Shutdown the system using java code

I want to shutdown my computer using java code. The following code has no error but its not shutting down my system.

import javax.swing.JOptionPane;
public class Shutdown{
public static void main(String[] args){
try{
Runtime rt = Runtime.getRuntime();
Process pr = rt.exec("cmd /c shutdown -s");
} catch (Exception ew){
JOptionPane.showMessageDialog(null, ew.toString());
ew.printStackTrace();
}
}
}

When Runtime.exec() won't

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