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Thread: Confusion about inheritance

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    Default Confusion about inheritance

    I am reading a SCJA study guide and it's explaining examples of classes that use inheritance, I saw one example and am confused why the wheelRPM variable is overriden, even with the explanation given below, it inherits the variable so why can't it be set by the methods of the TenSpeedBicycle class? Can anyone help me understand?
    // This is the base class, it is a basic bicycle
    public class Bicycle 
    {
    private float wheelRPM;
    private int degreeOfTurn;
    public void pedalRPM(float pedalRPM)
    {
    float gearRatio = 2f;
    this.wheelRPM = pedalRPM * gearRatio;
    }
    public void setDegreeOfTurn(int degreeOfTurn){
    this.degreeOfTurn = degreeOfTurn;
    }
    public float getWheelRPM() 
    {
    return this.wheelRPM;
    }
    public int getDegreeOfTurn() 
    {
    return this.degreeOfTurn;
    }
    }
    // This is the subclass, it is a special kind of bicycle.
    public class TenSpeedBicycle extends Bicycle 
    {
    private float gearRatio = 2f;
    private float wheelRPM; // why are we overriding this variable?  It is inherited, no?
    public void setGearRatio(float gearRatio) 
    {
    this.gearRatio = gearRatio;
    }
    public void pedalRPM(float pedalRPM) 
    {
    this.wheelRPM = pedalRPM * gearRatio;
    }
    public float getWheelRPM() 
    {
    return this.wheelRPM;
    }
    }
    This is a quote from the book:
    The TenSpeedBicycle class, listed in the preceding example, extends the
    Bicycle class. This class represents a bicycle that has ten different possible gear
    ratios. The regular Bicycle class cannot be used because it has a fixed gear ratio.
    The TenSpeedBicycle class adds a method and instance variable so a gear ratio
    can be set. It also overrides the wheelRPM variable. This must be done because the
    Bicycle class has no setter to set that variable directly. The TenSpeedBicycle
    class also overrides the pedalRPM(float pedalRPM) method. In the Bicycle
    class version of this method, the gear ratio was fixed. In the newer version, it uses
    the gear ratio that can be set. To retrieve the wheelRPM variable, the getter must
    also be overridden. This is because the original version of this method can only
    return the instance variable that is in its same class.
    ok, and to go through what I don't get:

    It also overrides the wheelRPM variable.This must be done because the
    Bicycle class has no setter to set that variable directly.
    To which I say, what does 'the Bicycle class has no setter to set that variable directly' have to do with the TenSpeedBicycle class's ability to access the wheelRPM variable, inherited from the Bicycle Class? It has a private access modifier, meaning any Bicycle objects can access their own wheelRPM variables and any TenSpeedBicycle objects created inherit this member that they can access privately too.
    Last edited by MeteoricDragon; May 16th, 2012 at 05:03 PM.


  2. #2
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    Default Re: Confusion about inheritance

    Not sure what you mean exactly, but you can't override private variables. In fact, you can't inherit them either.

    It appears actually to be overriding pedalRPM.
    Last edited by javapenguin; May 16th, 2012 at 04:50 PM.

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    Default Re: Confusion about inheritance

    private variables and methods are not directly inherited by subclasses, only public and protected variables and methods are. However, a subclass can access it's uninherited variables by using public or protected setters and getters created in the superclass.

    EDIT: This is a nice link that may help: http://docs.oracle.com/javase/tutori...sscontrol.html
    Last edited by aussiemcgr; May 16th, 2012 at 04:52 PM.
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    Default Re: Confusion about inheritance

    Quote Originally Posted by aussiemcgr View Post
    private variables and methods are not directly inherited by subclasses, only public and protected variables and methods are. However, a subclass can access it's uninherited variables by using public or protected setters and getters created in the superclass.

    EDIT: This is a nice link that may help: Controlling Access to Members of a Class (The Java™ Tutorials > Learning the Java Language > Classes and Objects)
    That link does help, but I am also confused between inheritance and access given by protected access modifiers. If I create a Bicycle object with protected wheelRPM, then I create a TenSpeedBicycle object that extends Bicycle, can the TenSpeedBicycle object access the Bicycle object's protected wheelRPM variable directly?

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    Default Re: Confusion about inheritance

    Quote Originally Posted by MeteoricDragon View Post
    That link does help, but I am also confused between inheritance and access given by protected access modifiers. If I create a Bicycle object with protected wheelRPM, then I create a TenSpeedBicycle object that extends Bicycle, can the TenSpeedBicycle object access the Bicycle object's protected wheelRPM variable directly?
    Yes. Protected variables and protected methods get directly inherited.
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    Default Re: Confusion about inheritance

    Quote Originally Posted by javapenguin
    In fact, you can't inherit them either.
    Quote Originally Posted by aussiemcgr
    private variables and methods are not directly inherited by subclasses, only public and protected variables and methods are. However, a subclass can access it's uninherited variables by using public or protected setters and getters created in the superclass.
    I must interject here and make a clarification in reference to the above statements

    Access modifiers do not necessarily define what is inherited - they define what can be accessed/overridden in child classes. One might argue if you cannot access something in a child class than you don't inherit - but I'd make the argument that just because you cannot access it does not mean you do not inherit its behavior. This in fact is a big distinction, because inheritance would not be inheritance if you could not inherit behavior defined by parent classes (regardless of how that behavior can be accessed/overridden from outside a class)
    Last edited by copeg; May 17th, 2012 at 05:35 PM. Reason: Clarification

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    Default Re: Confusion about inheritance

    Quote Originally Posted by copeg View Post
    I must interject here and make a clarification in reference to the above statements

    Access modifiers do not necessarily define what is inherited - they define what can be accessed/overridden in child classes. One might argue if you cannot access something in a child class than you don't inherit - but I'd make the argument that just because you cannot access it does not mean you do not inherit its behavior. This in fact is a big distinction, because inheritance would not be inheritance if you could not inherit behavior defined by parent classes (regardless of how that behavior can be accessed/overridden from outside a class)
    Thank you copeg, this was the more articulate way I sought to express my confusion. and helped me understand, thank you all.

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