public class GenFieldGroupHandler extends AbstractHandler {

	@Override
	public Object execute(ExecutionEvent event) throws ExecutionException {
		
		IEditorPart editor=HandlerUtil.getActiveEditor(event);
		ISelection selection=HandlerUtil.getCurrentSelection(event);
		IEditorInput input=HandlerUtil.getActiveEditorInput(event);
		if (editor instanceof XtextEditor & selection instanceof TextSelection)
		 {
			XtextEditor textEditor=(XtextEditor)editor;
			TextSelection textSelection = (TextSelection) selection;		
			
			
			System.out.println("Gen Field Group Handler is executed.");
			 
		 }
		 
		return null;
	}

	

}

XtextEditor textEditor=(XtextEditor)editor;
			TextSelection textSelection = (TextSelection) selection;		
			
			 IDocumentProvider dp = textEditor.getDocumentProvider();
			 IDocument doc = dp.getDocument(editor.getEditorInput());
			 int offset = textSelection.getOffset();
			 int startLine=textSelection.getStartLine();
			 int endLine=textSelection.getEndLine();
			 
			 try {
				String pasteText="replacement text\n";
				doc.replace(offset, 0, pasteText);
			} catch (BadLocationException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}