gwt class

/**
 * Entry point classes define <code>onModuleLoad()</code>.
 */
public class Testdialog2 implements EntryPoint {

    /**
     * This is the entry point method.
     */
    public void onModuleLoad() {

        final TextBox nameField = new TextBox();
        final Button sendButton = new Button("Send");
        sendButton.addStyleName("sendButton");
        sendButton.addClickHandler(new ClickHandler() {

            @Override
            public void onClick(ClickEvent event) {
                sendMessage(nameField.getValue());
            }
        });

        nameField.setText("Message To Send Via postMessage");

        RootPanel.get().add(sendButton);
        RootPanel.get().add(nameField);

        nameField.setFocus(true);
        nameField.selectAll();

    }

    /**
     * Specifies what the origin of otherWindow must be for the event to be dispatched, either as the literal string "*"
     * (indicating no * preference) or as a URI. If at the time the event is scheduled to be dispatched the scheme,
     * hostname, or port of otherWindow's document does not match that provided in targetOrigin, the event will not be dispatched;
     * only if all three match will the event be dispatched. This mechanism provides control over where messages are sent;
     * for example, if postMessage was used to transmit a password, it would be absolutely critical that this
     * argument be a URI whose origin is the same as the intended receiver of the message containing the password,
     * to prevent interception of the password by a malicious third party. Always provide a specific targetOrigin,
     * not *, if you know where the other window's document should be located. Failing to provide a specific
     * target discloses the data you send to any interested malicious site.
     * @param message
     */
    public native static void sendMessage(String message) /*-{
        $wnd.parent.postMessage(message, "*");
    }-*/;
}