Category Archives: Abstract

Can Abstract class can have Private Methods and Constructors

Posted on by

Yes.You can use any kind of method in an abstract class

public abstract class AbstractDAO{

public void save(){
  validate();
  //save
}
  private void validate(){ // we are hiding this method
  }
}

Private Constructors
If Private constructor is the only constructor of the class, then the reason is clear: to prevent subclassing. Some classes serve only as holders for static fields/methods and do not want to be either instantiated or subclassed. Note that the abstract modifier is in this case redundant—with or without it there would be no instantiation possible. The abstract modifier is also bad practice because it sends wrong signals to the class’s clients. The class should in fact have been final.

Abstract Class with Private Constructor is same as Final Class. Both serves the same purpose.

Can we instantiate abstract class

Posted on by

Is it possible to create Object for abstract class?

abstract class my {
    public void mymethod() {
        System.out.print("Abstract");
    }
}

class poly {
    public static void main(String a[]) {
        my m = new my() {};
        m.mymethod();
        System.out.println(m.getClass().getSuperclass());
    }
}

No, we can’t.The abstract super class is not instantiated by us but by java.

In the above code we are creating object for anonymous class.

my m = new my() {};

When the above code is compiled will result in following class file creation

My.class
Poly$1.class  // Class file corresponding to anonymous subclass
Poly.class

anonymous inner type allows you to create a no-name subclass of the abstract class

When the above code is run the output would be

Output

 Abstract
 class my

Now lets override the method in anonymous inner class like one below.

abstract class my {
    public void mymethod() {
        System.out.print("Abstract");
    }
}

class poly {
    public static void main(String a[]) {
        my m = new my() {
          public void mymethod() {
               System.out.print("Overridden in anonymous class");
           }
        };
        m.mymethod();
        System.out.println(m.getClass().getSuperclass());
    }
}

Output

 Overridden in anonymous class
 class my

Anonymous inner class with same name as abstract class are child classes of abstract class.

Abstract vs Interface

Posted on by

I will give you an example first:

public interface LoginAuth{
   public String encryptPassword(String pass);
   public void checkDBforUser();
}

Now suppose you have 3 databases in your application. Then each and every implementation for that database needs to define the above 2 methods:

public class DBMySQL implements LoginAuth{
          // Needs to implement both methods
}
public class DBOracle implements LoginAuth{
          // Needs to implement both methods
}
public class DBAbc implements LoginAuth{
          // Needs to implement both methods
}

But what if encryptPassword() is not database dependent, and it’s the same for each class? Then the above would not be a good approach.

Instead, consider this approach:

public abstract class LoginAuth{
   public String encryptPassword(String pass){
            // Implement the same default behavior here 
            // that is shared by all subclasses.
   }

   // Each subclass needs to provide their own implementation of this only:
   public abstract void checkDBforUser();
}

Now in each child class, we only need to implement one method – the method that is database dependent.

Technical Differences

  1. Implementing an interface consumes very little CPU, because it’s not a class, just a bunch of names, and therefore there is no expensive look-up to do. It’s great when it matters such as in embedded devices
  2. Abstract classes, unlike interfaces, are classes. They are more expensive to use because there is a look-up to do when you inherit from them.
  3. Abstract classes can have constants, members, method stubs (methods without a body) and defined methods, whereas interfaces can only have constants and methods stubs.
  4. Methods and members of an abstract class can be defined with any visibility, whereas all methods of an interface must be defined as public (they are defined public by default).
  5. When inheriting an abstract class, a concrete child class must define the abstract methods, whereas an an abstract class can extend another abstract class and abstract methods from the parent class don’t have to be defined.
  6. Similarly, an interface extending another interface is not responsible for implementing methods from the parent interface. This is because interfaces cannot define any implementation.
  7. A child class can only extend a single class (abstract or concrete), whereas an interface can extend or a class can implement multiple other interfaces.
  8. A child class can define abstract methods with the same or less restrictive visibility, whereas a class implementing an interface must define the methods with the exact same visibility (public).

Consider using abstract classes if :
You want to share code among several closely related classes.
You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
You want to declare non-static or non-final fields.

Consider using interfaces if :

You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface.
You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.
You want to take advantage of multiple inheritance of type.

abstract class establishes “is a” relation with concrete classes. interface provides “has a” capability for classes.