如何使用 java 编辑文本文件中的特定行？

Question

I am writing a program that lets the user enter up to 9999 accounts into a text file, however the issue i'm having is that they can be put in any order, but I have to print them in a sequential order. Here's my code

import java.nio.file.*;
import java.io.*;
import java.nio.channels.FileChannel;
import java.nio.ByteBuffer;
import static java.nio.file.StandardOpenOption.*;
import java.util.Scanner;
import java.text.*;
public class CreateBankFile {

    public static int lines = 0;

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        Path file = Paths.get("/root/sandbox/BankAccounts.txt");

        String line = "";
        int acctNum = 0;
        String lastName;
        double bal;
        final int QUIT = 9999;

        try
        {
            OutputStream output = new BufferedOutputStream(Files.newOutputStream(file));
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(output));

            while(acctNum != QUIT)
            {
                System.out.print("Enter the acct num less than 9999: ");
                acctNum = input.nextInt();
                if(acctNum == QUIT)
                {
                    continue;
                }
                System.out.print("Enter a last name: ");
                lastName = input.next();
                if(lastName.length() != 8)
                {
                    if(lastName.length() > 8)
                    {
                        lastName = lastName.substring(0, 8);
                    }
                    else if(lastName.length() < 8)
                    {
                        int diff = 8 - lastName.length();
                        for(int i = 0; i < diff; i++)
                        {
                            lastName += " ";
                        }
                    }
                }
                System.out.print("Enter balance: ");
                bal = input.nextDouble();

                line = "ID#" + acctNum + "  " + lastName + "$" + bal;

                writer.write(line);
                writer.newLine();

                lines++;
            }
            writer.close();
        }
        catch(IOException e)
        {
            System.out.println("Error");
        }
    }
}

My question being, how can I get it so that when the user inputs "55" for example, it is printed to the 55th line of the text file?

Answer 1

You can do something like this:

1) create a class that will store your line ( acctNum, lastName.. etc )

2) in your method, create an arraylist of the class you created, for a given number "n", your method will parse all the lines, if acctNum is less than "n", you will create a new instance using this line and add it to your arraylist

3) you will sort the arraylist using the acctNum and then print its content

Answer 2

Perhaps FileChannels would work for you:

RandomAccessFile writer = new RandomAccessFile(file, "rw");
FileChannel channel = writer.getChannel();
ByteBuffer buff = ByteBuffer.wrap("Test write".getBytes(StandardCharsets.UTF_8));

channel.write(buff,5);//5 is the distance in the file

Lots of good examples on the web.

Answer 3

In your question I think you are taking acctNum as line number and you want to add a line at this line number in the file so you can do something like this.

    List<String> readLines = Files.readAllLines(path, StandardCharsets.UTF_8);
    readLines.set(acctNum- 1, data);
    Files.write(path, lines, StandardCharsets.UTF_8);

I assumed that you are using Java 7 or higher so I did acctNum-1 because in Java 7 or higher version line number starts with 1 in rest it starts with 0 so you can change to acctNum .
Reference: List set() and NIO