About references.

[fatabass]

Hi,

I am pretty sure I have read in O'Reilly Series that if there are no references pointing to an object, that object is gets Garbage Collected.

I have this example code from a different book I am reading:

public LabelFrame(){
setLayout( new FlowLayout() )
...
}

So a new FlowLayout object is being created without a reference to it.

How is this possible ? How can I send commands to this new FlowLayout object if I ever require to ?

Thanks.

[Norm]

How can I send commands to this new FlowLayout object if I ever require to

You could ask the container for a reference to its layout manager.
Or you could add an extra line of source code that saves a reference to the layout manager when it is created:
FlowLayout flo = new FlowLayout();
then use flo to access the object.

[fatabass]

Thanks for the answer.

Yes I have tried that, and that helps me understand the way to create an object with a reference.

My question rather is, how do I create an object without a reference and use it ? How is it not garbage collected ?

[Norm]

how do I create an object without a reference and use it ?

If you do not save a reference to an object, you can not use it.

[fatabass]

When you say you can not use it, does it mean I can not reach it ?

Because it looks like i am using it in my LabelFrame.

[Norm]

Your posted code passes a reference to the object to a container. The container has the reference and saves it somewhere.

[sunde887]

To illustrate, LabelFrame will look something like this (or JFrame if it's subclassed):

public class JFrame {
  LayoutManager x;

  setLayoutManager(LayoutManager x){
    this.x = x;
  }
}

In other words -- it does have a reference somewhere. Just not one you declared explicitly.

Edit: This is not necessarily how JFrame actually handles this, but the approach is sure to be similar.

From the API docs:


setLayout

public void setLayout(LayoutManager manager)

Sets the LayoutManager. Overridden to conditionally forward the call to the contentPane. Refer to RootPaneContainer for more information.

Overrides:
setLayout in class Container

Parameters:
manager - the LayoutManager
See Also:
setRootPaneCheckingEnabled(boolean), RootPaneContainer

[fatabass]

Also see learn how Observable pattern works in JAVA.

Java Platform SE 6

public CurrentConditionsDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}

[endotherm]

@Op, if your still unclear on this, I think the issue is the distinction between names and references. Java supports anonymous members (objects that are referenced but have no name), so the distinction is important. in your initial post, the flow layout is being refered to by your main class, so it does not get garbage collected. it just doesnt have a name. as some one said, you can't directly manipulate an object that doesn't have a name. that doesn't mean your form can't though.