java - 尝试从 JDialog 框中将项目插入数组时出现 NullPointerException

Question

正如它所述，当我尝试通过 JDialog 弹出窗口将项目插入到对象数组中时，我收到 NullPointerException。我重新设计了一个现有应用程序来创建 JDialog，它从当前名为 Project6() 的另一个应用程序类打开。 JDialog 类称为 ProcessRec()，当它作为独立 GUI 运行时(在我将其设为 JDialog 之前)，它工作得很好。

在这种情况下，我的堆栈跟踪相当无用，因为它只指向尝试将数据插入数组的方法(正如我所说，之前工作得很好)以及与它们对应的 GUI 按钮。

ProcessRec() 的构造函数接受一个 ToolWarehouse() 对象，该对象与创建我的对象数组的类相匹配(这是我定义的另一个类 ToolItem() 中的对象数组)。

public ProcessRec(ToolWarehouse tools)

当它独立运行时，ProcessRec() 构造函数参数为空(默认)。

当 ProcessRec() 作为独立 GUI 运行以将数据输入到数组中时，我将使用默认构造函数创建一个对象，如下所示:

ToolWareHouse tools = new ToolWarehouse();

然后我将使用它的方法 insert() 将数据输入到数组中。

现在它是一个 JDialog，但我被指示更改为:

ToolWarehouse tools;

然而，这会导致 NullPointerException 因为，我猜测，编译器指向一个不存在的对象。当我在 ProcessRec() 中创建一个新对象时(就像我在独立对象时所做的那样)，我不再遇到此异常。但是，当我尝试使用 insert() 将数据存储到数组中时，它不起作用，并且找不到数据(尽管我收到提示说插入成功)。

我对类的实例知之甚少，但我假设我的数据未显示的原因是因为它使用的是 ToolWarehouse() 类的完全不同的实例？格式是:

public ProcessRec(ToolWarehouse tools)

接受之前使用的类的相同实例吗？我可能偏离了基地，所以我希望有人能帮助我理解可能发生的事情以及原因。

如果需要更多信息，我深表歉意，我会添加任何必要的信息。代码有点长，不想占用空间。万分感谢。

编辑:这是 ProcessRec() 类，它创建 JDialog 框，以及与其使用相对应的一些方法。

public class ProcessRec extends JDialog
{
     //global declarations
     private JButton insertBtn;
     private JButton deleteBtn;
     private JButton displayBtn;
     private JButton hideBtn;
     private JButton clearBtn;

     private JTextField toolFld;
     private JTextField idFld;
     private JTextField priceFld;
     private JTextField qualityFld;
     private JTextField numInStockFld;
     private JTextField messageFld;

     private JLabel messageLbl;

     private Font f1 = new Font("serif", Font.BOLD, 24);
     private Font f2 = new Font("serif", Font.PLAIN, 18);

     private Container c = getContentPane();

     private String input = "";
     private String toolInput = "";

     private int responseCode = 0;
     private int idInput = 0;
     private int qualityInput = 0;
     private int numInStockInput = 0;

     private double priceInput = 0.0;

     private ToolWarehouse tools = new ToolWarehouse();

    //constructor for GUI elements and event listeners
    public ProcessRec(ToolWarehouse tools)
    {
         setTitle("Project1");
         setSize(520,450);
             c.setLayout(new FlowLayout());

             JLabel title = new JLabel("Tiny Tim's Tool Warehouse, Inc.");
         title.setFont(f1);
         c.add(title);

         JTextField toolLbl = new JTextField("Enter tool name:", 15);
         toolLbl.setEditable(false);
         c.add(toolLbl);

         toolFld = new JTextField(25);
         c.add(toolFld);

         JTextField idLbl = new JTextField("Enter ID:", 15);
         idLbl.setEditable(false);
         c.add(idLbl);

         idFld = new JTextField(25);
         c.add(idFld);

         JTextField priceLbl = new JTextField("Base Price:", 15);
         priceLbl.setEditable(false);
         c.add(priceLbl);

         priceFld = new JTextField(25);
         c.add(priceFld);

         JTextField qualityLbl = new JTextField("Enter Quality:", 15);
         qualityLbl.setEditable(false);
         c.add(qualityLbl);

         qualityFld = new JTextField(25);
         c.add(qualityFld);

         JTextField numInStockLbl = new JTextField("Enter Number in Stock:", 15);
         numInStockLbl.setEditable(false);
         c.add(numInStockLbl);

         numInStockFld = new JTextField(25);
         c.add(numInStockFld);

         insertBtn = new JButton("Insert");
         c.add(insertBtn);

         deleteBtn = new JButton("Delete");
         c.add(deleteBtn);

         displayBtn = new JButton("Display");
         c.add(displayBtn);

         hideBtn = new JButton("Hide");
         c.add(hideBtn);

         clearBtn = new JButton("Clear");
         c.add(clearBtn);

         JLabel messageLbl = new JLabel("Messages:");
         messageLbl.setFont(f2);
         c.add(messageLbl);

         messageFld = new JTextField(30);
         c.add(messageFld);

         //button listeners
         insertBtn.addActionListener(new EventListener());
         deleteBtn.addActionListener(new EventListener());
         displayBtn.addActionListener(new EventListener());
         hideBtn.addActionListener(new EventListener());
         clearBtn.addActionListener(new EventListener());

    } //end constructor  

    public String getToolRecords(int index)
    {
            return tools.getRecord(index);
    }

    public int getNumberOfItems()
    {
            return tools.getNumberOfItems();
    }

//Action Listener 
private class EventListener implements ActionListener
{
    public void actionPerformed(ActionEvent ev)
    {
            if (ev.getSource() == insertBtn)
            {
                toolInput = toolFld.getText();

                input = idFld.getText();
                idInput = Integer.parseInt(input);

                input = priceFld.getText();
                priceInput = Double.parseDouble(input);

                input = qualityFld.getText();
                qualityInput = Integer.parseInt(input);

                input = numInStockFld.getText();
                numInStockInput = Integer.parseInt(input);

                responseCode = tools.insert(qualityInput, toolInput, idInput, numInStockInput,
                                                    priceInput);

                if (responseCode == 1)
                {
                    messageFld.setText(idInput + " - Successful insertion");
                }
                else if (responseCode == 0)
                {
                    messageFld.setText(idInput + " - Array is full");
                }
                else if (responseCode == -1)
                {
                    messageFld.setText(idInput + " - Duplicate/Invalid ID");
                }

                if (tools.getNumberOfItems() < 10)
                {   
                    JOptionPane.showMessageDialog(null, "Tool Name: " + toolInput  
                                + "\nTool ID: " + idInput + "\nTool Base Price: " + "$" + priceInput
                                + "\nQuality: " + qualityInput + "\nNumber In Stock: " 
                                + numInStockInput, "Insert Review", JOptionPane.INFORMATION_MESSAGE);
                }
                else if (tools.getNumberOfItems() == 10)
                {
                    JOptionPane.showMessageDialog(null, "The array is full, please delete an entry",
                                                  "Insert Review", JOptionPane.INFORMATION_MESSAGE);
                }

            }//end insert button

            else if (ev.getSource() == deleteBtn)
            {
                input = idFld.getText();
                idInput = Integer.parseInt(input);

                responseCode = tools.delete(idInput);

                if (responseCode == 1)
                {
                    messageFld.setText(idInput + " - Successful deletion");
                }
                else if (responseCode == -1)
                {
                    messageFld.setText(idInput + " - ID not found");
                }

            }//end delete button

            else if (ev.getSource() == displayBtn)
            {
                tools.display();
            }

            else if (ev.getSource() == hideBtn)
            {
                //setState(JFrame.ICONIFIED);
            }

            else if (ev.getSource() == clearBtn)
            {
                qualityFld.setText(null);
                toolFld.setText(null);
                idFld.setText(null);
                numInStockFld.setText(null);
                priceFld.setText(null);

                messageFld.setText(null);

                repaint();

            }//end clear button

    }//end actionPerformed

}//end ActionListener

}//end Project1

创建数组的ToolWarehouse类:

public class ToolWarehouse
{
    //Global declarations
    private int numberOfItems = 0;
    private int index = 0;
    private int returnCode = 0;

    protected ToolItem[] toolArray = new ToolItem[10];

    public ToolWarehouse()
    {
        numberOfItems = 0;

        //creating the array of ToolItems
        for (int i = 0; i < toolArray.length; i++)
        {
            toolArray[i] = new ToolItem();
            System.out.println(toolArray[i]);
        }

    }//end constructor

    public int searchArray(int id)
    {
        for (index = 0; index < toolArray.length; index++)
        {
            if (toolArray[index].getToolID() == id)
            {
                System.out.println("ID found at location " + index);
                return index;
            }
        }   
        return -1;
    }//end searchArray

    public int insert(int quality, String name, int id, int numInStock, double price)
    {
        returnCode = searchArray(id);

        if (numberOfItems == 10)
        {
            System.out.println("Array is full");
            return 0;
        }
        else if (returnCode == -1)
        {   
            boolean oK = toolArray[numberOfItems].assign(quality, name, id, numInStock, 
                                                            price);
            if (oK == true)
            {
                System.out.println("Successful insertion");
                numberOfItems++;   
                return 1;          
            }
        }
        return -1;

    }//end insert

    public int delete(int ID)
    {
        returnCode = searchArray(ID);   

        if (returnCode != -1)
        {   
            toolArray[returnCode] = new ToolItem();

            for (index = returnCode; index < numberOfItems; index++)
            {
                toolArray[index] = toolArray[index + 1];
            }

            numberOfItems--;
            System.out.println("Successful deletion");
            return 1;
        }
        else
            System.out.println("ID not found");
            return -1;

    }//end delete

    public void display()
    {
        for (index = 0; index < numberOfItems; index++)
        {
            toolArray[index].calcAdjustedPrice();
            toolArray[index].display();
        }
    }//end display

    public String getRecord(int index)
    {
        return "Tool Name:  " + toolArray[index].getName()
                + "| Tool ID:  " + toolArray[index].getToolID()
                + "| Tool Quality:  " + toolArray[index].getQuality()
                + "| Number in Stock:  " + toolArray[index].getNumberInStock()
                + "| Tool Price:  " + toolArray[index].getPrice();

    }//end getRecord

    public int getNumberOfItems()
    {
        return numberOfItems;
    }

}//end ToolWarehouse

我现在正在处理的当前部分，它从 ToolWarehouse() 类调用 insert()，但在使用默认构造函数创建 ToolWarehouse() 类的对象时，是否没有正确插入到我正在处理的数组中(即使我的 println 调试语句表明确实如此):

public void readBSAFile() throws IOException, 
                                     FileNotFoundException

    {

        FileInputStream fstream2 = new FileInputStream(filename);
        DataInputStream dstream2 = new DataInputStream(fstream2);

        while (dstream2.available() > 0)
        {
            try
            {
                toolName = dstream2.readUTF();
                id = dstream2.readInt();
                quality = dstream2.readInt();
                numInStock = dstream2.readInt();
                price = dstream2.readDouble();
                dstream2.close();

                System.out.println(toolName);
                System.out.println(id);
                System.out.println(quality);
                System.out.println(numInStock);
                System.out.println(price);

                //tools.insert(quality, toolName, id, numInStock, price);
            }   
            catch (EOFException e)
            {
                System.out.println("End of file reached");
            }
        }
        tools.insert(quality, toolName, id, numInStock, price);
     }//end readBSAFile

Answer 1

事实上，在尝试使用该对象之前，您必须确保该对象已初始化。你把

ToolWarehouse tools; 

作为类变量，在对话框方法中你可以说

tools = new ToolWareHouse();

创建对象并开始使用它。但也许更好的方法是将其直接初始化为类变量。例如写

ToolWarehouse tools = new ToolWareHouse(); 

在类(class)中名列前茅。 (所以它是一个类变量，并且在整个类中都是已知的)

您没有向我们展示太多代码，因此很难判断确切的问题是什么。但正如你所说，你没有初始化你的对象，所以当你试图将它传递给另一个方法并使用它时，你一定会得到一个 nullPointerException 。