About Java : Abstract Class vs Interface in Java Programming

Abstract and Interface:-

In Java technology, a java class can only extend another one class, but can implements one or more interfaces.

Example of Abstract and Interface Program:-

 abstract class Animal{

   public void type(){
     System.out.println(”Animal”);
   }

   public abstract void name();

 }

 interface Cat{
    public abstract void jump();
    void run(); // it will be automaticly public and abstract
 }

 interface Bird{
    void fly();
 }

 public class Tiger extends Animal implements Cat , Bird{

    public void fly(){
       System.out.println(”Tiger can’t fly”);
    }

    public void jump(){
       System.out.println(”Tiger can jump”);
    }

    public void run(){
       System.out.println(”Tiger can run”);
    }

    public void name(){
       System.out.println(”Tiger”);
 }

 public static  void main(String[]args){
     Tiger t = new Tiger();
     t.name();
     t.jump();
     t.run();
     t.fly();
   }
 }

A Excellent example of Abstract class vs Interface:-

One of the most frequent question in an interview for a Junior/Graduate Developer role is ‘What is the difference between Abstract class and Interface?’. I have to admit that (as a Graduate) I thought this was not a good question to gauge my skill as a developer. I thought my intrepidness or whatever that skill I thought I had was far more important than being able to answer the difference between abstract and interface.

A couple of promotions later, I am sitting at the other end of the table. Having the responsibility of asking that same question, I can start to see to why we need to know the answer. To know the difference between the two, a developer must think about abstraction and encapsulation; the two paradigm that Object Oriented Programming heavily relies on in modelling reality. Without inheritance and interfaces, we are stuck with complex trees of conditions, iterations and recursions that is probably duplicated again and again to describe a similar characteristic between two entities.

This post will discuss the difference between abstract and interface, along with an (awesome!!!) example – better than you’ve seen elsewhere.

Abstract Class:-

1. Cannot be instantiated.

2. Is a special type of class in which you can have members without implementation.

3. As we know in C#/VB/Java, a class can only inherit from 1 class. This also applies for Abstract Class.

4. Normally used for framework-type library classes: providing default behavior for some of its class members, but forcing the developer to implement others.

5. Is believed to be faster in Java, HOWEVER I cannot find the same claim in .Net. The speed difference is *probably* negligible and only relevant to the most academic field.

6. The aim: making sure something is *eventually* implemented.

7. A class can inherit an abstract class without implementing all its abstract method.

However only a class that has all its method implemented can be instantiated to an object.

1. IS-A relationship.

e.g. Student IS A Person, Employee IS A Person.

// 'framework library' for a person
// a person can enrol and submit
// however, the class that consume this framework library
// need to provide 'where' the paperwork need to be sent

public abstract Person{
    public abstract SendPaperWork(string paperwork)

    public void Enrol(){
        SendPaperWork("enrolment");
    }
    public void Submit(){
        SendPaperWork("report");
    }
}

// by inheriting Person abstract class
// we are enabling student to enrol and submit
// however, SendPaperWork need to be implemented
// because we need to tell it explicitly 'where'
// to send the enrolment/ submission

public class Student: Person{
    public override SendPaperWork(string paperwork){
        School.Send(paperwork);
    }
}

// an employee send the paperwork to a different 'place' than student

public class Employee : Person{
    public override SendPaperWork(string paperwork){
        Company.Send(paperwork);
    }
}

Interface:-

1. cannot be instantiated.

2. is a special type of abstract class in which all the members do not have any implementations.

3. enables polymorphism. A class can implement more than 1 Interfaces.

4. normally used for application classes: providing contract for ensuring interactibility.

5. the aim: making sure something is interchangeable.

6. A class that implements an Interface need to contain all the implementation, otherwise the compiler will throw an error.

7. CAN-DO relationship.

e.g. Student CAN enrol, Student CAN submit assignment.

public interface ICanEnrol{
    void Enrol();
}

public interface ICanSubmit{
    void Submit();
}

public class Student : ICanEnrol, ICanSubmit{
    public void Enrol(){
        School.Send("enrolment");
    }

    public void Submit(){
        School.Send("report");
    }
}

public class Employee : ICanEnrol, ICanSubmit{
    public void Enrol()
    {
        Company.Send("enrolment");
    }

    public void Submit(){
        Company.Send("report");
    }
}

public class MailServer{
    public void SendAllSubmissions(){

        // AllSubmitters is a collection of students and employees

        foreach (ICanSubmit submitter in AllSubmitters){

            // The MailServer does not care if
            // the submitter is a student
            // or an employee, as long as it can submit

            submitter.Submit()
        }
    }
}

With the rise of Aspect Oriented Programming and Domain Specific Language, we are starting to realise that Object Oriented Programming is not even rich enough (or too rich?) to model reality. It is difficult to handle cross cutting concerns using just inheritance and polymorphism. So it’s important to master these skills, because the answer is often not obvious. (Unfortunately) It took me a year after Uni (after I got my first job) to feel like I finally grasp these basic object oriented concept.

References:-

Geeks with blogs. Abstract Class vs Interface
Interfaces vs Abstract Class : Java Glossary

Declaring Interface Program in Java | Interface in Java

Declaring Interface:

In Java interfaces are declaring only the contract and no implementation, like a 100% abstract superclass. All methods declared in an interface are public and abstract (do not need to actually type the public and abstract modifiers in the method declaration, but the method is still always public and abstract). Interface methods must not be static. Because interface methods are abstract, they cannot be marked final, strictfp, or native. All variables in an interface are public, static, and final in interfaces. An interface can extend one or more other interfaces. An interface cannot implement another interface or class.

Declaring Interface Example:-

 interface Bounceable{
    public abstract void bounce();
    void setBounce(int b);
 }

 public class Tire implements Bounceable{
    public void bounce(){
      System.out.println(”Love”);
    }

    public void setBounce(int b){
      int a = b;
      System.out.println(a);
    }

    public static void main(String[]arge){
       Tire t = new Tire();
       t.bounce();
       t.setBounce(15);
   }
 }


Definition of Declaring Interfaces:

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

Example:-

Let us look at an example that depicts encapsulation:

/* 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 when 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();
}


Implementing Interfaces:

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

Aclass uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

/* File name : MammalInt.java */
public class MammalInt implements Animal{

   public void eat(){
      System.out.println("Mammal eats");
   }

   public void travel(){
      System.out.println("Mammal travels");
   }

   public int noOfLegs(){
      return 0;
   }

   public static void main(String args[]){
      MammalInt m = new MammalInt();
      m.eat();
      m.travel();
   }
}

This would produce following result:

Mammal eats
Mammal travels

When overriding methods defined in interfaces there are several rules to be followed:

Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:

    # A class can implement more than one interface at a time.

    # A class can extend only one class, but implement many interface.

    # An interface itself can extend another interface. An interface cannot extend another interface.

Extending Interfaces:

An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

//Filename: Sports.java
public interface Sports
{
   public void setHomeTeam(String name);
   public void setVisitingTeam(String name);
}

//Filename: Football.java
public interface Football extends Sports
{
   public void homeTeamScored(int points);
   public void visitingTeamScored(int points);
   public void endOfQuarter(int quarter);
}

//Filename: Hockey.java
public interface Hockey extends Sports
{
   public void homeGoalScored();
   public void visitingGoalScored();
   public void endOfPeriod(int period);
   public void overtimePeriod(int ot);
}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Extending Multiple Interfaces:

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as:

public interface Hockey extends Sports, Event

Tagging Interfaces:

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as:

package java.util;
public interface EventListener
{}

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces:

Creates a common parent: As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.

Adds a data type to a class: This situation is where the term tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.

Java Interface Example:-

    /*
    Java Interface example.
    This Java Interface example describes how interface is defined and
    being used in Java language.
    
    Syntax of defining java interface is,
    <modifier> interface <interface-name>{
      //members and methods()
    }
    */
    
    //declare an interface
    interface IntExample{
    
      /*
      Syntax to declare method in java interface is,
      <modifier> <return-type> methodName(<optional-parameters>);
      IMPORTANT : Methods declared in the interface are implicitly public and abstract.
      */
    
      public void sayHello();
      }
    }

    /*
    Classes are extended while interfaces are implemented.
    To implement an interface use implements keyword.
    IMPORTANT : A class can extend only one other class, while it
    can implement n number of interfaces.
    */
    
    public class JavaInterfaceExample implements IntExample{
      /*
      We have to define the method declared in implemented interface,
      or else we have to declare the implementing class as abstract class.
      */
    
      public void sayHello(){
        System.out.println("Hello Visitor !");
      }
    
      public static void main(String args[]){
        //create object of the class
        JavaInterfaceExample javaInterfaceExample = new JavaInterfaceExample();
        //invoke sayHello(), declared in IntExample interface.
        javaInterfaceExample.sayHello();
      }
    }
    
    /*
    OUTPUT of the above given Java Interface example would be :
    Hello Visitor !
    */