Each component has an identifier (the getId method). It is created automatically when a component is created. Developers could change it anytime. There is no limitation about how an identifier shall be named. However, if an alphabetical identifier is assigned, developers could access it directly in Java codes and EL expression embedded in the ZUML page.

<window title="Vote" border="normal">
    Do you like ZK? <label id="label"/>    
    <separator/>    
    <button label="Yes" onClick="label.value = self.label"/>    
    <button label="No" onClick="label.value = self.label"/>    
</window>