Question

这是一个大学课程作业，它创建一个ItemRecord类并使用getter和setter实现Serializable，然后覆盖toString方法。接下来，我创建了一个ItemRecordReport类，其中Scanner对象映射到输入文件“ItemRecord_text.txt”。创建了映射到二进制输出文件“ItemRecord_binary.txt”的ObjectOutputStream对象。创建了映射到二进制文件的ObjectInputStream对象。一切正常。我通过在方法main中放入一个数组并使用附加的文本文档更改字段值来修改此帖子。我的错误是NullPointerException：这是堆栈跟踪

Item   Cost     Quantity  Description
Number
A100  $ 99.99   10        Canon PowerShot-135
A200  $149.99   50        Panasonic-Lumix T55
A300  $349.99   20        Nikon- D3200 DSRL
A400  $280.99   30        Sony- DSC-W800
A500  $ 97.99   20        Samsung- WB35F
Exception in thread "main" java.lang.NullPointerException
at ItemRecordReport.endRecord(ItemRecordReport.java:165)
at ItemRecordReport.main(ItemRecordReport.java:156)

 ----jGRASP wedge2: exit code for process is 1.
 ----jGRASP: operation complete.

//创建程序，以便类的对象可以序列化，实现接口Serialiable //使用4个参数创建构造函数，并附带get和set方法，Override toString方法 //创建包含5条记录的文本文件，创建Scanner对象，ObjectOutputStream和ObjectInputStream //创建新的ItemRecord对象，使用对象的set方法更改ItemRecord对象中所有字段的值 //使用toString方法修改ItemRecord对象

 import java.io.Serializable;
 public class ItemRecord implements Serializable

 {  //declare 4 fields with private access
 private String itemNumber;
 private double cost;
 private int quantity;
 private String description;


 //constructor with no arguments call other constructor with default values
 public ItemRecord()
 {
 this("",0.0,0,"");

 }//end constructor with no arguments
//constructor with 4 arguments
public ItemRecord(String number, double price, int quant, String desc)
{
  setItemNumber(number);
  setCost(price);
  setQuantity(quant);
  setDescription(desc);
 }//end constructor

 public void setItemNumber(String number)
 {
  itemNumber = number;
 }//end setItemNumber
 public String getItemNumber()
 {
  return itemNumber;
 }//end getItemNumber

 public void setCost(double price)
{
  cost = price;
}//end seCost
public double getCost()
{
  return cost;
}//end getCost

public void setQuantity(int quant)
{
  quantity = quant;
}//end setQuantity
public int getQuantity()
{
  return quantity;
}//end getQuantity

public void setDescription(String desc)
{
  description = desc;
}//end setDescription
public String getDescription()
{
  return description;
}//end getDescription
public static void printHeading()
{
System.out.println("Item   Cost     Quantity  Description");
System.out.print("Number");
System.out.println();
}

public String toString()
{
 return this.getItemNumber()+String.format("  $%6.2f   ",+this.getCost())+String.format("%4d       ",+this.getQuantity())+this.getDescription();
}
}//end class ItemRecord

以下是另一个测试类：此类包含ObjectOutputStream和ObjectInputStream个对象。因此，方法main和我用来调用main中方法的ItemRecordReport对象。

import java.io.*;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.Scanner;

public class ItemRecordReport
{  private ObjectOutputStream output;//writes to binary 
private ObjectInputStream binInFile;//reads binary
private Scanner input;

public void openInputBinaryFile()
 {//create a ObjectInputStream object that will read a binary file
  try
  {
    binInFile  = new ObjectInputStream(new FileInputStream("ItemRecord_binary.txt"));

  }//end try
  catch (IOException ioException)
  {
     System.out.println("Error opening file.");
  }//end catch 
  }//end openInputBinaryFile

  public void openOutputBinaryFile()
  {  
 //creating a binary objectOutputStream text file
  try
  {
     output = new ObjectOutputStream(
     new FileOutputStream("ItemRecord_binary.txt"));
   }//end try
   catch (IOException ioException)
   {
     System.err.println("Error opening file");
  }

  }//end openOutputBinaryFile
  public void readInputTextRecords()
  {
     ItemRecord record;
     String itemNumber;
     double cost;
     int quantity;
     String description;


     try
     {
       input  = new Scanner(new File("ItemRecord_text.txt"));
     }

      catch (IOException e)
     {
        System.out.println("Error file not found");
     }//end catch

      while(input.hasNext())
        {
        try{
              itemNumber = input.next();
              cost = input.nextDouble();
              quantity = input.nextInt();
              description = input.nextLine();

              record = new ItemRecord(itemNumber,cost,quantity,description);

              //writing into the binary text file
              output.writeObject(record);
           }

           catch (IOException ioException)
           {
              System.out.println("Error writing to binary file.");
              return;
           }
        }  
    }//end readInputRecords

   public void readInputBinaryRecords()
   {
     openInputBinaryFile();
     ItemRecord otherRecord;

     try
     {   while (true)
        {
           otherRecord = (ItemRecord) binInFile.readObject(); //use down cast
           System.out.println(otherRecord.toString());
        }
     }
      catch (EOFException e)
     {
        return;
     }//end catch
      catch (ClassNotFoundException e)
     {
        System.err.println("Unable to create object.");
     }//end catch 
     catch (IOException ioException)
     {
        System.err.println("Error reading from binary file.");
     }//end catch 

   }//readInputBinaryRecords
   public void closeBinaryOutputFile()
  {
  try
  {  if (output !=null )
        output.close();
  }//end try
  catch ( IOException ioException)
  {
     System.out.println( "Error closing file");
     System.exit(1);
  }//end catch
   }//end closeBinaryOutputFile

  public void closeBinaryInputFile()
 {
  try
  {  if (binInFile != null )
        binInFile.close();
     else
        System.out.println("No records were written to the binary file!");
  }//end try
  catch ( IOException ioException)
  {
     System.out.println( "Error closing file");
     System.exit(1);
  }//end catch
  }//end closeBinaryFile

 public static void main(String[] args)
{
 ItemRecordReport object = new ItemRecordReport();

 ItemRecord.printHeading();//print heading
 object.openOutputBinaryFile();//opens the binary file
 object.readInputTextRecords();//reads text file and writes to binary file
 object.closeBinaryOutputFile();//closes the binaryOutputFile

 object.openInputBinaryFile();//opens binary file
 object.readInputBinaryRecords();
 object.closeBinaryInputFile();
 Scanner inFile;
 ItemRecord[] itemRecord = new ItemRecord[5];


  itemRecord[x] = new ItemRecord();


  for (int x = 0; x <= 4; x++){
     itemRecord[x].setItemNumber("");
     itemRecord[x].setCost(0);
     itemRecord[x].setQuantity(0);
     itemRecord[x].setDescription("");
  }

  try
  {
     inFile = new Scanner(new File("itemRecord.txt"));

     while(inFile.hasNext())
      {
      for(int count =0; count<=4; count++)
        {
          itemRecord[count].setItemNumber(inFile.next());
          itemRecord[count].setCost(inFile.nextDouble());
          itemRecord[count].setQuantity(inFile.nextInt());
          itemRecord[count].setDescription(inFile.next());

     }//end for loop 
   }//end while

  }
  catch (IOException e)
  {
     System.out.println("Error file not found");
  }//end catch
  try
  {
  ObjectOutputStream binOutFile = new ObjectOutputStream
  (new FileOutputStream("ItemRecord_binary.txt"));
  }//end try
  catch (IOException e)
  {
  System.err.println("error opening file");
  }
  System.out.println(itemRecord.toString());



 }//end method main 



}//end class ItemRecordReport

这是文本文件：

A100 99.99 10 Canon PowerShot-135
A200 149.99 50 Panasonic-Lumix T55
A300 349.99 20 Nikon- D3200 DSRL
A400 280.99 30 Sony- DSC-W800
A500 97.99 20 Samsung- WB35F

这是用于更改字段值的新文本文件：

B100 98.00 10 ABC1010
B200 97.00 15 DEF1020
B300 96.00 10 GHI1030
B400 95.00 05 JKL1040
B500 94.00 01 MNO1050

Error stack trace
Item   Cost     Quantity  Description
Number
A100  $ 99.99   10        Canon PowerShot-135
A200  $149.99   50        Panasonic-Lumix T55
A300  $349.99   20        Nikon- D3200 DSRL
A400  $280.99   30        Sony- DSC-W800
A500  $ 97.99   20        Samsung- WB35F
Exception in thread "main" java.lang.NullPointerException
at ItemRecordReport.endRecord(ItemRecordReport.java:165)
at ItemRecordReport.main(ItemRecordReport.java:156)

 ----jGRASP wedge2: exit code for process is 1.
 ----jGRASP: operation complete.

Answer 1

这里有很多问题。 NullPointerException来自于您已初始化ItemRecord[] 数组而非其任何元素的事实。你需要添加

itemRecord[x] = new ItemRecord();

以上第153行。

然而还有其他问题。

您正在编写自己的错误消息，而不是自己打印更有用的异常消息，或者，当您仍在调试时，打印堆栈跟踪。例如：
```
catch (IOException ioException)
{
    ioException.printStackTrace();
}
```

您正在捕捉异常并继续进行，就好像它们没有发生一样。取决于try块中代码成功的代码应该在同一个try块中。例如：

try
{
    input = new Scanner(new File("ItemRecord_text.txt"));
}
catch (IOException e)
{
    e.printStackTrace();
}//end catch
while (input.hasNext())
// ...

应该是

try
{
    input = new Scanner(new File("ItemRecord_text.txt"));
    while (input.hasNext())
    // ...
}
catch (IOException e)
{
    e.printStackTrace();
}//end catch

您无法扫描＆＃34; ItemRecord.txt＆＃34;文件正确。它有五列，但您只扫描其中四列。留给读者练习。
我建议你放弃写作的习惯
```
} // end catch
```
等等。现代的IDE和适当的缩进使得这种完整性变得不必要，并且它实际上在实践中受到妨碍，并且它们经常会出错。你可能已经在学校里教过它，你当然应该做你想做的任何事情来获得你想要的成绩，但是在这里人们不会做太多。几十年来，Haven没见过它。

Serializable对象中的NullPointerException

1 个答案: