ArrayList in Object Reference Confusion

[Caglar Cataloglu]

Hi dear javalovers;

I am so confused about an object reference. I am sending an object as parameter to another object`s function. Simply my 2 classes are described below...

Main Class
{
    public List show= new ArrayList();
    
    initComponents(){
    Worker w = new  Worker();
    w.print(this);

    System.out.println(show.size()); [b]---------> result is 2 instead of 0; how can this be happen??[/b]
 }
}

Worker Class
{
   public List showed= new ArrayList();
   
   print(Main mainobject){

     showed= this.show;
     showed.add(0);
     showed.add(1); //now goes back to main class
  }
}

result println should be 0 instead of 2; but result is 2; how can it be happen?

[Jim Hoglund]

Why do you think it should be zero? (Please provide the running code.)

Main Class {
    public List show = new ArrayList();
    initComponents() {
        Worker w = new Worker();
        w.print(this);
        System.out.println(w.size()); ---------> result is 2 instead of 0; how can this be happen??
    }
}
Worker Class {
    public List showed= new ArrayList();
    print(Main mainobject) {
        showed = this.show;
        showed.add(0);
        showed.add(1); //now goes back to main class
    }
} 

Jim ... ...

[Caglar Cataloglu]

Jim Hoglund wrote:Why do you think it should be zero?

Main Class {
    public List show = new ArrayList();
    initComponents() {
        Worker w = new Worker();
        w.print(this);
        System.out.println(w.size()); ---------> result is 2 instead of 0; how can this be happen??
    }
}
Worker Class {
    public List showed= new ArrayList();
    print(Main mainobject) {
        showed = this.show;
        showed.add(0);
        showed.add(1); //now goes back to main class
    }
} 

Jim ... ...

I am just copying the value of the arraylist in the main class. i am not trying to change it. but it just changes

If i do this.mainobject.show.add(0); inside Worker Class->print method, it will change the arraylist in the main class, but i am not doing that, and it still changes

[Henry Wong]

You need to show us the size() method of the worker class. After all, that is the method that is returning the two, isn't it?

Henry

[Caglar Cataloglu]

Henry Wong wrote:
You need to show us the size() method of the worker class. After all, that is the method that is returning the two, isn't it?

Henry

oh sorry my mistake, that should be show.size(); instead of w.size(). so size method is the method of an arraylist.

[Rajeev Rnair]

public class Main   {  
	public List show = new ArrayList();
    public static void main(String[] args) {   
    	new Main().initComponents();
    }   
        
    void initComponents() {
        Worker w = new Worker();
        w.print(this);
        System.out.println(show.size());// ---------> result is 2 instead of 0; how can this be happen??
    }
}

class Worker{
    public List showed= new ArrayList();
    void print(Main mainobject) {
        showed = mainobject.show;
        showed.add(0);
        showed.add(1); //now goes back to main class
    }
}

Ok , here is the working version of code after fixing the several compile errors!
You are trying to pass a complete object of Main{} class to print() method.
And you are assigning Main.show to Worker.showed. In effect you are passing the reference of the object "showed" to "show"
So whatever changes happen to "show" will happen to "showed" also because both references point to the same "ArrayList" object.
In your case you added two Integer objects to the ArrayList via auto boxing. So the size of the list would be two in both the references.

Hope this is clear and all the best!

[Caglar Cataloglu]

Hi Rajeev,

Thanks for your answer, i understood well. Now i tried to change a String variable in the reference to see if it will also change in the original object, but it seem doesnt change, i dont know why. Here is the working code:

   
import java.util.ArrayList;
import java.util.List;

public class Main   {
    public List show = new ArrayList();
    public String s="OriginalMainString";
    public static void main(String[] args) {
        new Main().initComponents();
    }

    void initComponents() {
        Worker w = new Worker();
        w.print(this);
        System.out.println(show.size());// ---------> result is 2 instead of 0; how can this be happen??
        System.out.println(s);//--------------->Result is OriginalMainString , Why ?
    }
}

class Worker{
    public List showed= new ArrayList();
    String s2="WorkerString";
    void print(Main mainobject) {
        s2=mainobject.s;
        s2="Changed@WorkerClass";
        showed = mainobject.show;
        showed.add(0);
        showed.add(1); //now goes back to main class
    }
}
 

[Rajeev Rnair]

Hi Caglar, String's are immutable and are an exception to the above rule. "String" is an Object, but doesn't work like other Objects. When you use

s2="Changed@WorkerClass";

it creates a new String "Changed@WorkerClass" in the pool and assign the reference to s2. The original String s="OriginalMainString"; doesn't change and will be still in the pool. Please go through JLS for String class, StringBuffer class, StringBuilder class etc

All the best!

[Caglar Cataloglu]

Rajeev Rnair wrote:Hi Caglar, String's are immutable and are an exception to the above rule. "String" is an Object, but doesn't work like other Objects. When you use

s2="Changed@WorkerClass";

it creates a new String "Changed@WorkerClass" in the pool and assign the reference to s2. The original String s="OriginalMainString"; doesn't change and will be still in the pool. Please go through JLS for String class, StringBuffer class, StringBuilder class etc

All the best!

Thanks so much!!