這個JFormattedTextField是如何工作的？

Question

我有下面的示例代碼，我有理解它的問題。所以我想要兩問兩個問題：

1. 爲什麼JFormattedTextField上有一個「值」（getValue()，setValue()）和一個JTextField不？

2. 在propertyChange()方法，爲什麼它足夠寫：

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

   ，使其顯示正確的方式來更新量，但對於paymentField我必須調用setValue()方法？

示例： 「（的getValue（）的setValue（））和一個JTextField不是 」

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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

「 爲什麼有一個的JFormattedTextField的」 值

JTextField和JFormattedTextField的文本屬性是您看到的每個文本顯示的文本屬性。但JFormattedTextField不只是顯示一些你給它的文字。它在你提供的格式化程序的幫助下，在一些對象之間進行轉換 - 在這種情況下是一個數字 - 以及在屏幕上看到的便於閱讀的文本表示。您正在顯示的值的對象是JFormattedTextField的「值」，它在屏幕上的外觀是「文本」。另一方面，一般的JTextField只顯示你告訴它顯示的任何文本，並返回你輸入的任何內容，所以不需要單獨的值屬性，因爲它總是與「文本」相同。

「爲什麼要寫」amount =（（Number）amountField.getValue（））。doubleValue（）;「來更新金額，以便以正確的方式顯示，但對於我必須調用的paymentField setValue-Methode？「

你不寫

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

更新amountField，你這樣做，找出哪些用戶輸入到amountField。 amountField正在更新自身以響應用戶輸入，因爲這就是JFormattedTextField的功能。

現在，在瞭解了用戶輸入到amountField後，您重新計算了付款金額，並通過調用setValue方法告訴paymentField顯示新付款。