z.B.:

import com.google.gwt.core.client.GWT;

   ...

   button.addClickHandler(new ClickHandler() {
      public void onClick(ClickEvent event) {
        GWT.log("User Pressed a button.", null); // Added debugging message
        if (label.getText().equals(""))
          label.setText("Hello World!");
        else
          label.setText("");
      }
    });