使用 GUI 的输入验证 (java)

Question

我正在使用 Java 中的 GUI 创建（嗯，已经创建）一个贷款支付计算器。有些计算是错误的，我会修复它们，所以不要介意。我真正需要帮助的是用户输入验证。我尝试使用 try/catch 块，但在捕获异常的 NetBeans 中不断出现红色下划线。为什么我会得到下划线，这是正确的验证类型吗？如果需要更多代码或信息，请告诉我。这是主要的类/驱动程序代码：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.TitledBorder;

public class LoanCalculator extends JFrame {
// Create text fields for interest rate, years
// loan amount, monthly payment, and total payment


private static final JTextField INTEREST_RATE = new JTextField();
private static final JTextField LOAN_YEARS = new JTextField();
private static final JTextField LOAN_AMOUNT = new JTextField();
private static final JTextField MONTHLY_PAYMENT = new JTextField();
private static final JTextField TOTAL_PAYMENT = new JTextField();

// Create a Compute Payment button
private static final JButton LOAN_COMPUTE = new JButton("Compute Payment");

public LoanCalculator() {
    // Panel p1 to hold labels and text fields
    //setSize (FRAME_WIDTH, FRAME_HEIGHT);
    JPanel labelPanel = new JPanel(new GridLayout(6, 2));
    labelPanel.add(new JLabel("Annual Interest Rate"));
    labelPanel.add(INTEREST_RATE);
    labelPanel.add(new JLabel("Number of Years"));
    labelPanel.add(LOAN_YEARS);
    labelPanel.add(new JLabel("Loan Amount"));
    labelPanel.add(LOAN_AMOUNT);
    labelPanel.add(new JLabel("Monthly Payment"));
    labelPanel.add(MONTHLY_PAYMENT);
    labelPanel.add(new JLabel("Total Payment"));
    labelPanel.add(TOTAL_PAYMENT);
    labelPanel.setBorder(new TitledBorder("Enter loan amount, interest rate, and years"));

    // Panel p2 to hold the button
    JPanel buttonPanel = new JPanel(new FlowLayout(FlowLayout.RIGHT));
    buttonPanel.add(LOAN_COMPUTE);

    // Add the panels to the frame
    add(labelPanel, BorderLayout.CENTER);
    add(buttonPanel, BorderLayout.SOUTH);

    // Register listener
    LOAN_COMPUTE.addActionListener(new ButtonListener());
}

/**
 * Handle the Compute Payment button
 */
private class ButtonListener implements ActionListener {

    @Override
    public void actionPerformed(ActionEvent e) {
        // Get values from text fields
    try{
        double interest = Double.parseDouble(INTEREST_RATE.getText());
        int year = Integer.parseInt(LOAN_YEARS.getText());
        double loanAmount = Double.parseDouble(LOAN_AMOUNT.getText());

        // Create a loan object
        Loan loan = new Loan(interest, year, loanAmount);

        // Display monthly payment and total payment
        MONTHLY_PAYMENT.setText(String.format("%.2f", loan.getMonthlyPayment()));
        TOTAL_PAYMENT.setText(String.format("%.2f", loan.getTotalPayment()));
    }catch (Exception ex){
        JOptionPane.showMessageDialog(this, ex.getMessage(), "Error," , JOptionPane.ERROR_MESSAGE));
    }           

    }
}

public static void main(String[] args) {
    LoanCalculator frame = new LoanCalculator();
    frame.pack();
    frame.setTitle("LoanCalculator");
    frame.setLocationRelativeTo(null); // Center the frame
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
}

}

Answer 1

JOptionPane showMessageDialog 方法的第一个参数应该是一个 GUI 组件，而你的不是它，而是内部侦听器类。所以改变这个：

    JOptionPane.showMessageDialog(this, ex.getMessage(), 
        "Error,", JOptionPane.ERROR_MESSAGE));

到这里：

    JOptionPane.showMessageDialog(LoanCalculator.this, ex.getMessage(), 
        "Error", JOptionPane.ERROR_MESSAGE); // extra parenthesis removed

顺便说一句，简单地捕捉Exception 通常不是一个好主意，因为那样会在太大的区域上撒网。更好的是捕获特定的和可能预期的异常。

此外，从这个问题中学到的一个教训是，您需要仔细阅读所有错误和异常消息，因为它们通常会准确地告诉您哪里出了问题，而且通常并不难理解。