What does a subclass inherit from its superclass?

In designing objects in information sytems, determining relationships between objects is a skill which a designer or analyst must master. One common relationship type is the Generalization/Specification type. The type defines a hierarchy which consists of a superclass and multiple subclasses. A subclass inherits everything from its superclass, which is referred to as inheritance in the object-orientation methodology and object-oriented programming. By inheritance, the superclass’s attributes will not repeat in any of its subclasses. When a new object of subclass is created, no extra object of the superclass is needed.

Class Diagram

The following Python code shows you how to implement two classes: Person and Student. Student inherits all the attributes, fname and lname, as well as the methods, initializer and toString, from Person. In addition, Student has its own attribute: year.

he Enhanced Entity Relationship Model contains all the features of the Entity Relationship model. In addition to all that, it also contains features of Subclasses, Superclasses and Inheritance.

All of these in detail are as follows −

A subclass is a class derived from the superclass. It inherits the properties of the superclass and also contains attributes of its own. An example is:

Car, Truck and Motorcycle are all subclasses of the superclass Vehicle. They all inherit common attributes from vehicle such as speed, colour etc. while they have different attributes also i.e Number of wheels in Car is 4 while in Motorcycle is 2.

A superclass is the class from which many subclasses can be created. The subclasses inherit the characteristics of a superclass. The superclass is also known as the parent class or base class.

In the above example, Vehicle is the Superclass and its subclasses are Car, Truck and Motorcycle.

Inheritance is basically the process of basing a class on another class i.e to build a class on a existing  class. The new class contains all the features and functionalities of the old class in addition to its own.

The class which is newly created is known as the subclass or child class and the original class is the parent class or the superclass.

You can make a class (called a subclass, derived class, or child class) a specialization of another class (called a superclass, base class, or parent class).

A subclass inherits the methods and instance data of its superclasses, and is related to its superclasses by an is-a relationship. For example, if subclass P inherits from superclass Q, and subclass Q inherits from superclass S, then an instance of P is an instance of Q and also (by transitivity) an instance of S. An instance of P inherits the methods and data of Q and S.

Using subclasses has several advantages:

• Reuse of code: Through inheritance, a subclass can reuse methods that already exist in a superclass.
• Specialization: In a subclass you can add new methods to handle cases that the superclass does not handle. You can also add new data items that the superclass does not need.
• Change in action: A subclass can override a method that it inherits from a superclass by defining a method of the same signature as that in the superclass. When you override a method, you might make only a few minor changes or completely change what the method does.

Restriction: You cannot use multiple inheritance in your COBOL programs. Each COBOL class that you define must have exactly one immediate superclass that is implemented in Java™ or COBOL, and each class must be derived directly or indirectly from java.lang.Object. The semantics of inheritance are as defined by Java.

The structure and syntax of a subclass definition are identical to those of a class definition: Define instance data and methods in the DATA DIVISION and PROCEDURE DIVISION, respectively, within the OBJECT paragraph of the subclass definition. In subclasses that require data and methods that are to be associated with the subclass itself rather than with individual object instances, define a separate DATA DIVISION and PROCEDURE DIVISION within the FACTORY paragraph of the subclass definition.

COBOL instance data is private. A subclass can access the instance data of a COBOL superclass only if the superclass defines attribute (get or set) instance methods for doing so.

In fact, in Java, all classes must be derived from some class. Which leads to the question "Where does it all begin?" The top-most class, the class from which all other classes are derived, is the Object class defined in java.lang. Object is the root of a hierarchy of classes, as illustrated in the following figure.

The subclass inherits state and behavior in the form of variables and methods from its superclass. The subclass can use just the items inherited from its superclass as is, or the subclass can modify or override it. So, as you drop down in the hierarchy, the classes become more and more specialized:

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Definition: A subclass is a class that derives from another class. A subclass inherits state and behavior from all of its ancestors. The term superclass refers to a class's direct ancestor as well as all of its ascendant classes.

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Now would be a good time to review the discussion in What Is Inheritance?.

Creating Subclasses

To create a subclass of another class use the extends clause in your class declaration. (The Class Declaration explains all of the components of a class declaration in detail.) As a subclass, your class inherits member variables and methods from its superclass. Your class can choose to hide variables or override methods inherited from its superclass.

Writing Final Classes and Methods

Sometimes, for security or design reasons, you want to prevent your class from being subclassed. Or, you may just wish to prevent certain methods within your class from being overriden. In Java, you can achieve either of these goals by marking the class or the method as final.

Writing Abstract Classes and Methods

On the other hand, some classes are written for the sole purpose of being subclassed (and are not intended to ever be instantiated). These classes are called abstract classes and often contain abstract methods.

The Object Class

All objects in the Java environment inherit either directly or indirectly from the Object class. This section talks about the interesting methods in Object--methods that you may wish to invoke or override.

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