Question

State whether cach of the statements that follows is true or false. If \(f\) alse, explain why. a) All methods in an abstract class must be declared as abstract methods. b) Invoking a subclass-only method through a subclass variable is not allowed. c) If a superclass declares an abstract method, a subclass must implement that method. d) An object of a class that implements an interface may be thought of as an object of that interface type.

Short answer

a) False, an abstract class can contain non-abstract methods. b) False, a subclass variable can invoke subclass-specific methods. c) True, if a superclass declares an abstract method, a subclass must implement that method unless the subclass is also abstract. d) True, an object of a class that implements an interface can be considered as an object of that interface type.

Step-by-step solution

Statement a Evaluation

Examine whether all methods in an abstract class must be abstract. An abstract class can have both abstract methods (methods without a body) and concrete methods (methods with an implementation). Therefore, it is not necessary for all methods in an abstract class to be declared as abstract.

Statement b Evaluation

Determine the legitimacy of invoking a subclass-specific method through a subclass variable. Since subclass variables have access to all methods of the subclass, including those not present in the superclass, it is allowed to invoke a subclass-only method through a subclass variable. Thus, the given statement is false.

Statement c Evaluation

Verify if a subclass must implement an abstract method declared by its superclass. According to the rules of Java, if a subclass does not abstract itself, it is obligated to provide concrete implementations for all abstract methods inherited from its superclasses. Hence, this statement is true.

Statement d Evaluation

Contemplate whether an object of a class that implements an interface can be regarded as an object of the interface type. In Java, an object of a class that implements an interface can indeed be referenced by a variable of the interface type. This kind of polymorphism is a fundamental concept in Java. Therefore, this statement is true.

Key concepts

Abstract Methods in Java

Understanding abstract methods in Java is crucial for grasping the foundations of abstraction in object-oriented programming. An abstract method serves as a blueprint for the methods that must be implemented in subclasses. It doesn’t contain any implementation; rather, it just outlines the method's signature—its return type, name, and parameters.

In Java, an abstract class can have a mix of abstract and concrete methods. The concrete methods can have a full implementation and can be called directly, while the abstract methods must be defined by any non-abstract subclass. This setup allows an abstract class to provide a common structure for its subclasses while enforcing the implementation of certain essential methods tailored to the specifics of each subclass.

For example:

• An abstract class Shape might have an abstract method calculateArea().
• Different subclasses such as Circle, Rectangle, and Triangle would provide their own implementation of the calculateArea() method.

This design paradigm promotes code reuse and ensures consistency across different types of objects, representing real-world entities.

Subclass Implementation of Abstract Methods

When dealing with abstract methods, a subclass holds the responsibility to provide specific implementations. The beauty of this approach lies in the flexibility it offers programmers to define subclass-specific behavior while adhering to a unified interface established by the abstract superclass.

Subclasses in Java can be thought of as concrete embodiments of their abstract superclasses. They fill in the 'missing pieces' by providing bodies for abstract methods. However, a subclass does not need to implement abstract methods if it is abstract itself. It can pass the responsibility further down the inheritance hierarchy.

Consider a subclass Circle of the abstract class Shape mentioned earlier. It must concretize the calculateArea() method to compute the area specific to a circle's attributes, like the radius:

• public class Circle extends Shape { override public double calculateArea() { return Math.PI * radius * radius; } }

This emphasizes that the subclass inherits not only the properties and methods of its superclass but also the obligations to fulfill the abstract declarations through suitable implementations.

Interface Polymorphism in Java

The concept of interface polymorphism is a pivotal aspect of Java's flexibility and power in building dynamic and adaptable systems. By leveraging interfaces, Java allows objects to be treated as instances of multiple types, enhancing reusability and interactivity among classes.

An interface in Java is a contract that specifies what methods a class must implement without providing the implementation itself. Interface polymorphism enables a class to be accessed through multiple types: the type of the class itself and the type(s) of any interface(s) it implements.

For instance, if we have an interface Drivable and a class Car that implements Drivable, we can use a Drivable reference type to interact with an object of Car. This approach allows different classes to be treated uniformly through the interface’s reference type.

• Drivable myCar = new Car();

In this case, the actual object is a Car, but through the lens of polymorphism, it’s also seen as a Drivable entity. This is instrumental in writing generalized code that can handle a variety of objects that share the same interface but may have different underlying implementations.