Question

4. What is an abstract class? Can you instantiate objects of an abstract class?

5. Give an example of an Inheritance (is-a) relationship. No code needed just give an example in a simple English sentence.

6. Give an example of a Composition (has-a) relationship. No code needed just give an example in a simple English sentence.

JAVA

Answer 1

4. Abstract comes from the concept of abstraction which means only showing only functionality to the user and hiding internal details.

Abstract Class

JAVA uses abstract keyword for abstract class.

may contain abstract and non abstract methods.

may contain final and non-final, static non- static methods.

should be extended using extends keyword.

A Java abstract class also cannot be instantiated, that is it's object can not be created but can be invoked through main() method.

5)

Inheritance (is -a) relationship which works on concept of base class and derived class where base class have common functionalities and derived class comes with distinct specific feature.

Supports code re-usability.

Derived class can use data members(instance variables) and methods of the base class.

Example:

Base Class - Person with attributes ( name, age, phone, weight, height. many features)

Derived Classes - Employee can have additional specific attributes (profession, qualification)

where Employee is -a person with distinct attributes

and every Person need not be an Employee but every Employee is a Person.

In above example, Person class contains all the data members and methods common to all Person

But Employee class will have specific feature of it's own and common feature of a person can be accessed in Person class.

(Code re-usability)

We have different type of inheritance in java

1) Single - A single base class and derived class.

2) Multilevel - A base class , derived class and derived class of derived class.

3) Hierarchical - A base class having more than one derived classes.

6) Composition

designed to implement has-a relationship in classes. Polymorphic behavior to handle many forms.

A class reuses the functionality by creating a reference to the object of the class it wants to reuse.

Example :

Base Class - Shape

Derived Classes - Triangle , Rectangle

Explanation :

Shape class can have its common feature of Shape .

Data member :- Area, Perimeter

Methods :- getArea(), getPerimeter()

Further, Derived classes can use these data member and methods to implement its own specific feature.

As Rectangle - 4 sides, Triangle - 3 sides

Formula to calculate area and perimeter of above two is different.

this implies, they can use methods of base class and implement own distinct definition to calculate

Area and Perimeter using getArea() and getPerimeter().  

Thus here, Rectangle and Triangle has method of Shape class.