code: public void setActivePage(DialogPage activePage) {
  		DialogPage oldPage = this.activePage;
  
  		Assert.isTrue(activePage == null || pages.contains(activePage));
  		if (oldPage == activePage) {
  			return;
  		}
  		this.activePage = activePage;
  
  		updateMessage();
  		if (oldPage != null) {
  			updatePageLabels(oldPage);
  		}
  		if (activePage != null) {
  			updatePageLabels(activePage);
  		}
  	}
docs: Sets the active page of this CompositeDialogPage.
  
  @param activePage the page to be made active. Must be one of the child
  pages.