从作为第一个对象中的字段的对象访问对象的字段

Question

I was wondering if it is possible in java to access the fields of the object from one of its own fields that is an object itself. Let me present a sample code:

Suppose I have a class (I am condensing the code here, so I know the presented code will not compile)

public class Driver{
// constructor
public Driver(){
}
// method
public double findArea(){
return ???; <<--line #1 in question  How do I get the length and width fields in here?
}
}// end driver class

public class Car{
 public double length; 
 public double width;
 public Driver me;
public Car(){
length=20.5;
width=11.2;
me = new Driver();
}
}

So in the main section, we would have something like

public static void main(String arg[]){

car = new Car();
car.me.findArea();  << ---line #2 in question
}

so in the line #2 in questions, a public method .findArea() would execute which then needs somehow to tap into the calling object's fields and pull the numeric values (line #1 in question).

Is it at all possible through Java? If not, what would be the best conceptual solution to this problem?

thanks!!

Answer 1

If I understand your question correctly...

No, there's no way of navigating backwards from an instance of Driver to a Car which has a driver field referring to it. After all, there could be several such Car instances, or none at all.

If you want the Driver to know which Car it's part of, you'll need to have a field in Driver to represent that information, and then set it by (say) passing in this from Car to the Driver constructor. Be careful though - allowing this to escape within a constructor (you call the Driver constructor from the Car constructor) can be dangerous, as you're advertising the existence of an object before it's fully initialized.

Answer 2

You need to have a reference to the first object. You could do something like:

public class Driver {

    private Car car;

    public Driver(Car car){
        this.car = car;
    }

    public double findArea(){
        return car.getLength() * car.getWidth();
    }

}

public class Car {

    public double length; 
    public double width;
    public Driver me;

    public Car(){
        length=20.5;
        width=11.2;
        me = new Driver(this);
    }

    // getters and setters

}

Answer 3

Other's have already answered your question. I would just like to point out that public data members are considered a Bad Thing(TM) in Object Oriented programming. As Francisco illustrates, you should use getters and setters instead and change the member variables themselves to private access.

With that said, I believe that your design is not the best. Why does the Driver calculate the area of the Car? Shouldn't the Car be responsible for calculating its own area, especially since it already knows its width and length? If you designed the code this way, you would completely erase the issue that causes the problem in your original question.