Difference between Abstract Class and Interface in java

Some of the popular interview questions are “What are the differences between abstract class and interface”, “When will you use abstract class and when will you use interface”. So in this article, we will go through this topic.

Before going through differences between them, let’s go through their introduction.

Abstract class

Abstract classes are created to capture common characteristics of subclasses. It can not be instantiated, it can be only used as super class by its subclasses. Abstract classes are used to create template  for its sub classes down the hierarchy.

Lets take example of JDK class GenericServlet:

public abstract class GenericServlet implements  Servlet, ServletConfig,Serializable{    
// abstract method     
    abstract  void     service(ServletRequest req, ServletResponse res) ; 
    void init()     {
         // Its implementation        
     }  
       // other method related to Servlet    
}

When HttpServlet extends Generic servlet, it provides implementation of service() method:

public class HttpServlet extends GenericServlet
{
       void  service(ServletRequest req, ServletResponse res)
       {
       // implementation 
}
     protected  void  doGet(HttpServletRequest req, HttpServletResponse resp) 
{
    // Implementation 
}

   protected  void  doPost(HttpServletRequest req, HttpServletResponse resp) 
{
    // Implementation 
}

// some other methods related to HttpServlet
}

Interface

An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface. So it is kind of signing a contract, you agree that if you implement this interface, then you have to use its methods. It is just a pattern, it can not do anything itself.

Lets take example of Externalizable Interface:

public interface Externalizable
extends Serializable
{

    void writeExternal(ObjectOutput out)
            throws IOException;
    void readExternal(ObjectInput in)
            throws IOException,
            ClassNotFoundException;
}

When you implement this interface, you have to implement the above two methods:

public class Employee implements Externalizable{ 

 int employeeId;
 String employeeName;


 @Override
 public void readExternal(ObjectInput in) throws IOException,
 ClassNotFoundException {  employeeId=in.readInt();
  employeeName=(String) in.readObject();
  
 } @Override
 public void writeExternal(ObjectOutput out) throws IOException {

  out.writeInt(employeeId);
  out.writeObject(employeeName); }
}

Abstract class vs Interface

When to use Abstract class and interface:

• If you have a lot of methods and want default implementation for some of them, then go with abstract class
• If you want to implement multiple inheritance then you have to use interface. As java does not support multiple inheritance, subclass can not extend more than one class but you can implement multiple interface so you can use interface for that.
• If your base contract keeps on changing, then you should use abstract class, as if you keep changing your base contract and use interface, then you have to change all the classes which implements that interface.

Introduction of default and static methods in java 8

Oracle has tried to bridge gap between abstract class and interface by introducing concept of default and static methods in interface. So now we can provide default implementation of a method in interface and will not enforce class to implement it. I will cover this topic in my next post.

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