當我單擊Action Listener的按鈕時，如何解決此運行時錯誤（java.lang.NumberFormatException：null）？

Question

我正在使用Java Ready編程1.4。這是說我沒有在JTextField中輸入任何東西，但是我在點擊按鈕之前輸入了變量。

public class MortgageCalculator extends JFrame implements ActionListener 
{ 
private String nameText, ageText, incomeText, priceText, downPayText, interestRateText, paymentText, amortText; 


public MortgageCalculator() 
{ //Frame 
    JFrame frame = new JFrame(); 
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); 
    frame.setSize (new Dimension (300, 250)); 
    frame.setTitle ("Mortgage Calculator"); 
    frame.getContentPane().setLayout (new BorderLayout()); 

    //User Information (Input) - Center Panel 
    JPanel centerPanel = new JPanel (new GridLayout (16, 1)); 
    //Personal Information 
    centerPanel.add (new JLabel ("       Personal Information")); 
    centerPanel.add (new JLabel ("Full Name:")); 
    JTextField nameField = new JTextField(); 
    centerPanel.add (nameField); 
    centerPanel.add (new JLabel ("Your Age:")); 
    JTextField ageField = new JTextField(); 
    centerPanel.add (ageField); 
    centerPanel.add (new JLabel ("Yearly Income:")); 
    JTextField incomeField = new JTextField(); 
    centerPanel.add (incomeField); 
    centerPanel.add (new JLabel()); 
    //Mortgage Information 
    centerPanel.add (new JLabel ("       Mortgage Information")); 
    centerPanel.add (new JLabel ("Price of Property:")); 
    JTextField priceField = new JTextField(); 
    centerPanel.add (priceField); 
    centerPanel.add (new JLabel ("Down Payment:")); 
    JTextField downPayField = new JTextField(); 
    centerPanel.add (downPayField); 
    centerPanel.add (new JLabel ("Interest Rate (%):")); 
    JTextField interestRateField = new JTextField(); 
    centerPanel.add (interestRateField); 
    centerPanel.add (new JLabel ("Amortization Period (Years):")); 
    frame.getContentPane().add (centerPanel, BorderLayout.CENTER); 

    //User Options (Input) - South Panel 
    JPanel southPanel = new JPanel (new GridLayout (8, 3)); 
    //Payment Option 
    southPanel.add (new JLabel ("Payment:")); 
    southPanel.add (new JLabel ("Type in 1, 2 or 3")); 
    southPanel.add (new JLabel()); 
    southPanel.add (new JLabel ("1. Weekly")); 
    southPanel.add (new JLabel ("2. Biweekly")); 
    southPanel.add (new JLabel ("3. Monthly")); 
    JTextField paymentField = new JTextField(); 
    southPanel.add (paymentField); 
    southPanel.add (new JLabel()); 
    southPanel.add (new JLabel()); 

    //Amortization Period Option 
    southPanel.add (new JLabel ("Amortization Period:")); 
    southPanel.add (new JLabel ("Type in 1, 2 or 3")); 
    southPanel.add (new JLabel()); 
    southPanel.add (new JLabel ("1. 20 years")); 
    southPanel.add (new JLabel ("2. 25 years")); 
    southPanel.add (new JLabel ("3. 30 years")); 
    JTextField amortField = new JTextField(); 
    southPanel.add (amortField); 
    southPanel.add (new JLabel()); 
    southPanel.add (new JLabel()); 
    //To add a blank line in frame, added three blank JLabels because it is 3 horiontal for grid layout 
    southPanel.add (new JLabel()); 
    southPanel.add (new JLabel()); 
    southPanel.add (new JLabel()); 
    southPanel.add (new JButton ("Clear Information")); 
    southPanel.add (new JLabel()); 
    JButton nextButton = new JButton ("Next"); 
    southPanel.add (nextButton); 

    frame.getContentPane().add (southPanel, BorderLayout.SOUTH); 


    frame.pack(); 
    frame.setVisible (true); 


    nextButton.addActionListener (this); 

    String nameText = nameField.getText(); 
    String ageText = ageField.getText(); 
    String incomeText = incomeField.getText(); 
    String priceText = priceField.getText(); 
    String downPayText = downPayField.getText(); 
    String interestRateText = interestRateField.getText(); 
    String paymentText = paymentField.getText(); 
    String amortText = amortField.getText(); 
} 

public static void main (String[] args) 
{ 
    MortgageCalculator mainApp = new MortgageCalculator(); 
} 

public void actionPerformed (ActionEvent e) 
{ 

    //Gets the personal information from the text fields 
    String name; 
    int age, payment, amort; 
    double income, price, downPay, interestRate; 

    name = nameText; 

    age = Integer.parseInt (ageText); 

    income = Double.parseDouble (incomeText); 
    //Gets the mortgage information from the text fields 

    price = Double.parseDouble (priceText); 

    downPay = Double.parseDouble (interestRateText); 

    interestRate = Double.parseDouble (interestRateText); 
    interestRate = interestRate/100; 
    payment = Integer.parseInt (paymentText); 

    amort = Integer.parseInt (amortText); 

    //Output frame 
    JFrame frame2 = new JFrame(); 
    frame2.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); 
    frame2.setSize (new Dimension (300, 250)); 
    frame2.setTitle ("Mortgage Calculator"); 
    frame2.getContentPane().setLayout (new BorderLayout()); 

    JPanel westPanel2 = new JPanel (new GridLayout (3, 1)); 
    JLabel nameLabel = new JLabel ("Name: " + name); 
    westPanel2.add (nameLabel); 
    JLabel ageLabel = new JLabel ("Age: " + age); 
    westPanel2.add (ageLabel); 
    JLabel incomeLabel = new JLabel ("Yearly Income: $" + income); 
    westPanel2.add (incomeLabel); 
    frame2.getContentPane().add (westPanel2, BorderLayout.WEST); 

    JPanel centerPanel2 = new JPanel (new GridLayout (3, 1)); 
    JLabel priceLabel = new JLabel (" Price of property: $" + price); 
    centerPanel2.add (priceLabel); 
    JLabel downPayLabel = new JLabel (" Down Payment: $" + downPay); 
    centerPanel2.add (downPayLabel); 
    JLabel interestRateLabel = new JLabel (" Interest Rate: " + income); 
    centerPanel2.add (interestRateLabel); 
    frame2.getContentPane().add (centerPanel2, BorderLayout.CENTER); 


    double interest, numerator, l, denominator, payPerPeriod, initialP, mortgageYearly; 
    if (payment==1) 
    { 
     interest = 1 + interestRate/52; 
     if (amort==1) 
     { 
      l = Math.pow (interest, 20 * 52); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     else if (amort==2) 
     { 
      l = Math.pow (interest, 25 * 52); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     else // 3. amort30 
     { 
      l = Math.pow (interest, 30 * 52); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     mortgageYearly = payPerPeriod * 52; 
    } 
    else if (payment == 2) 
    { 
     interest = 1 + interestRate/26; 
     if (amort==1) 
     { 
      l = Math.pow (interest, 20 * 26); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     else if (amort==2) 
     { 
      l = Math.pow (interest, 25 * 26); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     else // 3. amort30 
     { 
      l = Math.pow (interest, 30 * 26); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     mortgageYearly = payPerPeriod * 26; 


    } 
    else //3. monthly payment 
    { 
     interest = 1 + interestRate/12; 
     if (amort==1) 
     { 
      l = Math.pow (interest, 20 * 12); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     else if (amort==2) 
     { 
      l = Math.pow (interest, 25 * 12); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 
     } 
     else // 2. amort30 
     { 
      l = Math.pow (interest, 30 * 12); 
      numerator = l * interest; 
      denominator = l - 1; 
      initialP = price - downPay; 
      payPerPeriod = (numerator/denominator) * initialP; 

     } 
     mortgageYearly = payPerPeriod * 12; 
    } 


    JPanel eastPanel2 = new JPanel (new GridLayout (3, 1)); 
    JLabel payTotalLabel = new JLabel (" Payment Total: $" + initialP); 
    eastPanel2.add (priceLabel); 
    JLabel payPerPeriodLabel = new JLabel (" Payment per period: $" + payPerPeriod); 
    eastPanel2.add (payPerPeriodLabel); 

    frame2.getContentPane().add (eastPanel2, BorderLayout.EAST); 


    JPanel southPanel2 = new JPanel (new GridLayout (2, 1)); 
    double minimumIncome = mortgageYearly/0.28; 
    JLabel minimumLabel = new JLabel (" Minimum Yearly Income: $" + minimumIncome); 
    eastPanel2.add (priceLabel); 

    if (minimumIncome > income) 
    { 
     JLabel qualifiedLabel = new JLabel ("You are not qualified for the mortgage"); 
       eastPanel2.add (qualifiedLabel); 
    } 
    else 
    { 
     JLabel qualifiedLabel = new JLabel ("You are qualified for the mortgage"); 
       eastPanel2.add (qualifiedLabel); 
    } 


    frame2.getContentPane().add (eastPanel2, BorderLayout.EAST); 



    frame2.pack(); 
    frame2.setVisible (true); 



} 

} 

Answer 1

以下行中的一個（或多個）期望的數值爲存在字符串中，很顯然，就通過一些非數值（或多個）作爲參數來解析方法。

age = Integer.parseInt (ageText); 

income = Double.parseDouble (incomeText); 

price = Double.parseDouble (priceText); 

downPay = Double.parseDouble (interestRateText); 

interestRate = Double.parseDouble (interestRateText); 

payment = Integer.parseInt (paymentText); 

amort = Integer.parseInt (amortText);