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这是一个大学课程作业，它创建一个ItemRecord类并 Serializable使用 getter 和 setter 实现，然后覆盖该toString方法。接下来，我创建了一个ItemRecordReport类，其Scanner对象映射到输入文件“ItemRecord_text.txt”。创建了一个ObjectOutputStream映射到二进制输出文件“ItemRecord_binary.txt”的对象。创建了一个ObjectInputStream映射到二进制文件的对象。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这篇文章。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，使类的对象可以序列化，实现接口Serialiable //创建带有4个参数的构造函数，附带get和set方法，覆盖toString方法 //创建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方法改变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

这是另一个测试类：该类包含ObjectOutputStream和ObjectInputStream对象。因此 main 方法以及ItemRecordReport我用来调用 main 中的方法的对象都在这里。

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.
4

1 回答 1

0

这里有很多问题。这NullPointerException是因为您已经初始化了ItemRecord[] 数组，但没有初始化它的任何元素。您需要添加

itemRecord[x] = new ItemRecord();

在第 153 行之上。

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();
}

  • 您正在捕获异常并继续进行，就好像它们没有发生一样。依赖于 try 块中代码成功的代码应该在同一个 try 块内。例如：

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

    应该

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

  • 您没有正确扫描“ItemRecord.txt”文件。它有五列，但您只扫描其中的四列。留给读者作为练习。

  • 我建议你改掉写作的习惯

    } // end catch

    等等。现代 IDE 和适当的缩进使这完全没有必要，实际上它在实践中会妨碍它们，而且它们经常会出错。你可能在学校就被教过，你当然应该做任何你必须做的事情来获得你想要的成绩，但是这里的人做的不多。几十年没见过了。

于 2014-12-15T00:14:58.590 回答