这个JFormattedTextField如何工作？

Question

I have the sample code below, and I'm having problems understanding it. So I'd like two ask 2 questions:

1. Why does JFormattedTextField have a "value" ( getValue() , setValue() ) and a JTextField does not?

2. In the propertyChange() method, why is it sufficient to write:

   amount = ((Number)amountField.getValue()).doubleValue();

   to update the amount so that it is displayed the right way, but for the paymentField I have to call the setValue() method?

Sample:

/*
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 * modification, are permitted provided that the following conditions
 * are met:
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 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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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.*;

/**
 * FormattedTextFieldDemo.java requires no other files.
 *
 * It implements a mortgage calculator that uses four
 * JFormattedTextFields.
 */
public class FormattedTextFieldDemo extends JPanel
                                    implements PropertyChangeListener {
    //Values for the fields
    private double amount = 100000;
    private double rate = 7.5;  //7.5%
    private int numPeriods = 30;

    //Labels to identify the fields
    private JLabel amountLabel;
    private JLabel rateLabel;
    private JLabel numPeriodsLabel;
    private JLabel paymentLabel;

    //Strings for the labels
    private static String amountString = "Loan Amount: ";
    private static String rateString = "APR (%): ";
    private static String numPeriodsString = "Years: ";
    private static String paymentString = "Monthly Payment: ";

    //Fields for data entry
    private JFormattedTextField amountField;
    private JFormattedTextField rateField;
    private JFormattedTextField numPeriodsField;
    private JFormattedTextField paymentField;

    //Formats to format and parse numbers
    private NumberFormat amountFormat;
    private NumberFormat percentFormat;
    private NumberFormat paymentFormat;

    public FormattedTextFieldDemo() {
        super(new BorderLayout());
        setUpFormats();
        double payment = computePayment(amount,
                                        rate,
                                        numPeriods);

        //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 JFormattedTextField(amountFormat);
        amountField.setValue(new Double(amount));
        amountField.setColumns(10);
        amountField.addPropertyChangeListener("value", this);

        rateField = new JFormattedTextField(percentFormat);
        rateField.setValue(new Double(rate));
        rateField.setColumns(10);
        rateField.addPropertyChangeListener("value", this);

        numPeriodsField = new JFormattedTextField();
        numPeriodsField.setValue(new Integer(numPeriods));
        numPeriodsField.setColumns(10);
        numPeriodsField.addPropertyChangeListener("value", this);

        paymentField = new JFormattedTextField(paymentFormat);
        paymentField.setValue(new Double(payment));
        paymentField.setColumns(10);
        paymentField.setEditable(false);
        paymentField.setForeground(Color.red);

        //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);
    }

    /** Called when a field's "value" property changes. */
    public void propertyChange(PropertyChangeEvent e) {
        Object source = e.getSource();
        if (source == amountField) {
            amount = ((Number)amountField.getValue()).doubleValue();
        } else if (source == rateField) {
            rate = ((Number)rateField.getValue()).doubleValue();
        } else if (source == numPeriodsField) {
            numPeriods = ((Number)numPeriodsField.getValue()).intValue();
        }

        double payment = computePayment(amount, rate, numPeriods);
        paymentField.setValue(new Double(payment));
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event dispatch thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("FormattedTextFieldDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Add contents to the window.
        frame.add(new FormattedTextFieldDemo());

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event dispatch thread:
        //creating and showing this application's GUI.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                //Turn off metal's use of bold fonts
            UIManager.put("swing.boldMetal", Boolean.FALSE);
                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() {
        amountFormat = NumberFormat.getNumberInstance();

        percentFormat = NumberFormat.getNumberInstance();
        percentFormat.setMinimumFractionDigits(3);

        paymentFormat = NumberFormat.getCurrencyInstance();
    }
}

Answer 1

"Why has a JFormattedTextField an "value" (getValue(), setValue()) and a jTextField not?"

The text attributes of both JTextField and JFormattedTextField are the actual text you see displayed by each. But JFormattedTextField isn't just displaying some text that you give it. It, with the help of a Formatter that you provided, translates between some Object--in this case a Number--and the convenient human-readable text representation you see on the screen. The object whose value you're displaying is the "value" of the JFormattedTextField , how it looks on the screen is the "text". A regular JTextField , on the other hand, just shows whatever text you tell it to show and returns whatever the you type into it, so there's no need for a separate value attribute because it would always be identical to the "text".

"Why is it sufficient to write "amount = ((Number)amountField.getValue()).doubleValue();" to update the amount so that is is displayed the right way but for the paymentField i have to call the setValue-Methode?"

You're not writing

amount = ((Number)amountField.getValue()).doubleValue();

to update amountField , you're doing it to find out what to user typed into amountField . amountField is updating itself in response to user input because that's what JFormattedTextField s do.

Now, having learned what the user typed into amountField , you recalculate the payment amount and tell the paymentField to show the new payment by calling its setValue method.