Inheritance - Using extendsS2C Home « Inheritance - Using extends

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In the Inheritance - Basics lesson we took a first look at the concept of inheritance and how in Java terminology, the general class that holds the group of common characteristics is known as the superclass and the specialised subset of this is known as the subclass. We achieve this hierarchial connection in Java using the extends keyword, wherein the subclass extends the superclass.

In this lesson we use the extends keyword to create a couple of subclasses for the Vehicle superclass.

Following is a slideshow to digest the information we have written so far about inheritance:

All is good in Vehicle Town, we have our vehicle class and everyone is happy. But then, people without cars want to get on a bus and people want trucks to transport goods. Let's use inheritance, rather than write new classes and replicate a lot of things that all vehicles can do.

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The first thing we do is make a Car class, which is all our original Vehicle class held; so we don't need to change anything here.

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Our new Bus class comes next and we need a new instance variable conductor.

We will look at the Truck class in the Inheritance - Overriding Methods lesson.

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Using the extends keyword

As metioned earlier in the lesson we create an inheritance tree in Java using the extends keyword, wherein the subclass extends the superclass. By extending like this we create an inheritance hierarchy where we can use the more abstract superclass members, as well as more specific subclass member overrides where required. We will look at overriding methods in the next lesson.

Ok, time to code up the first two classes of the hierarchy shown in the slide show above, starting with the Car class.

Our Car subclass Top

Here we code and test the Car subclass where we use the extends keyword for the first time to subclass the Vehicle class. The Car class is very simple as it just uses all the same members as its superclass, the Vehicle class.

package com.server2client;
/*
  A Car Class
*/ 
public class Car extends Vehicle {

}

Save and compile our Car subclass in directory   c:\_OOConcepts in the usual way.

When using the extends keyword to extend a class an important point to remember is you can only extend one class in Java. You can't say for instance:

package com.server2client;
public class Car extends Vehicle, Motorized {

}

As you will get a compiler error. We will go into more detail of this later in the section, for now lets test the new Car subclass to make sure it works:

package com.server2client;
/*
  Test Class for Car
*/ 
public class TestCar {

    public static void main (String[] args) {
        Car estate = new Car();
        estate.setChassis("2-axle chassis");
        estate.setMotor("6 valve");
        estate.setWheels(4);
        System.out.println("Our car has a " + estate.getChassis() + ", " + estate.getMotor() 
                                            + " motor and has " + estate.getWheels() + " wheels.");
        estate.service(12); 
        estate.carry(6); 
        estate.load("shopping."); 
    }
}

The above screenshot shows the output of running our TestCar class.

The above screenshot shows the output of testing our Car subclass. The class works as intended and uses all its inherited members of the Vehicle superclass to output some messages. Time to code up our second subclass, the Bus class:

Our Bus subclass Top

Here we code and test the Bus subclass where we use the extends keyword again to subclass the Vehicle class. With the Bus class we need to add a new instance variable conductor to the class. The superclass Vehicle has the rest of the stuff we need.

package com.server2client;
/*
  A Bus Class
*/ 
public class Bus extends Vehicle {
    private boolean conductor;
    /*
      Public getters and Setters
    */ 
    public boolean getConductor() {
        return conductor;
    }
    public void setConductor(boolean  conductor) {
        this.conductor = conductor;
    }

}

Save and compile our Bus subclass in directory   c:\_OOConcepts in the usual way.

Lets test the new Bus subclass to make sure it works:

package com.server2client;
/*
  Test Class for Bus
*/ 
public class TestBus {

    public static void main (String[] args) {
        Bus a1 = new Bus();
        a1.setChassis("4-axle chassis");
        a1.setMotor("12 stroke");
        a1.setWheels(8);
        a1.setConductor(false);
        System.out.println("Our bus has a " + a1.getChassis() + ", " + a1.getMotor() 
                                            + " motor and has " + a1.getWheels() + " wheels.");
        if (a1.getConductor()) {
            System.out.println("This bus has a driver and a conductor."); 
        } else {
            System.out.println("This bus has a driver only."); 
        }                                    
        a1.service(3); 
        a1.carry(60); 
        a1.load("people."); 
    }
}

The above screenshot shows the output of testing our Bus subclass. The class works as intended and uses all its inherited members of the Vehicle superclass as well as the new instance varible conductor to output some messages.

Related Quiz

OO Concepts Quiz 4 - Inheritance - Using extends

Lesson 4 Complete

In this lesson we continued our investigation of Inheritance by using the extends keyword to subclass our classes.

What's Next?

We delve further into inheritance by looking at overriding methods.

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• Inheritance - Overriding Methods     >>