Abstract and concrete

Learn concrete methods in abstract classes.

Concrete methods in abstract classes

An abstract class can contain both abstract and concrete methods. Concrete methods have a body; subclasses use them as is. This is the way to gather common behavior in one place.

Example: abstract class Vehicle { abstract void start(); void stop() { System.out.println("Stopped"); } }. Here start is abstract; the subclass writes it. But stop is concrete; its body is given in the abstract class. The subclass can use stop without overriding it. This is where abstract class diverges from interface: unlike an interface it carries common state and behavior. It reduces code duplication.

Sharing concrete methods

Concrete methods are written once in an abstract class; all subclasses inherit them. This is a powerful way to keep common behavior in one place.

Example: if you define a void describe() concrete method in the Vehicle abstract class, both Car and Truck use it. If you change the method, all subclasses update. This makes maintenance easier. A subclass may override the method; but it is not forced to. The abstract class centralizes common behavior. This feature is the main difference between abstract class and interface. An interface had no bodyful methods except default ones.

Exercise Add a concrete method to the Vehicle class

Define an abstract class Vehicle. Add the abstract void start(); signature and a void stop() concrete method with a body that runs System.out.println("Stopped");. Write class Car extends Vehicle; write the public void start() method with a body that runs System.out.println("Started");. In main create Car c = new Car(); then call c.start() and c.stop().

Show example solution

abstract class Vehicle {
    abstract void start();
    void stop() {
        System.out.println("Stopped");
    }
}
class Car extends Vehicle {
    public void start() {
        System.out.println("Started");
    }
}
class Main {
    public static void main(String[] args) {
        Car c = new Car();
        c.start();
        c.stop();
    }
}

Test yourself

What is a concrete method?
- A bodyful method
- A bodyless method
- A static method
- A private method
Show answer

A bodyful method — A concrete method is written with a body.

Complete the subclass declaration

class Car ____ Vehicle {
    public void start() { System.out.println("Çalıştı"); }
}

Show answer

class Car extends Vehicle {
    public void start() { System.out.println("Çalıştı"); }
}

Match the abstract and concrete concepts

abstract

Bodyless signature
concrete

Bodyful method
extends

Inheritance keyword
super

Parent access

Next lesson: Abstract vs interface

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