Question

Fill in the blanks in each of the following statements: a) __________ is a form of software reusability in which new classes acquire the members of existing classes and embellish those classes with new capabilities. b) A superclass’s____________ members can be accessed in the superclass declaration and in subclass declarations. c) In a(n) _______ relationship, an object of a subclass can also be treated as an object of its superclass. d) In a(n) _______ relationship, a class object has references to objects of other classes as members. e) In single inheritance, a class exists in a(n) _______________ relationship with its subclasses. f) A superclass’s __________ members are accessible anywhere that the program has a reference to an object of that superclass or to an object of one of its subclasses. g) When an object of a subclass is instantiated, a superclass ______________ is called implicitly or explicitly.] h) Subclass constructors can call superclass constructors via the _______ keyword.

Short answer

a) Inheritance b) public c) is-a d) has-a e) hierarchical f) public g) constructor h) super

Step-by-step solution

Identifying the Concept of Software Reusability

Recognize that the form of software reusability where new classes acquire members of existing classes and add new capabilities is called 'Inheritance'.

Understanding Class Member Accessibility

Understand that members of a superclass that can be accessed in both the superclass and subclass declarations are called 'accessible' or 'public' members.

Recognizing Object Type Relationships

Determine that when an object of a subclass can be treated as an object of its superclass, this is referred to as an 'is-a' relationship.

Comprehending Component Relationships in Classes

Recognize that when a class contains references to objects of other classes as members, it's in a 'has-a' or 'composite' relationship.

Understanding Single Inheritance Relationships

Identify that in single inheritance, a class has a 'hierarchical' relationship with its subclasses.

Recognizing Accessibility of Superclass Members

Acknowledge that members of a superclass that are accessible anywhere a reference exists to an object of that superclass or its subclasses are 'public' members.

Understanding Superclass Constructor Calls

Realize that when an object of a subclass is instantiated, a 'superclass constructor' is invoked, which can be either implicit or explicit.

Identifying How Subclass Constructors Call Superclass Constructors

Understand that subclass constructors can call superclass constructors using the 'super' keyword.

Key concepts

Software Reusability

Software reusability is a key concept in object-oriented programming that allows developers to use existing code for new purposes, reducing the need to write code from scratch. Inheritance is a prime example of software reusability, where new classes, known as subclasses, can acquire the properties and behaviors (members) of existing classes, termed superclasses. This enables a hierarchy whereby subclasses inherit and enhance their superclasses with additional capabilities, promoting code efficiency and maintainability.

For instance, consider a basic class 'Vehicle', which includes properties like speed and methods like drive. In a real-world scenario, rather than redefining these properties for every type of vehicle, you could create subclasses like 'Car' and 'Motorcycle' that inherit these attributes from 'Vehicle' and add unique features, such as number of wheels or type of engine.

Class Member Accessibility

Understanding class member accessibility is crucial to managing how the properties and methods of a class can be accessed. Java offers several accessibility levels, including public, protected, package-private, and private.

Public members are designed to be accessible from any other class whereas private members are accessible only within the class they are declared. Protected members are accessible within the same package and also through subclass references. Package-private members (default accessibility) are accessible exclusively within the same package. The intentional setting of accessibility levels ensures encapsulation, one of the four fundamental OOP principles, which protects the integrity of the data.

Is-A Relationship

An is-a relationship symbolizes inheritance in Java. It implies that a subclass is a type of its superclass. This relationship establishes a hierarchy which states that an object of a subclass can also be treated as an object of its superclass.

For example, a 'Dog' is-an 'Animal', so a 'Dog' object can be treated as an 'Animal' object. This polymorphic behavior is a cornerstone of object-oriented design, enabling a more general and flexible approach to coding. Any method that can handle an 'Animal' object can also handle a 'Dog' object without need for any changes.

Has-A Relationship

Contrasting the is-a relationship, a has-a relationship (also known as composition or aggregation) indicates that a class contains one or more references to objects of other classes as its members. This represents a use relationship rather than an inheritance relationship.

For example, a 'Car' class might have a 'has-a' relationship with a 'Wheel' class, meaning that a 'Car' object contains 'Wheel' objects. Composition is a versatile tool for building complex types out of simpler ones and is another way to promote reusability and modular design.

Single Inheritance

Java supports single inheritance, meaning a class can inherit from only one superclass. This establishes a straightforward and streamlined 'hierarchical' relationship between subclasses and superclasses. Single inheritance reduces complexity and simplifies the class hierarchy but can also limit the flexibility when compared to multiple inheritance, which some other languages support.

However, Java provides interfaces to bypass some limitations of single inheritance. A class can implement multiple interfaces on top of the single inheritance model, allowing for flexibility while maintaining the robust structure of single inheritance.

Superclass Constructor

When you create an object of a subclass, Java requires that the constructor of its superclass is called to ensure proper initialization of all members inherited from the superclass. This is done through the superclass constructor, which can be invoked either implicitly or explicitly.

If a subclass constructor doesn’t explicitly call a superclass constructor, Java automatically inserts a call to the no-argument constructor of the superclass. If the superclass doesn’t have a no-argument constructor, an error occurs, and the programmer must then explicitly call an available superclass constructor using the 'super' keyword.

Super Keyword

The 'super' keyword has a critical role in Java's inheritance mechanism. It is used within a subclass to refer explicitly to members of its superclass, especially when the members are overridden. More importantly, 'super' can be used to invoke a superclass’s constructor from the subclass. This ensures all inherited attributes are appropriately setup before a subclass performs any actions that depend on those attributes.

Consider a 'Vehicle' superclass with a custom constructor. If a subclass 'Car' is created, the 'Car' constructor can call super() to invoke the 'Vehicle' constructor, potentially passing necessary parameters if the 'Vehicle' constructor requires them.