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actionPerformed处的NullPointerException.不知道为什么

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 …
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java swing nullpointerexception

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