protected data member

[javanoobe]

I have a project that requires me to use 'protected' for a few variables.
So I have something like this:
protected int myVariable1;
protected int myVariable2;
protected int myVariable3;

Everything works and passes the information from the parent class to the child classes. My question is how do I check this to demonstrate the protected data can be accessed directly by the child classes? The book we have to use has maybe 50 whole words on the subject of protected and no examples. I understand that protected is a level of security between public and private. I'm have some sort of brain fart on how to check this.

Thank You

javanoobe

[sunde887]

let's think about what private and public mean.

A private variable can only be used inside the class

class X{
  private int x; //has a private variable named x
  public int y;  //has a public variable named x
}
public class Y extends X{
  public static void main(String[] args){
    X x = new X();
    System.out.println(x.x); //illegal(x is private and can only be used inside class X
    System.out.println(x.y);//legal, y is public, it can be used outside of class x
  }
}

With the use of protected, you allow the variable to be used outside the original class, if and only if, the class you are using it is a subclass of x.

class X{
  protected x;
}
class Y extends X{
  public void printVars(){
    System.out.println(x);
  }
  public static void main(String[] args){
    Y y = new Y();
    y.printVars();
  }
}

Test those snippets and see how everything happens.

[dlorde]

With the use of protected, you allow the variable to be used outside the original class, if and only if, the class you are using it is a subclass of x.

In C++, yes. In Java, 'protected' access gives visibility to other classes in the same package (that may not be subclasses), as well as all subclasses.

[javanoobe]

Yes I under stand all of that so far and I tried the snippets you posted sunde887 and Eclipse gave me an error for 'x' and I had to add int or string before it to make it work. Now since I'm not completely sure as to what all I need to do to apply the 'protected' to a variable I need some way to check this to see if I implemented it correctly. Here is what I have:

//my vehicle class

public class Vehicle
{
protected int wheels;
protected int mpg;
//Constructor
public Vehicle(int wheels2, int mpg2)
{
wheels = wheels2;
mpg = mpg2;
}
//get statements
public int getWheels()
{
return wheels;
}
public double getMpg()
{
return mpg;
}
//toString display method
public String toString()
{
String vehicleString = "Number of wheels is: " + wheels + " mpg is: " + mpg;
return vehicleString;

}
}


************************************************** ********
//my car class


public class Car extends Vehicle
{
public Car(int wheels, int mpg)
{
super(wheels, mpg);

}
}

************************************************** ********

my motorcycle class

public class MotorCycle extends Vehicle
{
public MotorCycle(int wheels, int mpg)
{
super(wheels, mpg);

}

}

************************************************** ********

//the application


public class UseVehicle
{
public static void main(String[] args)
{
//instantiates car and motorcycle
Car myCar = new Car(4, 27);
MotorCycle myMotorCycle = new MotorCycle(2, 55);
//calls display method
System.out.println(myCar.toString());
System.out.println(myMotorCycle.toString());
}
}

I have a sneaking suspicion I'm not implementing 'protected' correctly for the variables.


Thank You

javanoobe

P.S. Sorry didn't notice the code tag option until after I posted.

[Tolls]

You've implemented it correctly.

It's just you're not using them as protected, they could just as easily be private and your code would still work. Of course that would actually be good design, but since your goal seems to be show protected working then I would create a toString() method in the subclasses that printed the subclass name, and both parent attributes.

[javanoobe]

OK. I set a value to one of the variables in my Vehicle class then put a print statement in and it worked. I'm still not sure this is right. What I have to do is have a constructor in my Vehicle class that requires a value for each field. I believe I have that. Then I need to set the values in the Car and motorcycle class, I believe I did that. But if the point of protected is to access the variable directly form the parent class, I think the constructor in Vehicle is still passing the variables. If this is the case how do I get the variable from Vehicle and then pass the value back to the constructor in Vehicle? This is my first programing class and I'm still trying to understand things.

public class Vehicle 
{
	protected int wheels;
	protected int mpg;
	//Constructor
	public Vehicle(int wheels2, int mpg2) 
	{
		wheels = wheels2;
		mpg = mpg2;
	}
	//toString display method
	public String toString()
	{
		String vehicleString = "Number of wheels is: " + wheels + " mpg is: " + mpg;
		return vehicleString;
	}
}

 public class Car extends Vehicle
{
	public Car(int wheels, int mpg) 
	{
		super(6, 90);
	}
	  public void printCars()
	  //testing
	  {
		    System.out.print("testing: " + wheels);
	  }
}

public class MotorCycle extends Vehicle
{
	public MotorCycle(int wheels, int mpg) 
	{
		super(3, 80);
	}
}

public class UseVehicle 
{
	public static void main(String[] args) 
	{
		//instantiates car and motorcycle 
		//I put garbage values in demonstrate the values are being set by the other 2 classes
	Car myCar = new Car(0, 0);
	MotorCycle myMotorCycle = new MotorCycle(0, 0);
	//calls display method
	System.out.println(myCar.toString());
	System.out.println(myMotorCycle.toString());
	myCar.printCars();
	}
}

Thank You

javanoobe(the name says it all)

[Tolls]

Sorry, I have no idea what this exercise is intended to show, so I can't say what it is you are supposed to do.

[j2me64]

Sorry, I have no idea what this exercise is intended to show, so I can't say what it is you are supposed to do.

from inside the same package protected works like public. but from an other package? the protected variable is not yet visible if you make an instance from the class with protected but if you subclass the class with protected you will have access. none of the examples above show the 'meccano' from outside the package.

[Tolls]

I know all that.
I have been doing this for over 10 years after all.

I was saying I didn't understand what the OP is actually after.

[javanoobe]

OK, these are the instructions for this project.
Create a Vehicle class. Include protected(2 at least) data members of your choice.
Include a constructor that requires a value for each data field and a toString() method that returns a String containing a complete description of the vehicle. Save file as Vehicle.java.

Create two classes Car and Motorcycle that decend from Vehicle.
Supply each with a constructor that sets the Vehicle data fields with values you choose.
Save files as Car.java and Motorcycle.java.

Create an application that instatiates one Car and one Motorcycle.
call the toString() method with each object and display the results.
Save the file as UseVehicle.java.

I think I've done all of that. However, because I'm using the protected word I don't think I'm doing it right. I would think the Car and Motorcycle classes have to access the variables directly from Vehicle and I don't think that's happening with the code I have.


Thank you

javanoobe

[Tolls]

OK.
Looks like you did everything that was asked for in the code in post #4. Stick with that code.

I'm not sure why you've been asked to make the attributes protected, but it could be something you use later on in the course.

[javanoobe]

Thank You. Like some of my other assignments I try to make them more difficult than they have to be.



javanoobe