GWT extending DecoratorPanel and adding to rootPanel

Question

I am new to using gwt, although I am starting to get the hang of things. One perplexing situation I came across today was the following. (This is abbreviated to simplify my question)

  public class MyPanel extends DecoratorPanel{

    public MyPanel(){

    final TextBox tb = new TextBox();
    tb .setText("test");

    this.add(tb);

    tb .setFocus(true);

         }

    }






public class ClassA implements EntryPoint(){

    public void onModuleLoad(){

        MyPanel mp = new MyPanel();
        RootPanel.get("reference").add(mp);
      }


    }

for some reason my problem occurs at the line:

RootPanel.get("reference").add(log);

I get a null pointer exception...the lines of the stack above the line that points at the line above is:

java.lang.NullPointerException: null
    at com.google.gwt.user.client.ui.Panel.doAttachChildren(Panel.java:163)
    at com.google.gwt.user.client.ui.Widget.onAttach(Widget.java:259)
    at com.google.gwt.user.client.ui.Widget.setParent(Widget.java:393)
    at com.google.gwt.user.client.ui.Panel.adopt(Panel.java:119)
    at com.google.gwt.user.client.ui.ComplexPanel.add(ComplexPanel.java:86)
    at com.google.gwt.user.client.ui.AbsolutePanel.add(AbsolutePanel.java:80)

So I thought this would just be a simple problem however, when I put all the code from MyPanel into ClassA in the onModuleLoad method and just created a DecoratorPanel in there, there was no null pointer exception. Why is this, and how can I fix it. It seems like a simple problem but I am not sure which direction to go. Anyone have an idea?

The null pointer exception happens on this line in Panel:

for (Iterator it = iterator(); it.hasNext();) {

Answer 1

The above code works for me without any problem.Can you post the version of gwt and OS you are using.Make sure that you are having following DIV in your html.

<div id="reference"></div>

If everything is fine still it is not working means try the following

 RootPanel.get().add(mp);

If above all fails log a bug in http://groups.google.com/group/Google-Web-Toolkit?pli=1 that website

Answer 2

As BlackPanther said, the most likely error is that you are missing <div id="reference"></div> or some other HTML element with that id in your base html page.

I've tried

    /**
 * This is the entry point method.
 */
public void onModuleLoad() {
 // MyPanel mp = new MyPanel();
 DecoratorPanel mp = new DecoratorPanel();
 final TextBox tb = new TextBox();
 tb.setText("test");
 mp.add(tb);
 tb.setFocus(true);
 RootPanel.get("reference").add(mp);
}

and it will throw the same error.

RootPanel.get("reference")

is returning null as the element is not found in the HTML page.

Answer 3

So, it was due to a an incorrect jar in the library I believe. Either way it works now. I have one other question though. How can you add a panel to the root panel from a class that doesnt have onModuleLoad. Is it possible?