Difference Between Interface vs Abstract Class In Java

Difference Between Interface vs Abstract Class In Java: In this article, we will list out the difference between an interface and an abstract class in Java. Let us detail out the difference between interface v/s abstract class in tabular form below,

Read Also: Method vs. Constructor In Java

Interface vs Abstract Class In Java

Abstract class vs. The interface in Java 8

At a very high level, it looks very similar, but actually, they are different in many ways. Also, because the default method in the interface helps us to achieve loose coupling and backward compatibility

Example of Interface and Abstract class

1) Example on Interface:

Variable are implicitly public, static, and final; and methods are implicitly public and abstract

Java7Interface.java

package in.bench.resources.itf.example;
public interface Java7Interface {
    // variables are implicitly public, static and final
    String ORGANIZATION_NAME = "ABC Pvt. Ltd.";
    // methods are implicitly public and abstract
    void display();
}

2) Example on Interface in Java 8

Variable are implicitly public, static, and final, and methods are implicitly public and abstract for methods without implementation. Other than abstract public methods; default and static method with implementation are allowed inside Java 8 interface

InterfaceInJava8.java

package in.bench.resources.itf.example;
 
// only public abstract are permitted
public interface InterfaceInJava8 {
 
    // old abstract method
    void displayAbstractMethod(); // by default, this is public and abstract
 
    // default method with concrete implementation from Java 8
    default void displayDefaultMethod() {
        System.out.println("InterfaceInJava8 : default method impl inside interface");
    }
 
    // static method with concrete implementation from Java 8
    static void displayStaticMethod() {
        System.out.println("InterfaceInJava8 : static method impl inside Java Interface");
    }
 
}

3) Example of an Abstract class and an abstract method

Variables can be both instance and static data members; methods can be abstract or concrete or static. Though constructor is allowed, instantiation of abstract classes are not allowed but can have the main() method and execute as well

AbstractExample.java

package in.bench.resources.abstractclass.example;
 
// abstract class
public abstract class AbstractExample extends DemoAbstract {
 
    String demoString;
    static int demoCounter = 1;
 
    // default no-arg constructor
    AbstractExample(){
        // do some initialization logic here
    }
 
    static void staticMethod() {
        System.out.println("AbstractExample: static methods are allowed inside abstract class");
    }
 
    // abstract method declaration in abstract class
    abstract void myAbstractMethod();
 
    // concrete method definition in abstract class
    void myConcreteMethod() {
        System.out.println("AbstractExample: This is my concrete method in abstract class");
    }
 
    // main() method - the entry point to JVM to begin execution
    public static void main(String args[]) {
 
        System.out.println("Accessing static field demoCounter : " + AbstractExample.demoCounter);
        staticMethod();
    }
}

Interface:

• An interface is 100% pure abstraction, but post-Java 8 release, default, and static methods are allowed
• An interface is a mean by which Java supports multiple inheritances
• The constructor is not allowed inside the interface
• Instantiation of an interface is not allowed
• All variables are implicitly public, static, and final
• Apart from default and static methods, the method with no implementation are implicitly public and abstract (old abstract method)
• An interface can be used as a reference variable for sub-class instantiation

Abstract Class:

• An abstract class is declared with an abstract keyword in a class declaration
• Extending or sub-classing abstract class must provide implementation details to all abstract methods
• Else make extending class as abstract which means next implementing class must provide concrete implementation for all abstract methods
• An abstract class can contain a mix of both, i.e., abstract methods & concrete methods
• It can have both default & parameterized constructor, but still, it cannot be instantiated
• An abstract class with no abstract method is denoted that this class cannot instantiate, rather we can create an object of the extending a class (subclass)
• An abstract method is not allowed inside a concrete class and the compiler throws an error stating “requires method body instead of a semicolon(;)”
• Both static and non-static fields can be declared inside an abstract class
• An abstract class can extend only one class, and it could be either an abstract class or a concrete class
• An abstract can have a main() method – the entry point to JVM to begin execution

Abstract Method:

• An abstract method has nobody.
• Method signature ends with a semicolon(;)
• An abstract method can throw an exception
• Abstract methods can be declared inside the abstract class only
• Abstract methods cannot be final or private or static
• Only protected, default, and public access modifiers allowed