Abstract Class and Methods in Java: Key Concepts, Examples and Best Practices

Java continues to stand tall as one of the most widely used programming languages, with nearly 49% of developers globally relying on it in 2024. Its versatility powers everything from mobile apps to enterprise solutions, solidifying its third rank among top programming languages.

But, without understanding core concepts like abstract classes and methods, you could lose out on the potential that makes Java so powerful. Abstract classes form the foundation of abstraction, helping you simplify complex applications.

This article dives deep into abstract classes in Java and methods. So, read on to unlock practical insights and examples to sharpen your programming expertise.

What Do You Mean by Abstract Class and Methods in Java? An Overview

Abstract classes in Java and methods allow you to simplify complex systems. By hiding unnecessary details, they let you focus on the essential functionalities. Abstraction enables you to work efficiently without diving into every intricate detail. 

Here are key points.

• Abstract class in Java is declared using the abstract keyword. For example, abstract class Vehicle {} defines an abstract class.
• Abstract methods in Java lack a body. For instance, abstract void move(); enforces subclasses to implement the method.
• Abstract classes in Java can have both abstract and concrete methods.
• They help in creating a blueprint for other classes while maintaining modularity.

Example: Abstract class Animal with abstract method sound() can be extended by Dog and Cat classes.

Ready to learn powerful languages like Java? Start your journey with upGrad’s online software engineering courses today!

Abstract Classes in Java: Definition and Syntax

Abstract classes in Java are declared with the abstract keyword. They serve as a template for other classes and cannot be instantiated directly.

Below are essential aspects.

• Abstract class syntax: Abstract class Shape {} defines a base class.
• Example: Abstract class Shape may have a concrete method draw() and an abstract method area().
• Use cases: Ideal for situations where you want to define common behaviors and enforce subclass-specific implementation.

Syntax of Abstract Class

Abstract classes in Java may include both abstract and concrete methods.

abstract class Shape {
    abstract void area();
    void draw() {
        System.out.println("Drawing Shape");
    }
}

Example Code: This code defines an abstract class Shape with both types of methods:

abstract class Shape {
    abstract void area();
    void draw() {
        System.out.println("Drawing Shape");
    }
}
class Circle extends Shape {
    void area() {
        System.out.println("Area of Circle");
    }
}

Output:

Drawing Shape
Area of Circle

Explanation:

This code defines an abstract class Shape with an abstract method area() (without implementation) and a concrete method draw() (with implementation). The Circle class extends Shape and provides its own implementation of the area() method, making it concrete. 

When an object of Circle is created, calling draw() executes the inherited method, and calling area() executes the overridden implementation in Circle, demonstrating the combination of abstraction and inheritance in Java. 

Also Read: Why is Java Platform Independent Language?

Rules for Working with Abstract Classes in Java

Abstract classes in Java come with clear rules. They simplify complex coding scenarios but need to be used appropriately.

Below are key points.

• Abstract classes cannot be instantiated.
• Subclasses must implement all abstract methods unless they are abstract too.
• Abstract classes can include static method, final, and constructors.

Now, let’s have a look at the rules for working with abstract classes in Java.

Abstract Class Instantiation

Abstract classes cannot be instantiated directly but serve as a template for subclasses. Here are key points.

• To use an abstract class, create a subclass and instantiate it.
• Abstract methods enforce subclass implementation, ensuring adherence to the design.

Example: You cannot write Shape obj = new Shape(); but can write Shape obj = new Circle();.

Also Read: How to Add Two Numbers in Java?

Abstract and Concrete Methods

Abstract methods lack implementation, while concrete methods include it. Here are key points.

• Abstract method syntax: abstract void display();.
• Concrete methods: Provide complete functionality like void show() {}.

Example: Abstract class Vehicle with method void start() implemented in subclasses like Car and Bike.

Also Read: Learn Data Abstraction in Java

Constructors in Abstract Classes

Abstract classes can have constructors. Subclasses invoke these constructors. Here are key points.

• Constructors initialize fields shared among subclasses.
• Subclasses call super() to utilize the constructor.

Example: Abstract class Device with constructor Device(String name) ensures all devices have a name.

Also Read: How Can I Use Pointers in Java?

Visibility of Abstract Methods

Abstract methods can be public, protected, or default. Here are key points.

• Public methods: Accessible everywhere.
• Protected methods: Accessible within the package and by subclasses.
• Default methods: Accessible within the package only.
• Example: Abstract class Tool with protected method void use(); ensures specific access levels.

Why You Should Use Abstract Classes in Java: Benefits and Purpose

Abstract classes in Java provide a blueprint for creating scalable and reusable code. They address complex coding challenges efficiently.

Below are key benefits.

• They ensure code reusability by defining common functionality.
• Abstract classes reduce redundancy in large projects.
• They enable abstraction and polymorphism, vital for designing clean and flexible systems.

Example: A Shape abstract class defines draw() and enforces area() in subclasses like Circle and Rectangle.

Real-World Use Case: Abstract classes help in designing a graphics application. For instance, Shape defines the draw() method, which is implemented by Circle and Rectangle.

Essential Features of Abstract Classes in Java

Abstract classes in Java come with various features that make coding efficient and manageable. Below are their key aspects.

Also Read: Loose Coupling vs Tight Coupling in Java: Difference Between Loose Coupling & Tight Coupling

Abstract Classes vs Interfaces in Java: Key Differences Explained

Abstract classes and interfaces serve distinct purposes. Understanding their differences ensures better design choices. Below are key differences.

Example Code: This example differentiates abstract classes and java interfaces:

abstract class Animal {
    abstract void sound();
    void eat() {
        System.out.println("Eating");
    }
}
interface Pet {
    void play();
}
class Dog extends Animal implements Pet {
    void sound() {
        System.out.println("Bark");
    }
    public void play() {
        System.out.println("Playing fetch");
    }
}

Output:

Bark
Playing fetch
Eating

Explanation:

This code demonstrates how a class can combine the functionalities of an abstract class and an interface. The abstract class Animal provides a mix of abstract (sound()) and concrete (eat()) methods, while the interface Pet declares the method play(). 

The Dog class extends Animal to implement the abstract method sound() and implements Pet to define the play() method. This showcases how Java allows a single class to inherit both abstract behavior from an abstract class and multiple interface functionalities, enabling flexibility in design. 

Also Read: Abstract Class vs Interface: The Differences and the Similarities

upGrad’s Exclusive Software and Tech Webinar for you –

SAAS Business – What is So Different?

Abstract Methods in Java: What They Are and How to Use Them

Abstract methods define a contract for subclasses without providing an implementation. Here are key points.

• Abstract methods must be declared within an abstract class.
• Subclasses must override abstract methods.
• Abstract method void move(); in Vehicle ensures subclasses implement movement behavior.

Syntax of Abstract Method: Abstract methods have no body and end with a semicolon.

abstract class Animal {
    abstract void sound();
}

Example Code: This example highlights an abstract method in an abstract class:

abstract class Car {
    abstract void drive();
}
class Sedan extends Car {
    void drive() {
        System.out.println("Driving Sedan");
    }
}

Output:

Driving Sedan

Explanation:

The code showcases an abstract method drive() within the abstract class Car. The abstract method has no body and must be implemented by any subclass. The Sedan class extends Car and provides a concrete implementation of the drive() method. 

When an object of Sedan is created and drive() is called, it executes the subclass's implementation, enforcing specific functionality while maintaining flexibility through abstraction.

Also Read: Top 6 Reasons Why Java Is So Popular With Developers in 2024

Inheritance and Overriding Abstract Methods

Subclasses in Java inherit abstract class and methods in java and must override abstract methods to provide concrete implementations. This process ensures that every subclass adheres to the defined structure of the abstract class.

Below are essential points about abstract class and methods in java.

• Subclasses inherit abstract classes using the extends keyword.
• Abstract methods in the parent class must be implemented in the subclass unless the subclass is abstract too, reinforcing the importance of abstract class and methods in java.
• Abstract class Animal with method sound() is overridden in Dog and Cat classes.

Example: Concrete Class Extending an Abstract Class

This example demonstrates how a concrete class inherits and overrides abstract methods.

abstract class Animal {
    abstract void sound();
    void eat() {
        System.out.println("This animal eats food");
    }
}
class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}
public class Main {
    public static void main(String[] args) {
        Animal dog = new Dog();
        dog.sound();
        dog.eat();
    }
}

Output:

Dog barks
This animal eats food

Explanation: 

The Dog class overrides the abstract method sound() and inherits the concrete method eat() from the Animal class.

Polymorphism: Method overriding also enables polymorphism. You can refer to an object by its parent type while calling overridden methods.

• Example: Assigning Dog object to Animal type and invoking sound() demonstrates polymorphic behavior.
• Benefit: This approach enhances flexibility and code reuse in complex systems.

Also Read: Types of Inheritance in Java: Single, Multiple, Multilevel & Hybrid

Common Mistakes to Avoid When Working with Abstract Class and Methods in Java

Mistakes with abstract classes in Java and methods can lead to inefficient code. Below are common pitfalls.

• Instantiating abstract classes directly, such as new Shape();.
• Forgetting to implement all abstract methods in subclasses.
• Misusing access modifiers in abstract methods, leading to compilation errors.
• Overcomplicating designs with excessive abstraction.

Also Read: How to Implement Data Abstraction in Java?

Top Best Practices for Using Abstract Class and Methods in Java

Abstract classes in Javascript work best when used with clear strategies. Here are practical approaches to make the most of them.

Also Read: Types of Polymorphism in Java [Static & Dynamic Polymorphism with Examples]

How upGrad Can Help You Master Java Programming

Advance your Java programming journey with upGrad. upGrad offers clarity and practical learning opportunities to strengthen your skills. Here are upGrad's free courses you should explore.

Boost your career with our popular Software Engineering courses, offering hands-on training and expert guidance to turn you into a skilled software developer.

Master in-demand Software Development skills like coding, system design, DevOps, and agile methodologies to excel in today’s competitive tech industry.

Stay informed with our widely-read Software Development articles, covering everything from coding techniques to the latest advancements in software engineering.

Reference(s):
https://www.infoq.com/articles/java-trends-report-2024/
https://emeritus.org/in/learn/top-programming-languages-2024/
https://thenewstack.io/java-usage-keeps-climbing-according-to-new-survey/