Using an int's value in one class in a different class

[mainy]

Hello,

Im stuck with something relativly simple, but it is causing me so many problems.

Basically, I have two classes in one package. Lets say classA has an integer called int number = 2. How do I in totally different class called classB put system.out.println(number);.

It will bring up an error as "number" is not recognised in classB. How do I make this point to classA? I have tried to read through the java website but it is making me mighty confused.

Thank you

[dswastik]

You either need to create an instance of class A in the method of Class B where you wish to access the variable or your class B should inherit class A, but in either class the variable should not be private.

e.g.

class A{
  int x=10;
}

class B{
   void show(){
        A a1=new A();
        System.out.println(a1.x);
   }
}

or

class A{
  int x=10;
}

class B extends A{
  void show(){
        System.out.println(x);
  }

}

[mainy]

Ok, I got that working in a small test enviroment, but when I apply it to my actual code it doesnt seem to work. Any ideas?

This is in one class, say class A

  
public class ClassA{
    public static void main(String[] args) {

String text = "40 3 2 4 2 end";
Scanner s = new Scanner(text);
double arg = 0;
double tot = 0;
  
while (s.hasNext() ) {             
if (s.hasNextDouble()) {       
            double x = s.nextDouble();
            arg += 1;
            tot += x;
            }
if (s.hasNext("end")){
s.close();
}

This is in the other, say ClassB.

public class ClassB{
   public static void main(String[] args) {

        ClassA ca = new ClassA();
        System.out.println(ca.tot);

}  }

I get an error saying "cannot resolve method println" and "cannot resolve symbol tot". I have no idea what is wrong.

[MK12]

Its because you declared tot in a local variable inside the main method, not as field. Try declaring outside any methods.
-MK12

[mainy]

Sorry to be a pain, I should really read up on my foundation java before tackling more indepth stuff. This is my first Object Oreintated language and I jumped in the deep end.

I need the main method otherwise for some reason, my scanner script won't work. How do I apply the tot variable outside of any "methods?"

Sorry.. I don't really understand the Java lingo at the moment, heh ^^ You have to spell it out for me even simpler.

[MK12]

like this:

class Example {
     [COLOR="orange"]private[/COLOR] double inaccessablefieldbesidesthisclass = 7;
     [COLOR="Orange"]public[/COLOR] double getField() {
          return inaccessablefieldbesidesthisclass;
     }
     public static void main(String[] args) {
          double inaccessablelocalvariablebesidesthismethod = 10;
     }
}

That shows local variables and fields. Note that the field is private, so it can only be used in the class, but then I provided a public accessor method which returns the field. You could just make the field public, but this way is better.
-MK12

[mainy]

lol, I am so confused right now. I have also read several java tutorials on the web about classes and in my BIG JAVA book by Cay Horstman. I just dont understand any of this.

I know public class contains data and methods. I know a class can be instantiated by other other classes. I also know the different between public/private ect. I just don't know why it cant be simple to just take a value in a variable from one class and put it in another.

I tried

public double getField(){
return tot;
}

But i still got errors with the tot not being recognised. It would help if I knew what public double getField() did, just ive been trying to read up how these "contructors" work.. . even if they are constructors.. I have no idea.

This is getting me really stressed :( I need a break

[CJSLMAN]

I thought that was going to happen... OK,the quick and dirty solution is:

public class ClassA{
[B][COLOR="Blue"]double tot = 0;[/COLOR][/B]
    public static void main(String[] args) {
.
.
.

The above puts the tot variable within the scope of all the class. Use the code that you had before in your original class B and it should work.

Now , what MK12 was trying to tell you was that the correct way to expose a variable (specially private type) to other classes is to use an accessor method ("accessor" comes from the word "access"). If you want to use it, you have to call the accessor method, not the variable:
Class A:

public class ClassA{
double tot;
     
public double [B][COLOR="Blue"]getTotal()[/COLOR][/B] {
return tot;
}

public static void main(String[] args) {
.
.
.

Class B:

public class ClassB{
   public static void main(String[] args) {

        ClassA ca = new ClassA();
        System.out.println(ca.[B][COLOR="Blue"]getTtotal()[/COLOR][/B]);

}  }

Hope this helped...

Luck,
CJSL

[mainy]

Thank you for your help. I have got it working, but only to some extent.

With the changes:

Class A:

public class ClassA{

        double tot = 0;

public static void main(String[] args) {

String text = "40 3 2 4 2 end";
Scanner s = new Scanner(text);
double arg = 0;
double tot = 0;
  
while (s.hasNext() ) {             
if (s.hasNextDouble()) {       
            double x = s.nextDouble();
            arg += 1;
            tot += x;
            }
if (s.hasNext("end")){
s.close();
}

Class B:

public class ClassB{
public static void main(String[] args) {

        ClassA ca = new ClassA();
        System.out.println(ca.tot);

}  }

This prints out 0.0, as the value of tot is intialised as 0. How do I print out the value of tot once it has gone through the while loop?


Edit: Just to let you know, it also does this if I use the other method suggested.

[CJSLMAN]

Call class B from class A after tot has processed.

Luck,
CJSL

[mainy]

How do I tell the program to do that?

This is what I have so far:

public class ClassA{

   double tot;

         public double getTotal() {
         return tot;
        }
        
public static void main(String[] args) {

String text = "40 3 2 4 2 end";
Scanner s = new Scanner(text);
double arg = 0;

ClassA ca = new ClassA();
 
while (s.hasNext() ) {             
if (s.hasNextDouble()) {       
            double x = s.nextDouble();
            arg += 1;
            ca.tot += x;
            }
if (s.hasNext("end")){
s.close();
}

I think this should work, but the problem is its ClassB that is actually executed. I think for some reason, Its pulling the value of tot without actually making ClassA do anything to it.

[mainy]

hello? sorry im a pain :( I am really trying to learn this stuff.

[CJSLMAN]

Sorry... but I'm working right now. I would suggest the following link and I'll try to get back to you later:
Understanding Instance and Class Members (The Java™ Tutorials > Learning the Java Language > Classes and Objects)

Luck,
CJSL

[StormyWaters]

It looks like to me you are trying to run two main methods, and although this is possible, this is not what you are looking for.

You should be able to reduce them so that there is only one, and the other should become a method.

Try pulling out the main method of class A and making it a public method that can be called on the instance of Class A in the main method of class B

[mainy]

Its ok, ive kind of given up on this project. It is due in tommorow, i did the most I could but I was stuck on a few of the questions, all of which required passing of information. Just a shame I couldn't get more marks.

I definatly think this is an area I need to look into though, I will make it my own task to try and master it.

Thank you for everybodys input.

[paul pasciak]

The request is simple:

Demonstrate how a method in one object can
manipulate, or at least print a value in
another object.

To do this I'll use three classes,

A class named Arc, which has a launching
point, universaly known as:

public static void main(String[] args){
  ..
}

and its two stooge classes, named A and B.

The code, so far, is below. If you are working
with Windows, you can put all three classes in
one file, in this order, and it will compile.

public class Arc{
  public static void main(String[] args){
  }
}


class A{
}


class B{
}

But you want an object 'A' to print a 'number'
in an object 'B'. So next, I'll add code for
that:

public class Arc{
  public static void main(String[] args){
  }
}


class A{
  public void aPrintB(){
    System.out.println(number);
  }
}


class B{
  int number = 102;
}

NOW IT WON'T COMPILE! The compiler tells
us class A has been given the impossible
task of printing a value "number", which
does not exist in its scope.

I'll fix this by adding a constructor to
class A. A constructor specifies how I
want an object to be initialized, or
'instanciated'.

Upon instanciation I want object 'A' to
take a reference to an object 'B', and
to store it in a variable: "referenceToB"

public class Arc{
  public static void main(String[] args){
  }
}


class A{
  B referenceToB;             // B reference

  A(B objTypeB){              // constructor
    referenceToB = objTypeB;  // constructor
  }                           // constructor

  public void aPrintB(){
    System.out.println(number);
  }
}


class B{
  int number = 102;
}

..and still the same compiler error,
because object 'A' still does not know where
the hey to find "number".

I have corrected this below. Object 'A' will
print out a value "number", specifically
from an object of type B. This code will
compile.

public class Arc{
  public static void main(String[] args){
  }
}


class A{
  B referenceToB;             // B reference

  A(B objTypeB){              // constructor
    referenceToB = objTypeB;  // constructor
  }                           // constructor

  public void aPrintB(){
    System.out.println(referenceToB.number);
  }
}


class B{
  int number = 102;
}

Now run it using the command:

java Arc

(Remember, "Arc" contains the entry point
for this code, "public static void main( etc."

You'll get absolutely no results. Look at Arc,
it has absolutely no references to A or B!
It has no code at all, besides the launch
point!
So I'll have Arc 'instanciate' an "a" and "b"
object, and in it's main I'll have it prod
the "a" object for output from "b".
The following code will print the value of
"number" when you run Arc.

public class Arc{
  B b = new B();
  A a = new A(b);

  public static void main(String[] args){
    Arc arc = new Arc();
    arc.a.aPrintB();
  }
}


class A{
  B referenceToB;             // B reference

  A(B objTypeB){              // constructor
    referenceToB = objTypeB;  // constructor
  }                           // constructor

  public void aPrintB(){
    System.out.println(referenceToB.number);
  }
}


class B{
  int number = 102;
}

Here is similar code using static values.

public class Arc{
  public static void main(String[] args){
    B b = new B();
    A a = new A(b);
    a.aPrintB();
  }
}


class A{
  B referenceToB;             // B reference

  A(B objTypeB){              // constructor
    referenceToB = objTypeB;  // constructor
  }                           // constructor

  public void aPrintB(){
    System.out.println(referenceToB.number);
  }
}


class B{
  int number = 102;
}