#053 [JAVA] - Abstract Class part 1 (Examples, Abstract rules)

Introduction

This tutorial will guide you through the concept of abstract classes in Java, as presented in the video by Adel Nasim. Abstract classes are fundamental in object-oriented programming and allow you to define a base class that cannot be instantiated on its own. Instead, they serve as a blueprint for other classes. Understanding abstract classes will enhance your ability to design flexible and maintainable code.

Step 1: Understand Abstract Classes

• Definition: An abstract class is a class that cannot be instantiated directly. It can contain abstract methods (methods without a body) and concrete methods (methods with a body).
• Purpose: Abstract classes are used to provide a common interface for subclasses. They allow you to define methods that must be implemented in derived classes.

Practical Advice

• Use abstract classes when you have a common base that should not be instantiated but shared among subclasses.
• Think of abstract classes as contracts for derived classes.

Step 2: Create an Abstract Class

• Syntax: Declare an abstract class using the abstract keyword.

abstract class Animal {
    abstract void sound(); // Abstract method
    void eat() {           // Concrete method
        System.out.println("Eating");
    }
}

• Example: In the example above, Animal is an abstract class with an abstract method sound() and a concrete method eat().

Practical Advice

• Always ensure that any class extending an abstract class implements all abstract methods unless it is also declared as abstract.

Step 3: Implementing Abstract Methods in Subclasses

• Creating a Subclass: A subclass must implement all abstract methods from its parent abstract class.

class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}

class Cat extends Animal {
    void sound() {
        System.out.println("Meow");
    }
}

• Usage: When you create an instance of Dog or Cat, you can invoke both sound() and eat() methods.

Common Pitfalls

• Forgetting to implement all abstract methods in a subclass can lead to compilation errors.

Step 4: Understanding Constructors in Abstract Classes

• Abstract Class Constructor: Abstract classes can have constructors, which are called when a subclass is instantiated.

abstract class Animal {
    Animal() {
        System.out.println("Animal created");
    }
}

• Purpose: Constructors in abstract classes can initialize common attributes for all subclasses.

Practical Advice

• Use constructors in abstract classes to perform initialization tasks that apply to all derived classes.

Step 5: Rules of the Abstract Keyword

• Cannot be Final or Static: An abstract class cannot be declared as final or static.
• Cannot be Instantiated: You cannot create an instance of an abstract class directly.

Additional Tips

• Abstract methods must be declared in an abstract class.
• An abstract class can have both abstract and concrete methods, allowing for a mix of defined behavior and required implementation.

Conclusion

In this tutorial, we've covered the essential concepts of abstract classes in Java. You learned how to create abstract classes, implement their methods in subclasses, and understand their rules and constructors. As you continue to practice, consider how abstract classes can help you design more robust and maintainable applications. Next, try implementing abstract classes in your own projects to solidify your understanding.