Question

Fill in the blanks in each of the following statements: a) If a class contains at least one abstract method, it's a(n) ______ class. b) Classes from which objects can be instantiated are called ______ classes. \(c\) ______ involves using a superclass variable to invoke methods on superclass and subclass objects, enabling you to "program in the general." d) Methods that are not interface methods and that do not provide implementations must be declared using keyword ______ e) Casting a reference stored in a superclass variable to a subclass type is called ______

Short answer

a) abstract class, b) concrete classes, c) Polymorphism, d) abstract, e) downcasting.

Step-by-step solution

Identify the Type of Class with Abstract Methods

A class that contains at least one abstract method is known as an abstract class. Abstract classes cannot be instantiated and are intended to be subclassed by other classes to provide implementations for the abstract methods.

Recognize Classes That Can Be Instantiated

Classes that can be instantiated, meaning objects can be created from them, are referred to as concrete classes. Concrete classes provide implementations for all their methods.

Define the Concept That Enables 'Programming in the General'

Polymorphism is the object-oriented programming concept that involves using a superclass variable to refer to a subclass object and invoke methods, which enables programmers to write more general and reusable code.

Identify the Keyword for Non-Implemented Methods

Methods that are part of an interface, or abstract methods in an abstract class that do not provide implementations, must be declared using the abstract keyword.

Understanding Type Casting of Class References

When you assign a subclass object to a variable of a superclass type and then you want to access subclass-specific members, you need to perform a type cast. This action is known as downcasting.

Key concepts

Concrete Class

In the world of object-oriented programming, a concrete class is akin to a fully built house, ready to be lived in. This type of class can be instantiated, which means you can create individual objects using this class as the blueprint. The defining feature of a concrete class is that all of its methods are complete and actionable. There are no placeholders or unfinished parts; every method has its behavior clearly defined within the class. So, when the exercise asks for classes from which objects can be instantiated, we're talking about concrete classes.

Concrete classes are foundational to programming because they allow developers to create specific instances of objects that can interact with one another, store data, and execute defined tasks. They can also extend abstract classes and implement interfaces, providing the necessary detail and functionality that abstract methods only outline.

Polymorphism

Polymorphism is like a shape-shifter in programming. It's a concept that allows objects to be treated as instances of their parent class rather than their actual class. What makes this powerful is that you can write code that operates on a general level (like dealing with all shapes) but you benefit from the specializations of the actual object (like circles, squares, etc.).

In more technical terms, when a superclass reference variable is used to refer to a subclass object, and that reference is used to invoke methods, the method that gets called is the one that pertains to the actual subclass object. This characteristic of polymorphism, known as dynamic method dispatch, is essential for one of the main principles of object-oriented design: code reusability. Essentially, you can code 'in the general,' meaning your code can apply to objects of the superclass and any of its subclasses, enhancing versatility and flexibility in software design.

Abstract Keyword

Imagine you're drafting the blueprint for a house, but instead of detailing every room, you leave some of them as open spaces to be filled in later—this is the essence of what the abstract keyword does in programming. When you declare a method or a class as abstract, you're saying, 'the details are not here, but they will be filled in later by subclasses that inherit from me.' An abstract class cannot be instantiated—it's not complete on its own and only serves as a foundation for other classes.

Inside an abstract class, methods declared with the abstract keyword set up a contract of sorts: They say to any subclass, 'If you extend me, you must provide specific implementations for these methods.' This feature is crucial for defining a consistent interface in hierarchies of classes, while still allowing for specialized behavior in subclasses.

Downcasting

In a family tree, downcasting is like identifying a person not just as a member of the family but by their specific role, like being the youngest child. Downcasting in object-oriented programming refers to treating a variable of a parent class type as if it's a variable of a subclass type. It's what you do when you know that an object, while referred to by a variable of a superclass type, actually belongs to a subclass and you need to access its specialized methods or fields.

For example, you have a reference to an Animal, but you're sure it's a Dog and you want to make it bark. You downcast the Animal reference to a Dog reference, and then you can call the bark method. It's a powerful feature, but it needs to be used carefully, as improper downcasting can lead to runtime errors. Always make sure you're certain of an object's type before you perform a downcast to access subclass-specific features.