JButton actionPerformed方法提供空

Question

所以我有一个程序，我正在写，以允许用户输入文本值的“小说”和“杂志”类的设计，但我有一个问题的属性用我的actionListener，当我触发这个动作的时候，它给了我一个空指针异常。我的猜测是它有话跟我有switch语句

switch(theList.getSelectedIndex()){ 
       case -1: JOptionPane.showMessageDialog(null, "You did not select an object type to create!", "Error!", JOptionPane.ERROR_MESSAGE); 
         break; 
       case 0: createObject(theMagazine, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField); 
         JOptionPane.showMessageDialog(null, theMagazine.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE); 
         break; 
       case 1: createObject(theNovel, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField); 
         theNovel.setHardCover(Boolean.parseBoolean(JOptionPane.showInputDialog(null,"Is this Book a hard cover or not?","Question!",JOptionPane.QUESTION_MESSAGE))); 
         JOptionPane.showMessageDialog(null, theNovel.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE); 
         break; 
      } 

本质上讲，我希望它做的是能够调用适当的属性值的CreateObject（）方法，然后显示的JOptionPane的消息对话框对于使用这两个类所包含的toString（）方法创建的对象。我知道这个问题不是来自任何一个类的方法语法，所以我不一定明白问题出在哪里。

这是源代码：

import javax.swing.*; 
import javax.swing.border.*; 
import java.awt.event.*; 
import java.awt.*; 
@SuppressWarnings({"serial"}) 
public class GUI extends JFrame{ 

static Magazine theMagazine; 
static Novel theNovel; 

public static void main(String[] args){ 
    final String[] objectTypes = {"Magazine","Novel"}; 
    final Border textFieldBorder = BorderFactory.createLineBorder(Color.black, 1); 
    final Font theFont = new Font("Times New Roman", 0, 25); 
    final JLabel typeSelect = new JLabel("Please select the literature type!"); 
    final JLabel titleEnter = new JLabel("Enter the title here!"); 
    final JLabel authorEnter = new JLabel("Enter the Author here!"); 
    final JLabel publisherEnter = new JLabel("Enter the publisher here!"); 
    final JLabel pubLocationEnter = new JLabel("Enter the Publishers location here"); 
    final JLabel enterCopyrightYear = new JLabel("Enter the copyright year here!"); 
    final JLabel priceEnter = new JLabel("Enter the price here!"); 
    final JLabel ISBNEnter = new JLabel("Enter the ISBN here!");  

    JList<String> theList = new JList<String>(objectTypes); 
    theList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION); 
    theList.setLayoutOrientation(JList.HORIZONTAL_WRAP); 
    theList.setVisibleRowCount(-1); 
    theList.setBorder(textFieldBorder); 
    theList.setFont(theFont); 

    JTextField titleField = new JTextField(); 
    titleField.setBorder(textFieldBorder); 
    titleField.setFont(theFont); 
    titleField.setHorizontalAlignment(JTextField.CENTER); 
    JTextField authorField = new JTextField(); 
    authorField.setBorder(textFieldBorder); 
    authorField.setFont(theFont); 
    authorField.setHorizontalAlignment(JTextField.CENTER); 
    JTextField publisherField = new JTextField(); 
    publisherField.setBorder(textFieldBorder); 
    publisherField.setFont(theFont); 
    publisherField.setHorizontalAlignment(JTextField.CENTER); 
    JTextField publisherLocationField = new JTextField(); 
    publisherLocationField.setBorder(textFieldBorder); 
    publisherLocationField.setFont(theFont); 
    publisherLocationField.setHorizontalAlignment(JTextField.CENTER); 
    JTextField copyrightYearField = new JTextField(); 
    copyrightYearField.setBorder(textFieldBorder); 
    copyrightYearField.setFont(theFont); 
    copyrightYearField.setHorizontalAlignment(JTextField.CENTER); 
    JTextField priceEnterField = new JTextField(); 
    priceEnterField.setBorder(textFieldBorder); 
    priceEnterField.setFont(theFont); 
    priceEnterField.setHorizontalAlignment(JTextField.CENTER); 
    JTextField ISBNEnterField = new JTextField(); 
    ISBNEnterField.setBorder(textFieldBorder); 
    ISBNEnterField.setFont(theFont); 
    ISBNEnterField.setHorizontalAlignment(JTextField.CENTER); 

    JButton createObject = new JButton("Create an Object!"); 
    createObject.addActionListener(new ActionListener(){ 
     public void actionPerformed(ActionEvent event){ 
      switch(theList.getSelectedIndex()){ 
       case -1: JOptionPane.showMessageDialog(null, "You did not select an object type to create!", "Error!", JOptionPane.ERROR_MESSAGE); 
         break; 
       case 0: createObject(theMagazine, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField); 
         JOptionPane.showMessageDialog(null, theMagazine.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE); 
         break; 
       case 1: createObject(theNovel, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField); 
         theNovel.setHardCover(Boolean.parseBoolean(JOptionPane.showInputDialog(null,"Is this Book a hard cover or not?","Question!",JOptionPane.QUESTION_MESSAGE))); 
         JOptionPane.showMessageDialog(null, theNovel.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE); 
         break; 
      } 
     } 
    }); 
    JButton closeProgram = new JButton("Close the program!"); 
    closeProgram.addActionListener(new ActionListener(){ 
     public void actionPerformed(ActionEvent event){ 
      System.exit(0); 
     } 
    }); 

    JPanel thePanel = new JPanel(); 
    thePanel.setLayout(new GridLayout(9,2)); 

    JFrame theFrame; 

    //first row 
    thePanel.add(typeSelect); 
    thePanel.add(theList); 

    //second row 
    thePanel.add(titleEnter); 
    thePanel.add(titleField); 

    //third row 
    thePanel.add(authorEnter); 
    thePanel.add(authorField); 

    //fourth row 
    thePanel.add(publisherEnter); 
    thePanel.add(publisherField); 

    //fifth row 
    thePanel.add(pubLocationEnter); 
    thePanel.add(publisherLocationField); 

    //sixth row 
    thePanel.add(enterCopyrightYear); 
    thePanel.add(copyrightYearField); 

    //seventh row 
    thePanel.add(priceEnter); 
    thePanel.add(priceEnterField); 

    //eighth row 
    thePanel.add(ISBNEnter); 
    thePanel.add(ISBNEnterField); 

    //ninth row 
    thePanel.add(createObject); 
    thePanel.add(closeProgram); 

    theFrame = new JFrame(); 
    theFrame.add(thePanel); 
    theFrame.setTitle("Literature Creator!"); 
    theFrame.setSize(500,500); 
    theFrame.setVisible(true); 
    theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    theFrame.setResizable(false); 

} 
public static void createObject(Literature theLiterature, JTextField titleField, JTextField authorField, JTextField publisherField, JTextField publisherLocationField, 
         JTextField copyrightYearField, JTextField priceEnterField, JTextField ISBNEnterField){ 
    theLiterature.setTitle(parseField(titleField)); 
    theLiterature.setAuthor(parseField(authorField)); 
    theLiterature.setPublisher(parseField(publisherField)); 
    theLiterature.setPublisherLocation(parseField(publisherLocationField)); 
    theLiterature.setCopyrightYear(parseNumField(copyrightYearField)); 
    theLiterature.setPrice(parseNumField(priceEnterField)); 
    theLiterature.setISBN(parseNumField(ISBNEnterField)); 
} 
public static String parseField(JTextField field){ 
    if(field.getText() == null){ 
     JOptionPane.showMessageDialog(null, "Error! " + field.getName() + " contains no text!", "Error", JOptionPane.ERROR_MESSAGE); 
     System.exit(0); 
    } 
    return field.getText(); 
} 
public static int parseNumField(JTextField field){ 
    if(field.getText() == null){ 
     JOptionPane.showMessageDialog(null, "Error! " + field.getName() + " contains no text!", "Error", JOptionPane.ERROR_MESSAGE); 
     System.exit(0); 
    } 
    try{ 
     Integer.parseInt(field.getText()); 
    }catch(NumberFormatException nfe){ 
     JOptionPane.showMessageDialog(null, "Error! Value in field " + field.getName() + " was not a valid number!", "Error!", JOptionPane.ERROR_MESSAGE); 
     System.exit(0); 
    } 
    return Integer.parseInt(field.getText()); 
} 

}

这里是从错误信息文本：http://pastebin.com/HRi9jbSD

任何帮助，不胜感激！

Answer 1

默认情况下，Java将null分配给新定义的变量。此静态变量

static Magazine theMagazine; 

当您将它传递给createObject(...)方法时为空。

您需要创建新的Magazine对象（或以某种方式加载它）并将其分配给theMagazine变量。