Java.Where and for what is the abstract modifier used?

The abstract Modifier in Java

The abstract modifier in Java is used for classes and methods to indicate that they are incomplete and must be implemented by subclasses.

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1. Where Can We Use abstract?

Where?	Purpose
Abstract Class	Prevents direct instantiation and provides a base for subclasses.
Abstract Method	Declares a method without an implementation, to be implemented by subclasses.

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2. Abstract Classes (abstract class)

An abstract class:

• Cannot be instantiated (objects cannot be created directly).
• Can contain abstract methods (methods without a body).
• Can have both abstract and concrete (implemented) methods.

Example: Abstract Class

abstract class Animal {
    abstract void makeSound(); // Abstract method (must be implemented in subclasses)

    void sleep() { // Concrete method (has implementation)
        System.out.println("Sleeping...");
    }
}

class Dog extends Animal {
    void makeSound() { // Implementing the abstract method
        System.out.println("Bark!");
    }
}

public class AbstractExample {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.makeSound(); // Output: Bark!
        myDog.sleep(); // Output: Sleeping...
    }
}

✔ Key Points:

• The Animal class is abstract because it has an abstract method (makeSound()).
• Subclasses (Dog) must implement the abstract method.
• Concrete methods (sleep()) are inherited by subclasses.

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3. Abstract Methods (abstract method)

• An abstract method has no body ({} is missing).
• Must be implemented by subclasses.
• Can only exist inside an abstract class.

Example: Abstract Method

abstract class Shape {
    abstract void draw(); // Abstract method (no implementation)
}

class Circle extends Shape {
    void draw() { // Implementation of abstract method
        System.out.println("Drawing a Circle");
    }
}

✔ The draw() method must be implemented in Circle.

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4. Why Use abstract?

Feature	Reason
Encapsulation of Common Behavior	Abstract classes allow defining common methods while forcing subclasses to implement specific behaviors.
Prevents Object Creation	You cannot create an instance of an abstract class.
Forces Implementation	Ensures that subclasses implement necessary methods.

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5. Can an Abstract Class Have a Constructor?

Yes! An abstract class can have a constructor, but it cannot be instantiated directly.

Example

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

class Car extends Vehicle {
    Car() {
        System.out.println("Car created");
    }
}

public class Main {
    public static void main(String[] args) {
        Car car = new Car();
    }
}

✔ Output:

Vehicle created
Car created

✔ Why use a constructor?

• It allows initialization of common fields.
• Subclasses can call super() to use it.

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6. Abstract Class vs. Interface

Feature	Abstract Class	Interface
Methods	Can have abstract and concrete methods	Only abstract methods (before Java 8), can have default/static methods (Java 8+)
Variables	Can have instance variables	Only public static final (constants)
Constructors	✅ Can have constructors	❌ No constructors
Inheritance	A class can extend only one abstract class	A class can implement multiple interfaces