Abstract Classes and Methods in Java: Explained!

Abstraction is the process of hiding implementation details and presenting only the essential features to the user. This allows us to focus on what an object is doing rather than how it is doing it.

In Java, abstract classes and methods are key tools for implementing abstraction. Abstract classes serve as templates for creating subclasses, while abstract methods provide a blueprint for the behavior of those subclasses.

If you’re new to Java or need a refresher on the differences between abstract classes and interfaces, be sure to check out our tutorial on the topic: Difference Between Interface and Abstract class in Java.

Abstract Classes

Definition and Syntax of Abstract Classes in Java

An abstract class is a class that cannot be instantiated on its own and is designed to serve as a base for other classes. It is declared using the abstract keyword in its class definition.

public abstract class Shape {
  // class body
}

Note that abstract classes may contain both abstract and non-abstract methods, as well as instance variables, constructors, and other members. However, abstract classes cannot be directly instantiated, which means they cannot be used to create objects.

Differences Between Abstract Classes and Concrete Classes

Unlike concrete classes, abstract classes cannot be instantiated directly. Instead, they are designed to be extended by other classes, which can then implement the abstract methods and inherit the non-abstract members and behavior of the abstract class.

In addition, abstract classes can have constructors, but they cannot be invoked directly from subclasses. Instead, a constructor of a concrete subclass must invoke a constructor of its superclass explicitly, either by calling super() or by calling a specific constructor of the superclass.

Examples of When to Use Abstract Classes

Abstract classes are often used to define a common interface or behavior that can be shared among multiple subclasses. For example, imagine you are designing a game that includes various types of shapes, such as circles, squares, and triangles. All of these shapes have certain properties in common, such as area and perimeter, but they may also have unique characteristics, such as number of sides or radius.

To represent these shapes in your game, you could create an abstract Shape class that defines the common behavior of all shapes:

public abstract class Shape {
  protected int x, y;

  public Shape(int x, int y) {
    this.x = x;
    this.y = y;
  }

  public abstract double getArea();
  public abstract double getPerimeter();
}

In this example, the Shape class includes instance variables for the x and y coordinates of the shape, as well as abstract methods for calculating the area and perimeter. By defining these methods as abstract, we allow each concrete subclass to implement them in its own way.

For example, we could create a Circle subclass that extends the Shape class and implements its own version of the getArea() and getPerimeter() methods:

public class Circle extends Shape {
  private double radius;

  public Circle(int x, int y, double radius) {
    super(x, y);
    this.radius = radius;
  }

  public double getArea() {
    return Math.PI * radius * radius;
  }

  public double getPerimeter() {
    return 2 * Math.PI * radius;
  }
}

Similarly, we could create Square and Triangle subclasses that implement their own versions of the getArea() and getPerimeter() methods, while inheriting the x and y coordinates from the Shape superclass.

By using abstract classes in this way, we can create a flexible and extensible system for representing various shapes in our game, while maintaining a common interface and behavior across all subclasses.

Abstract Methods

Definition and Syntax of Abstract Methods in Java

An abstract method is a method that is declared but does not provide an implementation. It is declared using the abstract keyword in its method signature, and it must be included in an abstract class.

public abstract double getArea();

Note that abstract methods do not include a body, but they may include a return type, parameters, and modifiers. The responsibility for implementing the abstract method is left to the concrete subclasses that extend the abstract class.

Rules and Restrictions for Abstract Methods

There are several rules and restrictions that apply to abstract methods in Java:

• An abstract method cannot be declared as final or private.
• An abstract method must be declared within an abstract class.
• A concrete subclass that extends an abstract class must provide an implementation for all of its inherited abstract methods.

Examples of How to Use Abstract Methods in Conjunction with Abstract Classes

Abstract methods are often used in conjunction with abstract classes to provide a common interface for a group of related classes. Continuing with our example of shapes from the previous section, let’s consider how we might use abstract methods to define a common interface for calculating the area and perimeter of various shapes.

We already defined the abstract Shape class, which includes abstract methods for calculating the area and perimeter of a shape:

public abstract class Shape {
  protected int x, y;

  public Shape(int x, int y) {
    this.x = x;
    this.y = y;
  }

  public abstract double getArea();
  public abstract double getPerimeter();
}

To create a concrete subclass of Shape, we must implement these abstract methods:

public class Circle extends Shape {
  private double radius;

  public Circle(int x, int y, double radius) {
    super(x, y);
    this.radius = radius;
  }

  public double getArea() {
    return Math.PI * radius * radius;
  }

  public double getPerimeter() {
    return 2 * Math.PI * radius;
  }
}

In this example, the Circle class implements the getArea() and getPerimeter() methods using its own unique formulae for calculating the area and perimeter of a circle.

Similarly, we could create Square and Triangle subclasses that implement their own versions of the getArea() and getPerimeter() methods.

By using abstract methods in conjunction with abstract classes, we can provide a common interface for calculating the area and perimeter of various shapes, while allowing each concrete subclass to provide its own implementation of these methods.

Best Practices

When working with abstract classes and methods in Java, there are a few best practices to keep in mind. Here are some guidelines to help you use these powerful tools effectively:

1. Use abstract classes to model hierarchies of related classes: Abstract classes are a great way to represent a group of related classes that share some common functionality. By creating an abstract class that defines the common methods and properties, you can avoid code duplication and make your code more modular and maintainable.

2. Use abstract methods to define common behavior: Abstract methods are useful when you want to enforce a particular behavior across multiple subclasses. By defining an abstract method in an abstract class, you can ensure that all subclasses implement the same behavior in their own way.

3. Don’t overuse abstract classes and methods: While abstract classes and methods can be powerful tools, it’s important not to overuse them. In general, you should only create an abstract class or method if you have a clear reason to do so. Overuse can lead to unnecessary complexity and make your code harder to maintain.

4. Follow naming conventions: When naming abstract classes and methods, it’s important to follow the standard Java naming conventions. Abstract classes should be named with an abstract noun (e.g. “Shape”), while abstract methods should be named with a verb (e.g. “draw”).

5. Document your code: As with any code, it’s important to document your abstract classes and methods to make them more understandable to other developers. Use clear, concise comments to explain the purpose and behavior of each class and method.

By following these best practices, you can ensure that your abstract classes and methods are effective tools for implementing abstraction and creating modular, maintainable code.

Conclusion

In conclusion, abstract classes and methods are powerful tools for implementing abstraction in Java. By using them effectively, you can create more modular, maintainable code that is easier to understand and extend. Whether you’re modeling hierarchies of related classes or defining common behavior across multiple subclasses, abstract classes and methods can help you achieve your programming goals. With the best practices we’ve discussed, you’ll be well on your way to mastering this important aspect of Java programming.

I trust that this tutorial has been useful to you. If you’re interested in learning more about Java programming, be sure to visit our Java Tutorials for Beginners page. There, you’ll find a wealth of information and resources to help you improve your skills and knowledge.

Frequently asked questions

• Can abstract classes have constructors?
  Yes, abstract classes can have constructors. However, they cannot be instantiated directly, so their constructors are usually called by constructors of their subclasses.
• Can abstract classes implement interfaces?
  Yes, abstract classes can implement interfaces, just like any other class. This can be useful when you want to provide some common behavior across multiple classes that implement the same interface.
• Can abstract methods be static?
  No, abstract methods cannot be static because they require an instance of the class to be invoked. However, static methods can call abstract methods if they are implemented in concrete subclasses.
• Can abstract classes be final?
  No, abstract classes cannot be final because they are intended to be extended by subclasses. Making an abstract class final would prevent this from happening.
• When should I use abstract classes versus interfaces?
  Abstract classes are a good choice when you want to provide some common functionality across multiple classes that share a common hierarchy. Interfaces are better suited for defining a set of methods that multiple classes can implement independently. In general, use abstract classes when you want to provide a base implementation for a group of related classes, and use interfaces when you want to define a set of methods that multiple classes can implement in their own way.