Question

what is abstract class in java?

for example, abstract class animal and subclass cat which extends abstract class, and the test class which will call cat class to show if it yawns, scratch and whatnot.

I used to create class animal but it was not abstract, so I am not sure the purpose of it.

Another question, I created interface, by right clicking the package, I think the teacher saying interface is acts independent? and not part of hierarchy like super class and subclass, but still wants to apply (implment) inside subclass so it can be called from Test class, something like that. What is the interface exactly?

simple explanation works the best! thnx in advance

Answer 1

Abstract Class:

An abstract class is just a simple class in java, except the following:

• an abstract class cannot be instantiated, i.e., object of an abstract class cannot be created;
• an abstract class can also contain abstract methods (abstract methods are those methods which do not have their definition and the child class of that abstract class have to override those abstract methods).

The keyword 'abstract' is used to create an abstract class. Example:

abstract class Animal

{

abstract void speak( );

}

Now, we can create a child class of Animal and can implement the speak( ) method in child class, as below:

class Cat

{

//--overridden speak method

public void speak( )

{

   System.out.println("yawns");

}

}

• Don't get confused between an abstract class and interface.
• Abstract class can also contain constructors, data members and member functions, while interface is referred to as a blue print of a class in java.

Interface:

• Interface is used for full abstraction.
• It is a blue print of class.
• By default, the data members of an interface are public, static and final.
• By default, the member functions of an interface are public and abstract, i.e., the class which will implement an interface have to override the abstract method of interface in it.
• Interface cannot be instantiated.
• They cannot have constructors.

To create an interface, following is the syntax:

interface Test

{

   void testMethod( );

}

Now, any class can implement the Test interface and have to override the testMethod() of the interface. Example:

class TestImplementation implements Test

{

   public void testMethod( )

   {

      System.out.println("Override interface method in class");

   }

}

Note that, one interface can extend one or more interface(s).