Java - Inheritance Concepts

Inheritance
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

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Inheritance
 Inheritance can be defined as the process where one class
acquires the properties (methods and fields) of another.
 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).
 Therefore, a subclass is a specialized version of a
superclass.
 It inherits all of the instance variables and methods
defined by the superclass and adds its own, unique
elements.

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Important terminologies
 Super Class: The class whose features are inherited.
 Sub Class: The class that inherits the other class.
 The subclass can add its own fields and methods in addition to the
superclass fields and methods.
 Reusability: Inheritance supports the concept of
“reusability”,
 i.e. when we want to create a new class and there is already a
class that includes some of the code that we want, we can derive
our new class from the existing class. By doing this, we are reusing
the fields and methods of the existing class.
 To inherit from a class, use the extends keyword.

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Syntax
 extends is the keyword used to inherit the properties of a
class.

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Syntax

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Inheritance Demo

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Inheritance Demo

Try it!!
 Although a subclass includes all of the members of its
superclass, it cannot access those members of the
superclass that have been declared as private.
 The final Keyword
 If you don't want other classes to inherit from a class, use the
final keyword.
8Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

Using super to Call Superclass
 A subclass can call a constructor defined by its superclass
by use of the following form of super:
 arg-list specifies any arguments needed by the constructor
in the superclass.
 super() must always be the first statement executed
inside a subclass’ constructor.

Using super to Call Superclass

Using super to access members
 A subclass can access the data members and methods of
superclass by use of the following form of super:
 member can be either a method or an instance variable.
 This second form of super is most applicable to situations
in which member names of a subclass hide (same name)
members by the same name in the superclass

Using super to access members

Types of Inheritance

Single Inheritance
 When a class inherits another class, it is known as a single inheritance.

Multi level Inheritance
 When there is a chain of inheritance, it is known as multilevel inheritance.

Hierarchical Inheritance
 When two or more classes inherits a single class, it is hierarchical inheritance

Multiple Inheritance
 To reduce the complexity and simplify the language, multiple
inheritance is not supported in java.
 Consider a scenario where A, B, and C are three classes.
 The C class inherits A and B classes.
 If A and B classes have the same method and you call it from child
class object, there will be ambiguity to call the method of A or B
class.
 In java, we can achieve multiple inheritance only through Interfaces.

Method Overriding
 In a class hierarchy, when a method in a subclass has the same name
and type signature as a method in its superclass,
 then the method in the subclass is said to override the method in the
superclass.
 When an overridden method is called from within a subclass, it will
always refer to the version of that method defined by the subclass.
 The version of the method defined by the superclass will be hidden.

Method Overriding
How to call the show()
method of Class A????
Two possible ways!!!!!

Final Keyword
 The keyword final has three uses.
 First, it can be used to create the equivalent of a named
constant.
 The other two uses of final apply to inheritance.
 Using final to Prevent Overriding
 Using final to Prevent Inheritance

Final Keyword

Important facts about inheritance
 Default superclass: Except Object class, which has no superclass,
every class has one and only one direct superclass (single
inheritance). In the absence of any other explicit superclass, every
class is implicitly a subclass of Object class.
 Superclass can only be one: A superclass can have any number of
subclasses. But a subclass can have only one superclass. This is
because Java does not support multiple inheritance with classes.
 Inheriting Constructors: 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.
 Private member inheritance: A subclass does not inherit the private
members of its parent class. However, if the superclass has public or
protected methods for accessing its private fields, these can also be
used by the subclass.

The instanceof Keyword
 The java instanceof
operator is used to test
whether the object is an
instance of the specified
type (class or subclass or
interface).
 The instanceof in java is
also known as type
comparison operator
because it compares the
instance with type. It
returns either true or
false.

The Object Class
 There is one special class, Object, defined by Java.
 All other classes are subclasses of Object.
 That is, Object is a superclass of all other classes. This
means that a reference variable of type
 Object can refer to an object of any other class.
Check what are the methods associated with Object class!!!!

The End…

Java - Inheritance Concepts

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