为什么在尝试从文件中读取时出现错误？

Question

我正在编写一个可以使用经典加密方法对消息进行编码和解码的程序。消息将从文件中读取并写入输出文件。以下程序是用 java 编写的，并且编译没有错误。当我运行程序以测试它是否在我给它输入和输出文件的名称之后到目前为止它运行到某种异常抛出错误。我认为问题在于代码中的 for 循环。这是一个将所有消息存储到字符数组中的循环。有什么建议可以修复它或其他更好的数据结构（如堆栈或队列）？

import java.io.*;
import java.util.*;

class CryptoProject1
{
static char eord;
static Scanner cin=new Scanner(System.in);
static char [] message=new char[10000];

public static void main (String [] args)
    throws IOException
{
    //getting the input txt file name from user
    String infilename;
    System.out.println("Please give the name of the input file.");
    infilename=cin.nextLine();
    Scanner fileread=new Scanner (new FileReader(infilename));  

    //getting the output txt file name from user
    String outfilename;
    System.out.println("Please give the name of the output file.");
    outfilename=cin.nextLine();
    PrintWriter filewrite=new PrintWriter(new FileWriter(outfilename));

    //saving the message into an array
    //construct/make it into a usable function??
    for(int i=0; i<message.length; i++)
    {
        message[i]=fileread.next().charAt(0);
    }

    //trial to make sure it reads and writes correctly
    //printing the message onto the output file
    for(int i=0; i<message.length; i++)
    {
        filewrite.print(message[i]);
    }

}   

Answer 1

for(int i=0; i<message.length; i++)
{
  message[i]=fileread.next().charAt(0);
}

在这里，您无法知道文件长度是否与消息相同或更长，文件可能有 10 个字符。你需要这样做：

for(int i=0; i<message.length; i++)
{
  if(!fileread.hasNext())
    break;
  message[i]=fileread.next().charAt(0);
}

简单检查一下是否还有要从文件中读取的内容，如果没有则停止读取。

此外，习惯上使用 Java 对象 File 来表示文件，而不是使用字符串来保存文件路径。示例：

private File output;
public void create file(String path)
{
    output = new File(path);
}

private BufferedWriter out = new BufferedWriter(new FileWriter(output));

Answer 2

此外，无论何时写入文件，请确保通过调用 close() 方法将其关闭，否则将不会保存其内容

Answer 3

//I ran this code without an error
//getting the input txt file name from user
String infilename;
System.out.println("Please give the name of the input file.");
infilename=cin.nextLine();
Scanner fileread=new Scanner (new FileReader(infilename));  

//getting the output txt file name from user
String outfilename;
System.out.println("Please give the name of the output file.");
outfilename=cin.nextLine();
PrintWriter filewrite=new PrintWriter(new FileWriter(outfilename));

//saving the message into an array
//construct/make it into a usable function??
for(int i=0; i<message.length; i++)
{
    if(fileread.hasNext())
    message[i]=fileread.next().charAt(0);
}
fileread.close();

//trial to make sure it reads and writes correctly
//printing the message onto the output file
for(int i=0; i<message.length; i++)
{
    filewrite.print(message[i]);
}
filewrite.close();
}