Java - Inheritance

Question 1

what is Inheritance?

Answer 1

Inheritance can be defined as the process where one class acquires the properties (methods and fields) of another. With the use of inheritance the information is made manageable in a hierarchical order.

The class which inherits the properties of other is known as subclass (derived class, child class) and the class whose properties are inherited is known as superclass (base class, parent class).

Question 1

extends Keyword

Answer 1

extends is the keyword used to inherit the properties of a class. Following is the syntax of extends keyword.

Syntax
class Super {
…..
…..
}
class Sub extends Super {
…..
…..
}
Sample Code

Question 2

The super keyword

Answer 2

The super keyword is similar to this keyword. Following are the scenarios where the super keyword is used.

It is used to differentiate the members of superclass from the members of subclass, if they have same names.

It is used to invoke the superclass constructor from subclass.

Differentiating the Members
If a class is inheriting the properties of another class. And if the members of the superclass have the names same as the sub class, to differentiate these variables we use super keyword as shown below.

super.variable
super.method();

Question 3

IS-A Relationship

Answer 3

IS-A is a way of saying: This object is a type of that object. Let us see how the extends keyword is used to achieve inheritance.

public class Animal {
}

public class Mammal extends Animal {
}

public class Reptile extends Animal {
}

public class Dog extends Mammal {
}
Now, based on the above example, in Object-Oriented terms, the following are true −

Animal is the superclass of Mammal class.
Animal is the superclass of Reptile class.
Mammal and Reptile are subclasses of Animal class.
Dog is the subclass of both Mammal and Animal classes.
Now, if we consider the IS-A relationship, we can say −

Mammal IS-A Animal
Reptile IS-A Animal
Dog IS-A Mammal
Hence: Dog IS-A Animal as well
With the use of the extends keyword, the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass.

We can assure that Mammal is actually an Animal with the use of the instance operator.

Example
class Animal {
}

class Mammal extends Animal {
}

class Reptile extends Animal {
}

public class Dog extends Mammal {

public static void main(String args[]) {
Animal a = new Animal();
Mammal m = new Mammal();
Dog d = new Dog();

  System.out.println(m instanceof Animal);
  System.out.println(d instanceof Mammal);
  System.out.println(d instanceof Animal);    } } Output true true true Since we have a good understanding of the extends keyword, let us look into how the implements keyword is used to get the IS-A relationship.

Generally, the implements keyword is used with classes to inherit the properties of an interface. Interfaces can never be extended by a class.

Example

public interface Animal {
}

public class Mammal implements Animal {
}

public class Dog extends Mammal {
}
The instanceof Keyword
Let us use the instanceof operator to check determine whether Mammal is actually an Animal, and dog is actually an Animal.

Example
interface Animal{}
class Mammal implements Animal{}

public class Dog extends Mammal {

public static void main(String args[]) {
Mammal m = new Mammal();
Dog d = new Dog();

  System.out.println(m instanceof Animal);
  System.out.println(d instanceof Mammal);
  System.out.println(d instanceof Animal);    } } Output true true true HAS-A relationship These relationships are mainly based on the usage. This determines whether a certain class HAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.

Lets look into an example −

Example
public class Vehicle{}
public class Speed{}

public class Van extends Vehicle {
private Speed sp;
}
This shows that class Van HAS-A Speed. By having a separate class for Speed, we do not have to put the entire code that belongs to speed inside the Van class, which makes it possible to reuse the Speed class in multiple applications.

In Object-Oriented feature, the users do not need to bother about which object is doing the real work. To achieve this, the Van class hides the implementation details from the users of the Van class. So, basically what happens is the users would ask the Van class to do a certain action and the Van class will either do the work by itself or ask another class to perform the action.

Types of Inheritance
There are various types of inheritance as demonstrated below.

Types of Inheritance
A very important fact to remember is that Java does not support multiple inheritance. This means that a class cannot extend more than one class. Therefore following is illegal −

Example
public class extends Animal, Mammal{}
However, a class can implement one or more interfaces, which has helped Java get rid of the impossibility of multiple inheritance.

Question 4

The instanceof Keyword

Answer 4

Let us use the instanceof operator to check determine whether Mammal is actually an Animal, and dog is actually an Animal.

Example
interface Animal{}
class Mammal implements Animal{}

public class Dog extends Mammal {

public static void main(String args[]) {
Mammal m = new Mammal();
Dog d = new Dog();

  System.out.println(m instanceof Animal);
  System.out.println(d instanceof Mammal);
  System.out.println(d instanceof Animal);    } } Output true true true HAS-A relationship These relationships are mainly based on the usage. This determines whether a certain class HAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.

Question 5

Types of Inheritance

Answer 5

There are various types of inheritance as demonstrated below.

Types of Inheritance
A very important fact to remember is that Java does not support multiple inheritance. This means that a class cannot extend more than one class. Therefore following is illegal −

Example
public class extends Animal, Mammal{}
However, a class can implement one or more interfaces, which has helped Java get rid of the impossibility of multiple inheritance.