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How to Write a Focus Listener

Focus events are fired whenever a component gains or loses the keyboard focus. import java.awt.event.FocusEvent; import java.awt.event.FocusListener; import javax.swing.JFrame; import javax.swing.JTextField; public class UsingFocusListener {   public static void main(String[] a) {     JFrame frame = new JFrame();     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);     JTextField textField = new JTextField("A TextField");     textField.addFocusListener(new FocusListener() {       public void focusGained(FocusEvent e) {         displayMessage("Focus gained", e);       }       public void focusLost(FocusEvent e) {         displayMessage("Focus lost", e);       }       void displayMessage(String prefix, FocusEvent e) {         System.out.println(prefix             + (e.isTemporary() ? " (temporary):" : ":")             + e.getComponent().getClass().getName()             + "; Opposite component: "             + (e.getOppositeComponent() != null ? e.getOppositeComponent().getClass().getName()                 : "null"));       }     });     frame.add(textField,"North");     frame.add(new JTextField(),"South");     frame.setSize(300, 200);     frame.setVisible(true);   } }