actionPerformed处的NullPointerException.不知道为什么

Question

我正在创建一个使用GUI来显示抵押付款的Java程序.我正在尝试向textArea输出将用于抵押的付款.这是我的代码:

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.text.DecimalFormat;

public class MortgageGui extends JFrame implements ActionListener {

// Set two-places for decimal format
DecimalFormat twoPlaces = new DecimalFormat("$0.00");

// Declare variable for calculation
Double loanAmt;
double interestRate;
double monthlyPayment;
int payment;
String amount;

JTextField loanAmount;
JComboBox loanTypeBox;
JLabel paymentOutput;
JTextArea paymentList;

// Build arrays for mortgages
double[] mortgage1 = {7.0, 5.35, 0.0}; // (years, interest, monthly payment)
double[] mortgage2 = {15.0, 5.5, 0.0}; // (years, interest, monthly payment)
double[] mortgage3 = {30.0, 5.75, 0.0}; // (years, interest, monthly payment)


public MortgageGui() {

    super("Mortgage Calculator");
    setLookAndFeel();
    setSize(350, 500);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // Loan Amount Panel
    JPanel loanAmtPanel = new JPanel();
    JLabel loanAmtLabel = new JLabel("Loan Amount: ", JLabel.LEFT);
    JTextField loanAmount = new JTextField(10);
    loanAmtPanel.add(loanAmtLabel);
    loanAmtPanel.add(loanAmount);

    // Loan Type Panel
    JPanel loanTypePanel = new JPanel();
    JLabel loanTypeLabel = new JLabel("Loan Type: ", JLabel.LEFT);
    String[] items = {"7 years at 5.35%", "15 years at 5.5%", "30 years at 5.75%"};
    JComboBox loanTypeBox = new JComboBox(items);
    loanTypePanel.add(loanTypeLabel);
    loanTypePanel.add(loanTypeBox);

    // Calculate Button Panel
    JPanel calculatePanel = new JPanel();
    JButton calcButton = new JButton("Calculate Paytment");
    calcButton.addActionListener(this);
    calculatePanel.add(calcButton);

    // Monthly Payment Panel
    JPanel paymentPanel = new JPanel();
    JLabel paymentLabel = new JLabel("Monthly Payment: ", JLabel.LEFT);
    JLabel paymentOutput = new JLabel("Calculated Payment");
    paymentPanel.add(paymentLabel);
    paymentPanel.add(paymentOutput);

    // View Payments Panel
    JPanel viewPayments = new JPanel();
    JTextArea paymentList = new JTextArea("", 17, 27);
    paymentList.setEditable(false);
    paymentList.setLineWrap(true);
    viewPayments.add(paymentList);

    // Add panels to win Panel
    JPanel win = new JPanel();
    BoxLayout box = new BoxLayout(win, BoxLayout.Y_AXIS);
    win.setLayout(box);
    win.add(loanAmtPanel);
    win.add(loanTypePanel);
    win.add(calculatePanel);
    win.add(paymentPanel);
    win.add(viewPayments);
    add(win);

    // Make window visible
    setVisible(true);
}


private void setLookAndFeel() {
    try {
        UIManager.setLookAndFeel(
            "com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel"
        );
    } catch (Exception exc) {
        // ignore
      }
}


public void actionPerformed(ActionEvent e) {

    // Clear payment list
    paymentList.setText("");

    // Get loan amount from textfield
    String amount = loanAmount.getText();
    loanAmt = Double.valueOf(amount).doubleValue();

    // Find which mortgate array to use from combobox
    Object obj = loanTypeBox.getSelectedItem();
    String loanSelected = obj.toString();

    // Run the calculation based on the mortgage arrays

    // 7-year loan
    if (loanSelected.equals("7 years at 5.35%")) {
        // Calculate payment, interest, and remaining
        mortgage1[2] = (loanAmt + (loanAmt * (mortgage1[1] / 100))) / (mortgage1[0] * 12);
        double interest1 = (loanAmt * (mortgage1[1] / 100)) / 84;
        double amountRemaining1 = loanAmt + (loanAmt * (mortgage1[1] / 100));

        // Loop through payments
        for (int payment = 1; payment <=84; payment++) {

            // Deduct one payment from the balance
            amountRemaining1 = amountRemaining1 - mortgage1[2];

            // Write payment to textArea
            paymentList.append("Payment " + payment + ": $" + twoPlaces.format(mortgage1[2]) +
                               " / " + "Interest: $" + twoPlaces.format(interest1) + " / " +
                               "Remaining: $" + twoPlaces.format(amountRemaining1) + "\n");
        }

    } else {
        // 15-year loan
        if (loanSelected.equals("15 years at 5.5%")) {

            // Calculate payment, interest, and remaining
            mortgage2[2] = (loanAmt + (loanAmt * (mortgage2[1] / 100))) / (mortgage2[0] * 12);
            double interest2 = (loanAmt * (mortgage2[1] / 100)) / 180;
            double amountRemaining2 = loanAmt + (loanAmt * (mortgage2[1] / 100));

            // Loop through payments
            for (int payment = 1; payment <=180; payment++) {

                // Deduct one payment from the balance
                amountRemaining2 = amountRemaining2 - mortgage2[2];

                // Write payment to textArea
                paymentList.append("Payment " + payment + ": $" + twoPlaces.format(mortgage2[2]) +
                                   " / " + "Interest: $" + twoPlaces.format(interest2) + " / " +
                                   "Remaining: $" + twoPlaces.format(amountRemaining2) + "\n");
            }

        } else {
            //30-year loan

            //Calculate payment, interest, and remaining
            mortgage3[2] = (loanAmt + (loanAmt * (mortgage3[1] / 100))) / (mortgage3[0] * 12);
            double interest3 = (loanAmt * (mortgage3[1] / 100)) / 360;
            double amountRemaining3 = loanAmt + (loanAmt * (mortgage3[1] / 100));

            // Loop through payments
            for (int payment = 1; payment <=360; payment++) {

                // Deduct one payment from the balance
                amountRemaining3 = amountRemaining3 - mortgage3[2];

                // Write payment to textArea
                paymentList.append("Payment " + payment + ": $" + twoPlaces.format(mortgage3[2]) +
                                   " / " + "Interest: $" + twoPlaces.format(interest3) + " / " +
                                   "Remaining: $" + twoPlaces.format(amountRemaining3) + "\n");
            }
        }
    }
}


public static void main(String[] arguments) {
    MortgageGui calc = new MortgageGui();
}
}

当我运行程序时,我看到了GUI但是当我按下按钮来计算时我在控制台中得到了这个:

异常的线程 "AWT-EventQueue的-0" MortgageGui.actionPerformed(MortgageGui.java:100)在javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)在javax.swing.AbstractButton中$处理器显示java.lang.NullPointerException.在javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)的actionPerformed(AbstractButton.java:2341)在javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)在javax.swing.plaf.basic.BasicButtonListener.mouseReleased (BasicButtonListener.java:252)在java.awt.Component.processMouseEvent(Component.java:6505)在javax.swing.JComponent.processMouseEvent(JComponent.java:3321)在java.awt.Component.processEvent(Component.java: 6270)java.awt.Container.processEvent(Container.java:2229)at java.awt.Component.dispatchEventImpl(Component.java:4861)at java.awt.Container.dispatchEventImpl(Container.java:2287)at java. awt.Component.dispatchEvent(Component.java:4687)at java.awt.LightweightDispatcher.retargetMouseEvent(Con tainer.java:4832)java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)at java.awt.Container.dispatchEventImpl(Container.java:2273) )java.awt.Window.dispatchEventImpl(Window.java:2713)at java.awt.Component.dispatchEvent(Component.java:4687)at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)at java.awt java.security.AccessController上的java.awt.EventQueue $ 3.run(EventQueue.java:664)java.awt.EventQueue $ 3.run(EventQueue.java:666)上的.EventQueue.access $ 000(EventQueue.java:101) .doPrivileged(本地方法)在java.security.ProtectionDomain $ 1.doIntersectionPrivilege(ProtectionDomain.java:76)在java.security.ProtectionDomain $ 1.doIntersectionPrivilege(ProtectionDomain.java:87)在java.awt.EventQueue中$ 4.run(EventQueue中. java:680)at java.awt.EventQueue $ 4.run(EventQueue.java:678)at java.security.AccessController.doPrivileged(Native Method)at java.security.ProtectionDomain $ 1.doIntersectionPriv ilege(ProtectionDomain.java:76)在java.awt.EventQueue.dispatchEvent(EventQueue.java:677)处java.awt.EventDispatchThread.pumpEventsForFilter java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)(EventDispatchThread.java :128)在java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)在java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)在java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)在java的.awt.EventDispatchThread.run(EventDispatchThread.java:90)

我无法弄清楚我哪里出错了.请帮忙!

Answer 1

您将在类的构造函数中重新声明paymentList变量

  JTextArea paymentList = new JTextArea("", 17, 27);

构造函数内部的变量"阴影"了类字段,因此构造函数不会看到类字段.此局部变量仅在构造函数中可见(因为它在其中声明),并且类字段将保持为null.

解决方案:不要在构造函数中重新声明变量.可以在那里初始化它,但不要重新声明它:

  paymentList = new JTextArea("", 17, 27);

更一般地说,您应该始终检查抛出NPE的行上的变量,因为其中一个变量为null.如果你看到它是paymentList,你很快就能识别出我刚给你看的东西,你不需要我们的帮助,这是本论坛的目标 - 让你能够解决问题靠自己.

编辑:
另请注意,您在代码中多次执行此错误.我会让你找到其他事件.

编辑2:
考虑将您的JTextArea放在JScrollPane中.考虑使用一些不错的布局管理器而不是设置GUI的大小,而是让布局管理器为您执行此操作.