如何在不创建新文件的情况下更新文件

Question

我的代码没有更新现有文件。所以我创建了新文件，但我想使用同一个文件并更新它。有人知道怎么做吗？我的代码在这里和我在做什么的图片

    try (BufferedWriter writer = new BufferedWriter(new FileWriter("project-output.csv"))) {
    try (BufferedReader reader = new BufferedReader(new FileReader("project.csv"))) {
    String line;
    while ((line = reader.readLine()) != null) {
    String[] cols = line.split(",");
    System.out.println("Please choose a criteria (2-7): ");
    final int subjectToGiveMark = in.nextInt(); // for creativity is 2
    System.out.println("Please enter a mark: ");
    final int mark = in.nextInt(); // which mark should be given
    cols[subjectToGiveMark] = Integer.toString(mark);
    // Here is where you write the output:
    writer.write(String.join(",", cols));
    writer.newLine();
}
writer.flush();

} }

enter image description here

Answer 1

按如下方式进行：

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        String line;
        List<String> dataLines = new ArrayList<String>();
        final int COLS = 6;
        String[][] data = null;
        try (BufferedReader br = new BufferedReader(new FileReader("project.csv"))) {
            while ((line = br.readLine()) != null) {
                dataLines.add(line);
            }
            // Initialise data[][] with the data from project.csv
            data = new String[dataLines.size()][COLS];
            for (int i = 0; i < dataLines.size(); i++) {
                data[i] = dataLines.get(i).split(",");// Split on comma
            }

            // Display Sarah's marks in Achievement (15)
            System.out.println(data[2][1] + "'s marks in Achievement (15) is " + data[2][3]);

            // Display Harry's marks in Knowledge (25)
            System.out.println(data[3][1] + "'s marks in Knowledge (25) is " + data[3][4]);
        } catch (Exception e) {
            e.printStackTrace();
        }

        // Update the file
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("project.csv"))) {
            // Increasing Sarah's marks in Achievement by 1
            int m = Integer.parseInt(data[2][3]) + 1;
            data[2][3] = String.valueOf(m);

            // Decreasing Harry's marks in Knowledge by 1
            m = Integer.parseInt(data[3][4]) - 1;
            data[3][4] = String.valueOf(m);

            //Write the updated data to file
            for (String[] row : data) {
                writer.write(String.join(",", row) + System.lineSeparator());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

输出：

Sarah's marks in Achievement (15) is 13
Harry's marks in Knowledge (25) is 24

project.csv的原始内容：

Student Id,Student Name,Creativity (10),Achievement (15),Knowledge (25),Documentation (25)
F1233,Bill,8,12,20,18
F2345,Sarah,9,13,22,23
F3456,Harry,9,14,24,24

project.csv的新内容：

Student Id,Student Name,Creativity (10),Achievement (15),Knowledge (25),Documentation (25)
F1233,Bill,8,12,20,18
F2345,Sarah,9,14,22,23
F3456,Harry,9,14,23,24

交互式更新数据示例：

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws FileNotFoundException, IOException {
        String line;
        List<String> dataLines = new ArrayList<String>();
        final int COLS = 6;
        String[][] data = null;
        try (BufferedReader br = new BufferedReader(new FileReader("project.csv"))) {
            while ((line = br.readLine()) != null) {
                dataLines.add(line);
            }
            // Initialise data[][] with the data from project.csv
            data = new String[dataLines.size()][COLS];
            for (int i = 0; i < dataLines.size(); i++) {
                data[i] = dataLines.get(i).split(",");// Split on comma
            }
        }

        // Update the file
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("project.csv"))) {
            Scanner in = new Scanner(System.in);

            // Updating existing record
            System.out.println("Updating " + data[2][1] + "'s marks in a subject...");
            System.out.print(
                    "Enter the subject number[2 for Creativity, 3 for Achievement, 4 for Knowledge, 5 for Documentation]: ");
            int col = Integer.parseInt(in.nextLine());
            if (col >= 2 && col <= 5) {
                System.out.print("Enter marks in the subject: ");

                data[2][col] = in.nextLine();

                // Write the updated data to file
                for (String[] row : data) {
                    writer.write(String.join(",", row) + System.lineSeparator());
                }
            }

            // Adding a new record
            System.out.println("Adding a new record...");
            String[] record = new String[COLS];
            System.out.print("Enter student ID: ");
            record[0] = in.nextLine();
            System.out.print("Enter student name: ");
            record[1] = in.nextLine();
            System.out.print(
                    "Enter marks in Creativity (10), Achievement (15), Knowledge (25), and Documentation (25): ");
            System.arraycopy(in.nextLine().split("\\s+"), 0, record, 2, COLS - 2);
            writer.write(String.join(",", record) + System.lineSeparator());
        }
    }
}

示例运行：

Updating Sarah's marks in a subject...
Enter the subject number[2 for Creativity, 3 for Achievement, 4 for Knowledge, 5 for Documentation]: 2
Enter marks in the subject: 7
Adding a new record...
Enter student ID: F4567
Enter student name: Richard
Enter marks in Creativity (10), Achievement (15), Knowledge (25), and Documentation (25): 8 12 20 21

project.csv的新内容：

Student Id,Student Name,Creativity (10),Achievement (15),Knowledge (25),Documentation (25)
F1233,Bill,8,12,20,18
F2345,Sarah,7,14,22,23
F3456,Harry,9,14,23,24
F4567,Richard,8,12,20,21

Answer 2

你可以试试

// Read existing file 
CSVReader reader = new CSVReader(new FileReader(inputFile), ',');
List<String[]> csvBody = reader.readAll();
// get CSV row column  and replace with by using row and column
csvBody.get(row)[col] = replace;
reader.close();

// Write to CSV file which is open
CSVWriter writer = new CSVWriter(new FileWriter(inputFile), ',');
writer.writeAll(csvBody);
writer.flush();
writer.close();