java - Adding same widget to two panel causing issue -

i have created 2 verticalpanel(mainvp,subvp) , 1 textbox(box).textbox(box) added mainvp subvp adding mainvp rootpanel here if lauch application nothing visible though have added textbox(box) verticalpanel(mainvp) added main rootpanel.

 verticalpanel mainvp=new verticalpanel();  verticalpanel subvp=new verticalpanel();  textbox box=new textbox();   mainvp.add(box); //textbox added verticalpanel  subvp.add(box);   rootpanel.get().add(mainvp);//mainvp contains textbox 

can explain me how above code working internally?

your panel subvp isn't added anything, , when box added subvp, removed mainvp. widgets can 1 place @ time.

--

(adding more detail comment posted)

this part of basic assumption of how widget works - isn't "stamp" can put on page in several places, instead represents 1 or more dom elements, , wraps them easier use , reuse. each widget exposes event handlers can listen to, , ways change widget looks like.

imagine if had 2 buttons on page, , need them same thing. when looking @ page, there 2 buttons, , while same , cause same action, aren't same thing.

consider how button might work internally - lets has check see if mouse hovered, , if display tooltip, or change color. if same widget rendered in 2 places, both change.

from api side: widget has method getparent(), returns parent widget. if add widget more 1 place @ time, either getparent() wouldn't possible, or need return list.

panel likewise has indexof, if add widget more 1 parent, follow add same widget same parent multiple times - indexof return?

finally, implementation achieve this. javadoc panel.add:

/**  * adds child widget.  *   * <p>  * <b>how override method</b>  * </p>  * <p>  * there several important things must take place in correct  * order add or insert widget panel. not of these steps  * relevant every panel, of steps must considered.  * <ol>  * <li><b>validate:</b> perform sanity checks ensure panel can  * accept new widget. examples: checking valid index on insertion;  * checking panel not full if there max capacity.</li>  * <li><b>adjust reinsertion:</b> panels need handle case  * widget child of panel. example: when performing  * reinsert, index might need adjusted account widget's  * removal. see {@link complexpanel#adjustindex(widget, int)}.</li>  * <li><b>detach child:</b> remove widget existing parent, if  * any. panels call {@link widget#removefromparent()} on  * widget.</li>  * <li><b>logical attach:</b> state variables of panel should  * updated reflect addition of new widget. example: widget  * added panel's {@link widgetcollection} @ appropriate index.</li>  * <li><b>physical attach:</b> widget's element must physically  * attached panel's element, either directly or indirectly.</li>  * <li><b>adopt:</b> call {@link #adopt(widget)} finalize add  * last step.</li>  * </ol>  * </p>  *   * @param child widget added  * @throws unsupportedoperationexception if method not supported (most  *           means specific overload must called)  * @see haswidgets#add(widget)  */ public void add(widget child) 

finally, default implementation used panels in gwt @ heart (complexpanel.add):

// detach new child. child.removefromparent();  // logical attach. getchildren().add(child);  // physical attach. dom.appendchild(container, child.getelement());  // adopt. adopt(child); 

there other implementations well, boil down this, in keeping guidelines outlined in panel.