Name: rl43681 Date: 06/17/2003


A DESCRIPTION OF THE REQUEST :
The method java.awt.CardLayout.show(Container,String) does nothing when the String (the key) does not match any Component in the Container. Furthermore, no exception is thrown, and the method has no return value. Thus, the client code using show() cannot know whether a call were successful or not.


JUSTIFICATION :
A Container may be used to display a potentially large number of Components in a mutually-exclusinve way using a CardLayout. When the number of Components is large, and only a subset are likely to be displayed during program lifetime, an application can optimize memory- and window-system-resource usage by only creating Components (and adding them to the Container) as required/on-the-fly.

However, since the CardLayout gives no feedback about a failed show()-attempt, it does not support such an optimizing strategy (if it did, then the application program could react to the failure by creating and adding the missing Component, and then retrying the show()).

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
It would be antisocial to modify the existing show()-method (a change to return type, or throwing a checked exception would both break existing code; throwing an unchecked exception, such as IllegalArgumentException, would cause existing applications to suddenly die), but two backwards-compatible possibilities would be:

1. Add a checkedShow(Container,String) method, which either returns a boolean value, or throws a CheckedException, should the wished key not be found.

2. Add a checkWhetherPresent(String) method which simply returns a boolean, indicating the success or failure of the key-lookup.

[I might be wrong about changing the return type of the existing show() from void to boolean -- that probably wouldn't break existing code per se, but it seems bad style to modify return types, even when no ill effects ensue.]
ACTUAL -
  From the Javadoc for show():

* Flips to the component that was added to this layout with the
* specified name, using addLayoutComponent.
* If no such component exists, then nothing happens.



---------- BEGIN SOURCE ----------
import java.awt.CardLayout;
import java.awt.Container;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class CardLayoutTester extends JFrame {
  public final static int EXIT_ON_CLOSE = 3;
  public CardLayoutTester(){
    final Container c = getContentPane();
    final CardLayout l = new CardLayout();

    c.setLayout(l);
    c.add(new JLabel("Hello"), "Hello");
    c.add(new JLabel("World"), "World");

    // Intentional misspelling of key
    l.show(c, "XXX");
    // We have no way of knowing, that the show() had no visual effect
  }

  public static void main(final String[] args) {
    final JFrame f = new CardLayoutTester();
    f.pack();
    f.setDefaultCloseOperation(EXIT_ON_CLOSE);
    f.setVisible(true);
  }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Clearly, it's possible for the application code to implement its own monitoring of whether components have been added or not, but this is precisely the sort of information which the CardLayout could provide to its client code at little extra cost.
(Review ID: 188314)
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