Java abstract Keyword

Java is an object-oriented programming language that provides the “abstract” keyword to allow programmers to define incomplete classes that cannot be instantiated. Abstract classes are used as templates for creating concrete classes and are meant to be extended by subclasses. They provide a blueprint for creating objects that can be customized to suit specific needs. It is important to understand the use of abstract classes for Java programming as it allows for code reusability and saves time.

This article explains the “abstract” keyword in Java, its benefits and limitations. It also covers how to use it in Java programming. Additionally, the article compares abstract classes and interfaces in Java. It also illustrates when to use each.

Abstract Keyword in Java: Key Characteristics

Instantiation of abstract classes:

An abstract class can’t be instantiated directly. Instead, it is designed to be extended by other classes, which can provide concrete implementations of its abstract methods.

Method body:

A method without an implementation is said to be abstract. It terminates with a semicolon rather than a method body and is declared using the abstract keyword. Subclasses of an abstract class must provide a concrete implementation of all abstract methods defined in the parent class.

Abstract and concrete methods:

Abstract classes can contain both abstract and concrete methods. Concrete methods are implemented in the abstract class itself and can be used by both the abstract class and its subclasses.

Constructors:

Abstract classes can have constructors, which are used to initialize instance variables and perform other initialization tasks. Constructors in concrete subclasses often invoke the constructors of abstract classes as they cannot be constructed directly.

Instance variables:

Instance variables contained in the abstract classes may be utilized by both the abstract class and its subclasses. Subclasses can access these variables directly, just like any other instance variables.

Interfaces:

Abstract classes can implement interfaces, which define a set of methods that must be implemented by any class that implements the interface. In this case, the abstract class must provide concrete implementations of all methods defined in the interface.

Abstract classes

Definition and explanation of abstract classes

In Java, an abstract class is a class that cannot be instantiated and is used as a base class for other classes. It’s designed to be extended by other classes, and it contains abstract methods, concrete methods, and instance variables. An abstract class provides a common interface for a set of subclasses, which allows for polymorphism in Java programming.

Examples of abstract classes and their implementation in Java

An example of an abstract class in Java is the Shape class. A Shape class can be extended by various other classes, like Circles, rectangles, Triangles, etc. Each of these classes can define its methods while still inheriting the properties of the Shape class. Here’s an example of how to define an abstract class in Java:

    public abstract class Shape {
   private String color;

   public Shape(String color) {
      this.color = color;
   }

   public String getColor() {
      return color;
   }

   public void setColor(String color) {
      this.color = color;
   }

   public abstract double getArea();
}

In the above code, the Shape class is declared as abstract using the abstract keyword. It has a constructor and two methods, getColor() and setColor(), which are not abstract. It also has an abstract method, getArea().

Advantages of using abstract classes

The advantages of using abstract classes in Java are:

• It provides a common interface for a set of subclasses, which allows for polymorphism in Java programming.
• It can implement common functionality and state shared by all subclasses.
• It can enforce a contract that subclasses must implement abstract methods.
• It can simplify the design of large and complex software systems by providing a structure for inheritance.

How to use Abstract Classes and Methods

To use the abstract class in Java, you need to create a subclass that extends the abstract class and implements the abstract methods. Here is an example:

public class Circle extends Shape {
   private double radius;

   public Circle(String color, double radius) {
      super(color);
      this.radius = radius;
   }

   public double getRadius() {
      return radius;
   }

   public void setRadius(double radius) {
      this.radius = radius;
   }

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

In the above code, the Circle class extends the Shape abstract class and implements the getArea() method.

Abstract classes and methods are useful when you want to create a group of related classes that share some common functionality. They also have their own unique features. They provide a way to enforce consistency among the related classes while still allowing for customization.

Abstract keyword rules and best practices:

There are several rules and best practices to keep in mind when working with the abstract keyword in Java.

Best Practices:

• The abstract keyword can only be used with classes and methods.
• An abstract class can contain constructors and static methods.
• If a class extends an abstract class, it must implement at least one of the abstract methods.
• An abstract class can contain overloaded abstract methods.
• The local inner class may be declared abstract.
• The throw clause can be used to declare the abstract method.

Avoid:

• Do not use the abstract keyword with variables and constructors.
• An abstract class cannot be instantiated.
• An abstract method does not contain the body.
• Do not use the abstract keyword with the final.
• Do not declare abstract methods as private or static.
• An abstract method cannot be synchronized.

Abstract Classes vs. Interfaces

In Java, both abstract classes and interfaces are used to define abstractions. Abstract classes are used to define a common structure and behaviour for a group of classes that share a common set of features. On the other hand, interfaces are used to define a common set of methods that can be implemented by multiple unrelated classes.

Abstract classes can have both abstract and non-abstract methods. At the same time, interfaces can only have abstract methods. This is the key difference between abstract classes and interfaces. A class is capable of extending only one abstract class, but it can implement multiple interfaces.

When must an interface be used over an abstract class:

• When you need to define a common behaviour for a group of unrelated classes
• When you need to provide multiple inheritances to a class

Examples of when to use an abstract class over an interface:

• When you need to define a common structure and behaviour for a group of related classes
• When you need to provide default implementations for some of the methods

Conclusion

In conclusion, understanding the “abstract” keyword is crucial for effective Java programming. Abstract classes and methods allow developers to define and enforce a blueprint for the behaviour of child classes. They provide structure and abstraction to code, making it more efficient and easier to maintain. By using abstract classes and methods, developers can write more flexible, extensible, and reusable code, reducing development time and cost.