尝试从文件中读取文件时为什么会出现错误？

Question

I am writing a program that can encode and decode messages using classical encryption methods. The message is to be read in from a file and written to an output file. The following program is written in java and compiles with no errors. When I run the program to test if it works so far after I give it the names of the input and output file it runs into some kind of exception throwing error. I assume the issue lies in the for loop that follows in the code. It is a loop to store all of the message into an array of characters. Any suggestions to fix it or another data structure that would work better (like a stack or queue)?

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);
}

Here, you cannot know if file length is same or higher than message, file may as well have 10 characters. You need to do that:

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

Just a simple check if there is still something to be read from file, if not then stop reading.

Also it is customary to use the Java object File to represent files rather than using string to hold the file path. Example:

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

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

Answer 2

另外，每当写入文件时，请确保通过调用close（）方法将其关闭，否则将不保存其内容

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();
}