Add override to actionPerformed().

src/main/java/net/themusicinnoise/vaccalc/VacCalc.java

@@ -20,6 +20,7 @@ public class VacCalc extends JFrame {
         JMenu appMenu = new JMenu("VacCalc");
         JMenuItem exitButton = new JMenuItem("Exit");
         exitButton.addActionListener(new ActionListener() {
+            @Override
             public void actionPerformed(ActionEvent e) {
                 System.exit(0);
             }