Which of the Following Class Definitions Defines a Legal Abstract Class
In object-oriented programming, an abstract class serves as a blueprint for other classes that inherit from it. It cannot be instantiated on its own and is designed to be extended by subclasses. However, not all class definitions can be considered legal abstract classes. This article will explore the concept of abstract classes and analyze which of the following class definitions define a legal abstract class.
To understand the concept of abstract classes, let’s first define what an abstract class is. An abstract class is a class that cannot be instantiated but can be subclassed. It often includes abstract methods, which are methods without an implementation. Subclasses that inherit from an abstract class must implement these abstract methods. Abstract classes are used to provide a common interface or behavior for a group of related classes.
Now, let’s examine the following class definitions and determine if they define a legal abstract class:
1. Class A {
public void method1() {
// implementation
}
}
This class definition does not define an abstract class. It has a method with an implementation and lacks the keyword “abstract.” Abstract classes cannot have concrete methods; they can only have abstract methods or no methods at all.
2. Abstract class B {
public abstract void method1();
}
This class definition defines a legal abstract class. It uses the keyword “abstract” to declare the class as abstract and includes an abstract method, “method1.” Subclasses that inherit from this abstract class will be required to implement this method.
3. Class C extends B {
public void method1() {
// implementation
}
}
This class definition does not define an abstract class. It extends class B, which is an abstract class, but it provides an implementation for the abstract method “method1.” An abstract class cannot have concrete methods, so this class definition violates that rule.
4. Abstract class D {
public abstract void method1();
public void method2() {
// implementation
}
}
This class definition does not define an abstract class. It includes both an abstract method, “method1,” and a concrete method, “method2.” Abstract classes cannot have concrete methods, so this class definition is not legal.
Based on the above analysis, the only class definition that defines a legal abstract class is option 2, “Abstract class B.” It properly uses the keyword “abstract” to declare the class as abstract and includes an abstract method. This definition conforms to the rules of abstract classes and can be used as a blueprint for subclasses.
Frequently Asked Questions (FAQs):
Q: Can an abstract class be instantiated?
A: No, an abstract class cannot be instantiated. It is designed to be subclassed and provides a common interface or behavior for its subclasses.
Q: Can an abstract class have concrete methods?
A: No, an abstract class cannot have concrete methods. It can only have abstract methods or no methods at all. Concrete methods should be implemented in subclasses.
Q: Can a class be both abstract and final?
A: No, a class cannot be both abstract and final. An abstract class is meant to be subclassed, while a final class cannot be subclassed. These two keywords have conflicting purposes.
Q: Can an abstract class have variables?
A: Yes, an abstract class can have variables. These variables can be accessed by both the abstract class and its subclasses.
Q: Can an abstract class implement an interface?
A: Yes, an abstract class can implement an interface. It can provide default implementations for the methods defined in the interface and leave the abstract methods to be implemented by its subclasses.
In conclusion, an abstract class is a fundamental concept in object-oriented programming. It cannot be instantiated on its own and is designed to be subclassed. Only class definition 2, “Abstract class B,” defines a legal abstract class as it declares the class as abstract and includes an abstract method. The other class definitions violate the rules of abstract classes by including concrete methods or providing implementations for abstract methods. Understanding abstract classes is essential for creating well-structured and reusable code in object-oriented programming.