Suffering around a class name notation

Question

I have downloaded a simple package source files from the Net.
Their original package structure was:
CWD - aa - bb - example - TestApp.java
              - xx - yy - Model.java
                        - Widget.java
                        - WidgetUI.javaTestApp.java had a package declaration:
package aa.bb.example;
// and import stmts for Model and WidgetOther three had another package declaration:
package aa.bb.xx.yy;And, in the Widget.java:
  public static final String uiClassID = "aa.bb.xx.yy.WidgetUI";
  public String getUIClassID(){
    return uiClassID;
  }They were all fine. They compiled and ran normally in the specified
package/directory structure. But a small silly surprise was: the command
line javac aa/bb/example/TestApp.java didn't compile WidgetUI.
WidgetUI is logically referenced from Widget but the compiler didn't
care that type of referencing.
Here follows the story of my long silly suffering. I had flattened the
whole package structure of the package in order to make them simple
teaching material. The new structure was:
CWD - TestApp.java
    - Model.java
    - Widget.java
    - WidgetUI.javaAnd in the Widget.java:
  public static final String uiClassID = "WidgetUI";They compiled fine without smallest glitches. However, got runtime error:
UIDefaults.getUI() failed: no ComponentUI class for: Widget
......The Widget wasn't drawn on the screen because Java could not find the UI.
After several hours of futile struggling, I tried below without any ground
nor confidence:
  public static final String uiClassID = "/WidgetUI";Then, suddenly the success came.
Why oh why "WidgetUI" failed and "/WidgetUI" succeeded for a class name
from the unnamed package? Why?

Answer 1

Thanks for the feedback.
Another user Pete had the same issue
(see SQL format problem
and it was fixed today.
As for the format width(132 lines) Ill see if I can get this fixed for the next release.
Regards,
Dermot.