InputVerifier setup

Below you find the example code for an InputVerifier from the swing tutorial, slightly modified to compile error-free under java 25.

//package misc; import java.util.Set; import java.util.HashSet; import java.awt.*; import java.awt.event.*; import javax.swing.*; import javax.swing.event.*; import javax.swing.text.*; import java.beans.PropertyChangeListener; import java.beans.PropertyChangeEvent; import java.text.*; /** * InputVerificationDialogDemo.java is a 1.4 example that * requires no other files. * * Yet another mortgage calculator. * However, instead of using a formatted text field, * as shown in FormattedTextFieldDemo, this example * uses input verification to validate user input. * This one uses a dialog to warn people when their input is bad. */ public class InputVerificationDialogDemo extends JPanel { //Default values private static double DEFAULT_AMOUNT = 100000; private static double DEFAULT_RATE = 7.5; //7.5 % private static int DEFAULT_PERIOD = 30; //Labels to identify the text fields private JLabel amountLabel; private JLabel rateLabel; private JLabel numPeriodsLabel; private JLabel paymentLabel; //Strings for the labels private static String amountString = "Loan Amount (10,000 - 10,000,000): "; private static String rateString = "APR (>= 0%): "; private static String numPeriodsString = "Years (1-40): "; private static String paymentString = "Monthly Payment: "; //Text fields for data entry private JTextField amountField; private JTextField rateField; private JTextField numPeriodsField; private JTextField paymentField; //Formats to format and parse numbers private NumberFormat moneyFormat; private NumberFormat percentFormat; private DecimalFormat decimalFormat; private DecimalFormat paymentFormat; private NumberFormat integerFormat; private MyVerifier verifier = new MyVerifier(); public InputVerificationDialogDemo() { super(new BorderLayout()); setUpFormats(); double payment = computePayment(DEFAULT_AMOUNT, DEFAULT_RATE, DEFAULT_PERIOD); //Create the labels. amountLabel = new JLabel(amountString); rateLabel = new JLabel(rateString); numPeriodsLabel = new JLabel(numPeriodsString); paymentLabel = new JLabel(paymentString); //Create the text fields and set them up. amountField = new JTextField(moneyFormat.format(DEFAULT_AMOUNT), 10); amountField.setInputVerifier(verifier); rateField = new JTextField(percentFormat.format(DEFAULT_RATE), 10); rateField.setInputVerifier(verifier); numPeriodsField = new JTextField(decimalFormat.format(DEFAULT_PERIOD), 10); numPeriodsField.setInputVerifier(verifier); paymentField = new JTextField(paymentFormat.format(payment), 10); paymentField.setInputVerifier(verifier); paymentField.setEditable(false); //Remove this component from the focus cycle. paymentField.setFocusable(false); paymentField.setForeground(Color.red); //Register an action listener to handle Return. amountField.addActionListener(verifier); rateField.addActionListener(verifier); numPeriodsField.addActionListener(verifier); //Tell accessibility tools about label/textfield pairs. amountLabel.setLabelFor(amountField); rateLabel.setLabelFor(rateField); numPeriodsLabel.setLabelFor(numPeriodsField); paymentLabel.setLabelFor(paymentField); //Lay out the labels in a panel. JPanel labelPane = new JPanel(new GridLayout(0,1)); labelPane.add(amountLabel); labelPane.add(rateLabel); labelPane.add(numPeriodsLabel); labelPane.add(paymentLabel); //Layout the text fields in a panel. JPanel fieldPane = new JPanel(new GridLayout(0,1)); fieldPane.add(amountField); fieldPane.add(rateField); fieldPane.add(numPeriodsField); fieldPane.add(paymentField); //Put the panels in this panel, labels on left, //text fields on right. setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20)); add(labelPane, BorderLayout.CENTER); add(fieldPane, BorderLayout.LINE_END); } class MyVerifier extends InputVerifier implements ActionListener { double MIN_AMOUNT = 10000.0; double MAX_AMOUNT = 10000000.0; double MIN_RATE = 0.0; int MIN_PERIOD = 1; int MAX_PERIOD = 40; String message = null; public boolean shouldYieldFocus(JComponent input, JComponent target) { boolean inputOK = verify(input); makeItPretty(input); updatePayment(); if (inputOK) { return true; } //Beep and then tell whoever called us that we don't //want to yield focus. Toolkit.getDefaultToolkit().beep(); //Pop up the message dialog. message += ".\nPlease try again."; JOptionPane.showMessageDialog(null, //no owner frame message, //text to display "Invalid Value", //title JOptionPane.WARNING_MESSAGE); // vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv //Reinstall the input verifier. - - WHY?? // input.setInputVerifier(this); // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ /* Following lines moved up to beep before the error message. //Beep and then tell whoever called us that we don't //want to yield focus. Toolkit.getDefaultToolkit().beep(); */ return false; } protected void updatePayment() { double amount = DEFAULT_AMOUNT; double rate = DEFAULT_RATE; int numPeriods = DEFAULT_PERIOD; double payment = 0.0; //Parse the values. try { amount = moneyFormat.parse(amountField.getText()). doubleValue(); } catch (ParseException pe) {} try { rate = percentFormat.parse(rateField.getText()). doubleValue(); } catch (ParseException pe) {} try { numPeriods = decimalFormat.parse(numPeriodsField.getText()). intValue(); } catch (ParseException pe) {} //Calculate the result and update the GUI. payment = computePayment(amount, rate, numPeriods); paymentField.setText(paymentFormat.format(payment)); } //This method checks input, but should cause no side effects. public boolean verify(JComponent input) { return checkField(input, false); } protected void makeItPretty(JComponent input) { checkField(input, true); } protected boolean checkField(JComponent input, boolean changeIt) { if (input == amountField) { return checkAmountField(changeIt); } else if (input == rateField) { return checkRateField(changeIt); } else if (input == numPeriodsField) { return checkNumPeriodsField(changeIt); } else { return true; //shouldn't happen } } //Checks that the amount field is valid. If it is valid, //it returns true, otherwise it sets the message field and //returns false. If the change argument is true, set //the textfield to the parsed number so that it looks //good -- no letters, for example. public boolean checkAmountField(boolean change) { boolean wasValid = true; double amount = DEFAULT_AMOUNT; //Parse the value. try { amount = moneyFormat.parse(amountField.getText()). doubleValue(); } catch (ParseException pe) { message = "Invalid money format in Loan Amount field"; return false; } //Value was invalid. if ((amount < MIN_AMOUNT) || (amount > MAX_AMOUNT)) { wasValid = false; if (amount < MIN_AMOUNT) { message = "Loan Amount was < " + integerFormat.format(MIN_AMOUNT); } else { //amount is greater than MAX_AMOUNT message = "Loan Amount was > " + integerFormat.format(MAX_AMOUNT); } } //Whether value was valid or not, format it nicely. if (change) { amountField.setText(moneyFormat.format(amount)); amountField.selectAll(); } return wasValid; } //Checks that the rate field is valid. If it is valid, //it returns true, otherwise it sets the message field and //returns false. If the change argument is true, set the //textfield to the parsed number so that it looks good -- no //letters, for example. public boolean checkRateField(boolean change) { boolean wasValid = true; double rate = DEFAULT_RATE; //Parse the value. try { rate = percentFormat.parse(rateField.getText()). doubleValue(); } catch (ParseException pe) { message = "Invalid percent format in APR field"; return false; } //Value was invalid. if (rate < MIN_RATE) { wasValid = false; message = "Bad value: APR was < " + MIN_RATE; } //Whether value was valid or not, format it nicely. if (change) { rateField.setText(percentFormat.format(rate)); rateField.selectAll(); } return wasValid; } //Checks that the numPeriods field is valid. If it is valid, //it returns true, otherwise it sets the message field and //returns false. If the change argument is true, set the //textfield to the parsed number so that it looks good -- no //letters, for example. public boolean checkNumPeriodsField(boolean change) { boolean wasValid = true; int numPeriods = DEFAULT_PERIOD; //Parse the value. try { numPeriods = decimalFormat.parse(numPeriodsField.getText()). intValue(); } catch (ParseException pe) { message = "Invalid decimal format in Years field"; return false; } //Value was invalid. if (numPeriods < MIN_PERIOD) { wasValid = false; message = "Bad value: Number of years was < " + integerFormat.format(MIN_PERIOD); } else if (numPeriods > MAX_PERIOD) { wasValid = false; message = "Bad value: Number of years was > " + integerFormat.format(MAX_PERIOD); } //Whether value was valid or not, format it nicely. if (change) { numPeriodsField.setText(decimalFormat.format(numPeriods)); numPeriodsField.selectAll(); } return wasValid; } public void actionPerformed(ActionEvent e) { JTextField source = (JTextField)e.getSource(); shouldYieldFocus(source); //ignore return value source.selectAll(); } } /** * Create the GUI and show it. For thread safety, * this method should be invoked from the * event-dispatching thread. */ private static void createAndShowGUI() { //Create and set up the window. JFrame frame = new JFrame("InputVerificationDialogDemo"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); JComponent newContentPane = new InputVerificationDialogDemo(); newContentPane.setOpaque(true); //content panes must be opaque frame.setContentPane(newContentPane); //Display the window. frame.pack(); frame.setVisible(true); } public static void main(String[] args) { /* Use an appropriate Look and Feel */ try { //UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel"); //UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel"); UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel"); } catch (UnsupportedLookAndFeelException ex) { ex.printStackTrace(); } catch (IllegalAccessException ex) { ex.printStackTrace(); } catch (InstantiationException ex) { ex.printStackTrace(); } catch (ClassNotFoundException ex) { ex.printStackTrace(); } /* Turn off metal's use of bold fonts */ UIManager.put("swing.boldMetal", Boolean.FALSE); //Schedule a job for the event-dispatching thread: //creating and showing this application's GUI. javax.swing.SwingUtilities.invokeLater(new Runnable() { public void run() { createAndShowGUI(); } }); } //Compute the monthly payment based on the loan amount, //APR, and length of loan. double computePayment(double loanAmt, double rate, int numPeriods) { double I, partial1, denominator, answer; numPeriods *= 12; //get number of months if (rate > 0.01) { I = rate / 100.0 / 12.0; //get monthly rate from annual partial1 = Math.pow((1 + I), (0.0 - numPeriods)); denominator = (1 - partial1) / I; } else { //rate ~= 0 denominator = numPeriods; } answer = (-1 * loanAmt) / denominator; return answer; } //Create and set up number formats. These objects also //parse numbers input by user. private void setUpFormats() { moneyFormat = (NumberFormat)NumberFormat.getNumberInstance(); percentFormat = NumberFormat.getNumberInstance(); percentFormat.setMinimumFractionDigits(3); decimalFormat = (DecimalFormat)NumberFormat.getNumberInstance(); decimalFormat.setParseIntegerOnly(true); paymentFormat = (DecimalFormat)NumberFormat.getNumberInstance(); paymentFormat.setMaximumFractionDigits(2); paymentFormat.setNegativePrefix("("); paymentFormat.setNegativeSuffix(")"); integerFormat = NumberFormat.getIntegerInstance(); } }

The code creates an InputVerifer (class MyVerifier), which is set to the four JTextFields. Nowhere in the code the InputVerifier is "removed" (set to null), but in its shouldYieldFocus(...) method it is set anew with every verification (input.setInputVerifier(this)). Commenting out this line shows no different behaviour on my side. So is it at least recommendable or maybe even necessary for reasons I cannot figure out?