Listen to ChangeListener
package com.java2s.gwt.client; import com.google.gwt.core.client.EntryPoint; import com.google.gwt.user.client.Window; import com.google.gwt.user.client.ui.Button; import com.google.gwt.user.client.ui.ClickListener; import com.google.gwt.user.client.ui.RootPanel; import com.google.gwt.user.client.ui.Widget; import com.google.gwt.user.client.ui.ChangeListener; import com.google.gwt.user.client.ui.ListBox; public class GWTClient implements EntryPoint,ChangeListener { ListBox list = new ListBox(); public void onModuleLoad() { list.setVisibleItemCount(10); list.setMultipleSelect(true); list.addItem("A"); list.addItem("B"); list.addItem("C"); list.addItem("D"); list.addChangeListener(this); RootPanel.get().add(list); } public void onChange(Widget sender) { if (sender == list) { Window.alert(Integer.toString(list.getSelectedIndex())); } } }