Passing a refrence to another Class vs method

[Chadd Franck]

Hello thanks for your time,

How come when I pass a copy of a refernce to a method, I don't have to create a new type of the refernce inside the method and set it equal to the name of the refrence?

It won't compile this way,

 
import javax.swing.*;
 
public class ClassOne 
{    
    public static void main (String[] args)
    {
        ClassTwo class2 = new ClassTwo();
        ClassOne class1 = new ClassOne(class2);
        class1.methodOne();
    }
 

    public ClassOne (ClassTwo class2)
      {
           
      {
   
    public void methodOne()
    {
        System.out.println(class2.buttonOne.getText());
    }
}

 
import javax.swing.*;
 
   public class ClassTwo
{
    JButton buttonOne = new JButton("Button Clicked");
}
 

It does compile this way;

public class ClassOne 
{
    
    
    public static void main (String[] args)
    {
        ClassTwo class2 = new ClassTwo();
        ClassOne class1 = new ClassOne();
        class1.methodOne(class2);
    }
   
    public void methodOne(ClassTwo class2)
    {
        System.out.println(class2.buttonOne.getText());
    }
}

[Henry Wong]

It won't compile this way,

public ClassOne (ClassTwo class2)
{
}
 
public void methodOne()
{
    System.out.println(class2.buttonOne.getText());
}

The class2 parameter to the constructor, is local to the constructor. It goes out of scope, once the constructor finishes. It is certainly not in scope during the methodOne() call.

If you want the methodOne() method to have access to the object passed in the constructor, perhaps you should consider saving it -- maybe in an instance variable.

Henry

[Henry Wong]

Meaning, something like this will work...

import javax.swing.*;
 
public class ClassOne 
{    
    public static void main (String[] args)
    {
        ClassTwo class2 = new ClassTwo();
        ClassOne class1 = new ClassOne(class2);
        class1.methodOne();
    }
 
    private ClassTwo class2;
    public ClassOne (ClassTwo class2)
    {
        this.class2 = class2;
    }
   
    public void methodOne()
    {
        System.out.println(class2.buttonOne.getText());
    }
}

Henry

[Chadd Franck]

I couldn't understand why it would be local to the constructor,
but that most certainly is the way it acts. So if I understand it correctly
while a constructor helps build the class, it's scope is limited to the duration of the construction phase.

Thanks for your time;

[Ernest Friedman-Hill]

Originally posted by Chadd Franck:
I couldn't understand why it would be local to the constructor,
but that most certainly is the way it acts. So if I understand it correctly
while a constructor helps build the class, it's scope is limited to the duration of the construction phase.

Names declared as constructor parameters or local variables in a constructor are all local to the constructor. In this sense, constructors are exactly like any other method. Why is this surprising?

[Henry Wong]

Originally posted by Ernest Friedman-Hill:

Names declared as constructor parameters or local variables in a constructor are all local to the constructor. In this sense, constructors are exactly like any other method. Why is this surprising?

Agreed. This has nothing to do with constructors. Methods can break too, if you try to use something that is not in scope. For example, this won't compile either...

public class ClassOne 
{
    public static void main (String[] args)
    {
        ClassTwo class2 = new ClassTwo();
        ClassOne class1 = new ClassOne();
        class1.methodOne(class2);
        class1.methodTwo();
    }
   
    public void methodOne(ClassTwo class2)
    {
    }
 
    public void methodTwo()
    {
        System.out.println(class2.buttonOne.getText());
    }
}

Henry

[Chadd Franck]

Names declared as constructor parameters or local variables in a constructor are all local to the constructor. In this sense, constructors are exactly like any other method. Why is this surprising?

Because if you pass a parameter to a method the parameter lasts for the life of the method, so when you pass a parameter as an argument to a class I thought it should pass a parameter to the class that would be used for the life of the class. I see now that it doesn't, I thought the constructor was used as a workaround to allow the class have the same scope
parameters of the method. I see that it can still be used this way but you have to create a objRefernce or a new object, and just set this parameter in the constructor. (probably with a this. constructor).

Thanks again.