java class and implement multiple Java interfaces.
- Multiple Inheritance: Interface supports multiple inheritance; an abstract class does not support multiple inheritance.
- Accessibility of Data Members: Members of a Java interface are public by default. A Java abstract class can have class members like private, protected, etc.
Example 1-A: Abstract Class :
Java
abstract class sunstar { abstract void printInfo(); } class employee extends sunstar { void printInfo() { String name = "avinash" ; int age = 21 ; float salary = 222 .2F; System.out.println(name); System.out.println(age); System.out.println(salary); } } class base { public static void main(String args[]) { sunstar s = new employee(); s.printInfo(); } } |
avinash 21 222.2
example 2:Abstract class
Java
// Java Program to Illustrate Concept of // Abstract Class // Importing required classes import java.io.*; // Class 1 // Helper abstract class abstract class Shape { // Declare fields String objectName = " " ; // Constructor of this class Shape(String name) { this .objectName = name; } // Method // Non-abstract methods // Having as default implementation public void moveTo( int x, int y) { System.out.println( this .objectName + " " + "has been moved to" + " x = " + x + " and y = " + y); } // Method 2 // Abstract methods which will be // implemented by its subclass(es) abstract public double area(); abstract public void draw(); } // Class 2 // Helper class extending Class 1 class Rectangle extends Shape { // Attributes of rectangle int length, width; // Constructor Rectangle( int length, int width, String name) { // Super keyword refers to current instance itself super (name); // this keyword refers to current instance itself this .length = length; this .width = width; } // Method 1 // To draw rectangle @Override public void draw() { System.out.println( "Rectangle has been drawn " ); } // Method 2 // To compute rectangle area @Override public double area() { // Length * Breadth return ( double )(length * width); } } // Class 3 // Helper class extending Class 1 class Circle extends Shape { // Attributes of a Circle double pi = 3.14 ; int radius; // Constructor Circle( int radius, String name) { // Super keyword refers to parent class super (name); // This keyword refers to current instance itself this .radius = radius; } // Method 1 // To draw circle @Override public void draw() { // Print statement System.out.println( "Circle has been drawn " ); } // Method 2 // To compute circle area @Override public double area() { return ( double )((pi * radius * radius)); } } // Class 4 // Main class class GFG { // Main driver method public static void main(String[] args) { // Creating the Object of Rectangle class // and using shape class reference. Shape rect = new Rectangle( 2 , 3 , "Rectangle" ); System.out.println( "Area of rectangle: " + rect.area()); rect.moveTo( 1 , 2 ); System.out.println( " " ); // Creating the Objects of circle class Shape circle = new Circle( 2 , "Circle" ); System.out.println( "Area of circle: " + circle.area()); circle.moveTo( 2 , 4 ); } } |
Area of rectangle: 6.0 Rectangle has been moved to x = 1 and y = 2 Area of circle: 12.56 Circle has been moved to x = 2 and y = 4
What if we don’t have any common code between rectangle and circle then go with the interface.
Interface :
Example 1-:
Java
// Java Program to Illustrate Concept of Interface // Importing I/O classes import java.io.*; // Interface interface Shape { // Abstract method void draw(); double area(); } // Class 1 // Helper class class Rectangle implements Shape { int length, width; // constructor Rectangle( int length, int width) { this .length = length; this .width = width; } @Override public void draw() { System.out.println( "Rectangle has been drawn " ); } @Override public double area() { return ( double )(length * width); } } // Class 2 // Helper class class Circle implements Shape { double pi = 3.14 ; int radius; // constructor Circle( int radius) { this .radius = radius; } @Override public void draw() { System.out.println( "Circle has been drawn " ); } @Override public double area() { return ( double )((pi * radius * radius)); } } // Class 3 // Main class class GFG { // Main driver method public static void main(String[] args) { // Creating the Object of Rectangle class // and using shape interface reference. Shape rect = new Rectangle( 2 , 3 ); System.out.println( "Area of rectangle: " + rect.area()); // Creating the Objects of circle class Shape circle = new Circle( 2 ); System.out.println( "Area of circle: " + circle.area()); } } |
Area of rectangle: 6.0 Area of circle: 12.56
Example 2 :
Java
interface student { void printInfo(); } class avi implements student { public void printInfo() { String name= "avi" ; int age= 23 ; System.out.println(name); System.out.println(age); } } class interfacesss { public static void main (String args[]) { avi s = new avi(); s.printInfo(); } } |
avi 23
When to use what?
Consider using abstract classes if any of these statements apply to your situation:
- In the java application, there are some related classes that need to share some lines of code then you can put these lines of code within the abstract class and this abstract class should be extended by all these related classes.
- You can define the non-static or non-final field(s) in the abstract class so that via a method you can access and modify the state of the object to which they belong.
- You can expect that the classes that extend an abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
Consider using interfaces if any of these statements apply to your situation:
- It is a total abstraction, All methods declared within an interface must be implemented by the class(es) that implements this interface.
- A class can implement more than one interface. It is called multiple inheritances.
- You want to specify the behavior of a particular data type but are not concerned about who implements its behavior.
You can also go for Quiz on this topic.
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