referencing an object

[jon ninpoja]

just toying with reference variables etc

class makeVehicles {
    public static void main(String[] args) {
        Vehicle fourByFour = new Vehicle();
        Vehicle sportsCar = new Vehicle();
        fourByFour.engineSize = 20;
        sportsCar.engineSize = 18;
        fourByFour.passengerCount = 6;
        sportsCar.passengerCount = 2;

        System.out.println("the sports car can carry " + sportsCar.passengerCount + " and has an engine size of " + sportsCar.engineSize);
        System.out.println("the 4x4 car can carry " + fourByFour.passengerCount + " and has an engine size of " + fourByFour.engineSize);

        sportsCar = fourByFour;

        System.out.println("the sports car can carry " + sportsCar.passengerCount + " and has an engine size of " + sportsCar.engineSize);
        //sports car now points to 4x4
    }
}

how would i get sportsCar reference variable to point to where it was originally pointing to?

[jon ninpoja]

sorry full code here

package frombook;

public class Vehicle {
int passengerCount;
int engineSize;
}

class makeVehicles {
    public static void main(String[] args) {
        Vehicle fourByFour = new Vehicle();
        Vehicle sportsCar = new Vehicle();
        fourByFour.engineSize = 20;
        sportsCar.engineSize = 18;
        fourByFour.passengerCount = 6;
        sportsCar.passengerCount = 2;

        System.out.println("the sports car can carry " + sportsCar.passengerCount + " and has an engine size of " + sportsCar.engineSize);
        System.out.println("the 4x4 car can carry " + fourByFour.passengerCount + " and has an engine size of " + fourByFour.engineSize);

        sportsCar = fourByFour;

        System.out.println("the sports car can carry " + sportsCar.passengerCount + " and has an engine size of " + sportsCar.engineSize);
        //sports car now points to 4x4
    }
}

obviously i tried sportsCar = Sportscar but i knew that wouldnt do it as it is already pointing to fourByFour,so its almost like a duplicate

[Jim Ryan]

With this code, there is no way to get sportsCar to point to its original object, because that object's location is no longer known to the code.

You would have to make a significant change. You would have to keep track of the original object like this:

Vehicle tempReference = sportsCar;// This will keep track of the original object.

sportsCar = fourByFour;
System.out.println("the sports car can carry " + sportsCar.passengerCount + " and has an engine size of " + sportsCar.engineSize);
//sports car now points to 4x4

//Point sportsCar back at its original object by making it point at what tempReference is pointing at:
sportsCar = tempReference;

Now sportsCar points at its original object.

[jon ninpoja]

thanks jim...so basically you pointing it to another (temporary) variable...that then can be pointed back...makes sense

[Jim Ryan]

Think of sportsCar as a pointer, as a finger pointing at a vehicle. You want to point it at something else for a while, so you create another finger, "tempReference," and point it at that vehicle. Now you have two fingers pointing at the same vehicle. Now you are free to point sportsCar at something else for a while. Later, you can come back and point sportsCar at what tempReference has been patiently pointing at while waiting for sportsCar to return. Now you have the two fingers pointing at the same vehicle again. At this point tempReference is no longer needed.

[jon ninpoja]

would you typically call it tempreference? or could ou call it originalSportsCar ...or it doesnt really matter..just trying to stick to convention
thanks for the help...

[Paul Clapham]

jon ninpoja wrote:would you typically call it tempreference?

I'd be unlikely to write that code, so I wouldn't have to make that decision. If I had to write some code which changed a variable to refer to object X and which then changed it back to refer to the original object -- which is very unlikely -- I would decide then. Made-up examples which don't bear much relationship to reality aren't good tools for learning that sort of thing.

[salvin francis]

I agree with Paul. Why exactly would you need such a scenario ?

Here is a equivalent question to what you are asking without the use of OOP :

public class Main {
    public static void main(String[] args) {
        int number = 10; //-- some original value
        number = (int) (Math.random() * 100);
        System.out.println("Number is " + number);

        //-- How do I get the original value of number ?
    }
}

The answer is simple, if you want a reference/value, you keep it.

public class Main {
    public static void main(String[] args) {
        int number = 10; // -- some original value
        int originalValue = number;
        number = (int) (Math.random() * 100);

        System.out.println("Number is " + number);
        System.out.println("Number was " + originalValue);
    }
}

[jon ninpoja]

hey guys

i m self studying and dont work in the industry...i was watching a tutorial that explained these concepts
it used a reference to one object and then another
i had no idea that you would never have to point a reference variable back to the original object,i was just wondering if this could be done...but after what you guys are saying, it doesnt seem necessary...
thanks

[salvin francis]

Hi Jon,
In languages such as C, C++ the programmer is responsible to allocate memory, access them using pointers and then deallocate memory.

In Java, an object can be pointed to by using a reference. If an object does not have any references pointing to it, its eligible to be "deallocated" or garbage collected. Hence, we loose the reference to it. This frees the programmer from worrying about memory allocation and leaves it to java.

On a side note, it's cool to know that you are self studying Java. I would suggest that you start writing small programs to help you understand java better.
You could start with simple "Guess a number" program, then increase complexity as you go further. You could soon catchup with the industry guys ;)

[Paul Clapham]

jon ninpoja wrote:i had no idea that you would never have to point a reference variable back to the original object,i was just wondering if this could be done...but after what you guys are saying, it doesnt seem necessary...

I'm not saying "never"... but the example you posted wasn't all that realistic. And in Java it's the objects which are the things you need to work with. The variables are just tools which help you access the objects. So typically you set a variable to refer to an object and then you work with the object. Then using that variable to refer to another object and then using it to refer to the first object again, that just seems like why wouldn't you just use two variables? But my imagination isn't infinite so there certainly could be use cases for that.

[jon ninpoja]

thanks guys

yes written the guess the number game

written a few programs now

thanks for the encouragement