parameter.....

[mark smith]

hi

i use a variable in a class
i pass it to a method to another class, the method change the value


first class

    
public class FirstApp {
   public int m_age=15; 
   public void setAge(int p_age){
     m_age=p_age;
   }
   public int getAge(){
     return m_age;
   }
   public static void main (String[] args){
       FirstApp a = new FirstApp();
       SecondApp b = new SecondApp();
       b.setAgeB(a.m_age);
       System.out.println(a.getAge());
   }
}

second class

    
public class SecondApp {
   private int m_ageB;
   public void setAgeB(int p_age){
     m_ageB=p_age * 2 ;
     p_age = m_ageB;
   }
 
   public int getAgeB(){
     return m_ageB;
   }
}

why FirstApp display 15 and not 30?

thanks

[Keith Lynn]

Well the parameter you send to the method is a primitive, and the value of the actual parameter is unaffected by what you do in the method.

[mark smith]

primitive is passing by value and object by reference...

that work with this code

public class FirstApp {
   public int m_age; 
   public void setAge(int p_age){
     m_age=p_age;
   }
   public int getAge(){
     return m_age;
   }
   public static void main (String[] args){
       FirstApp a = new FirstApp();
       a.setAge(15);
       System.out.println(a.getAge());
       SecondApp b = new SecondApp();
       b.setAgeB(a);
       System.out.println(a.getAge());
   }
}
 
public class SecondApp {
   private int m_ageB;
   public void setAgeB(FirstApp tmp){
     m_ageB=tmp.getAge() * 2 ;
     tmp.setAge(m_ageB);
   }
 
   public int getAgeB(){
     return m_ageB;
   }
}

with this code, i need to know the class name for setAgeB.....
if i create a third class with a main i can use SecondApp.... i tried to change FirstApp type in the setAgeB method by Object but i get

SecondApp.java:7: cannot find symbol
symbol : method getAge()
location: class java.lang.Object
m_ageB=tmp.getAge() * 2 ;
^
SecondApp.java:8: cannot find symbol
symbol : method setAge(int)
location: class java.lang.Object
tmp.setAge(m_ageB);

with

m_ageB=tmp.getAge() * 2 ;
tmp.setAge(m_ageB);

any idea?

[Keith Lynn]

Originally posted by mark smith:
primitive is passing by value and object by reference...

All parameter passing in Java is by value.

Also notice that since SecondApp does not extend FirstApp, it does not have the methods of FirstApp.
[ May 15, 2006: Message edited by: Keith Lynn ]

[Henry Wong]

There is a JavaRanch FAQ page that addresses this very issue.

[mark smith]

ok but that don't resolve my problem of my second post...

public class SecondApp {
   private int m_ageB;
   public void setAgeB(FirstApp tmp){
     m_ageB=tmp.getAge() * 2 ;
     tmp.setAge(m_ageB);
   }

setAgeB can only be used by FirstApp object.....
what can i do if i want another objet type use this method (setAgeB)?

thanks

[Keith Lynn]

Do you mean you want to send another object to the method. If the parameter type is FirstApp, then you can only send an object whose type is FirstApp or a subclass of FirstApp.

[mark smith]

Originally posted by Keith Lynn:
Do you mean you want to send another object to the method. If the parameter type is FirstApp, then you can only send an object whose type is FirstApp or a subclass of FirstApp.

ya another object to the method.....

i can't do something like that

public class ThirdApp {
   public int m_age; 
   public void setAge(int p_age){
     m_age=p_age;
   }
   public int getAge(){
     return m_age;
   }
   public static void main (String[] args){
       ThirdApp a = new ThirdApp();
       a.setAge(15);
       System.out.println(a.getAge());
       SecondApp b = new SecondApp();
       b.setAgeB(a);
       System.out.println(a.getAge());
   }
}

[wise owen]

you need to downcast the input parameter which is type "Object" to FirstApp type first.

[mark smith]

Originally posted by wise owen:
you need to downcast the input parameter which is type "Object" to FirstApp type first.

import java.lang.*;
public class SecondApp {
   private int m_ageB;
   public void setAgeB(Object tmp){
     m_ageB=tmp.getAge() * 2 ;
     tmp.setAge(m_ageB);
   }
   public int getAgeB(){
     return m_ageB;
   }
}

i get

SecondApp.java:7: cannot find symbol
symbol : method getAge()
location: class java.lang.Object
m_ageB=tmp.getAge() * 2 ;
^
SecondApp.java:8: cannot find symbol
symbol : method setAge(int)
location: class java.lang.Object
tmp.setAge(m_ageB);

[Jesper de Jong]

Mark, the reason you get that error is because you did not do what wise owen told you:

Originally posted by wise owen:
you need to downcast the input parameter which is type "Object" to FirstApp type first.

You have an Object and you are trying to call the method getAge() on it. This method does not exist in class Object. You need to cast the Object to FirstApp:

((FirstApp) tmp).getAge()

Another solution is to change the type of the parameter of the method setAgeB to FirstApp instead of Object:

public class SecondApp {
    private int m_ageB;
    public void setAgeB(FirstApp tmp){
    // ...

[Henry Wong]

You have an Object and you are trying to call the method getAge() on it. This method does not exist in class Object. You need to cast the Object to FirstApp:

I am not convinced that this is what Mark wants... From his question, I got the impression that he wants to use the methods for any object -- not just firstApp objects that is referenced as Object objects.

The cast will allow it to compile, but will throw an exception at runtime, if he passes any object that is not an instanceof FirstApp object.

Henry