Java Inheritance Pdf

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Java - Inheritance - 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 i. Below given is an example demonstrating Java inheritance. In this example you can observe two classes namely Calculation and MyCalculation. Using extends keyword the MyCalculation inherits the methods addition and Subtraction of Calculation class. Copy and paste the program given below in a file with name MyCalculation.java class Calculation. Inheritance and Polymorphism. In Java, the keyword extends serves this purpose. In this chapter, we study Java’s extends mechanism, and see how it can be used to save. Inheritance is implemented in Java using the extends clause. A class Subclass1 can inherit.

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What Is Inheritance In Java

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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).

extends Keyword

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

Syntax

Sample Code

Following is an example demonstrating Java inheritance. In this example, you can observe two classes namely Calculation and My_Calculation.

Using extends keyword, the My_Calculation inherits the methods addition() and Subtraction() of Calculation class.

Copy and paste the following program in a file with name My_Calculation.java

Example

Compile and execute the above code as shown below.

After executing the program, it will produce the following result −

Output

In the given program, when an object to My_Calculation class is created, a copy of the contents of the superclass is made within it. That is why, using the object of the subclass you can access the members of a superclass.

The Superclass reference variable can hold the subclass object, but using that variable you can access only the members of the superclass, so to access the members of both classes it is recommended to always create reference variable to the subclass.

If you consider the above program, you can instantiate the class as given below. But using the superclass reference variable ( cal in this case) you cannot call the method multiplication(), which belongs to the subclass My_Calculation.

Note − A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

The super keyword

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.

What is inheritance in java

Sample Code

This section provides you a program that demonstrates the usage of the super keyword.

In the given program, you have two classes namely Sub_class and Super_class, both have a method named display() with different implementations, and a variable named num with different values. We are invoking display() method of both classes and printing the value of the variable num of both classes. Here you can observe that we have used super keyword to differentiate the members of superclass from subclass.

Copy and paste the program in a file with name Sub_class.java.

Example

Compile and execute the above code using the following syntax.

On executing the program, you will get the following result −

Output

Invoking Superclass Constructor

If a class is inheriting the properties of another class, the subclass automatically acquires the default constructor of the superclass. But if you want to call a parameterized constructor of the superclass, you need to use the super keyword as shown below.

Sample Code

Inheritance In Java Pdf

The program given in this section demonstrates how to use the super keyword to invoke the parametrized constructor of the superclass. This program contains a superclass and a subclass, where the superclass contains a parameterized constructor which accepts a integer value, and we used the super keyword to invoke the parameterized constructor of the superclass.

Copy and paste the following program in a file with the name Subclass.java

Example

Compile and execute the above code using the following syntax.

On executing the program, you will get the following result −

Output

IS-A Relationship

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.

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

This will produce the following result −

Output

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

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

This will produce the following result −

Java Inheritance Notes Pdf

Output

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

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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.

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 −

Java Inheritance Pdf Free

Example

Single Inheritance In Java Example

However, a class can implement one or more interfaces, which has helped Java get rid of the impossibility of multiple inheritance.