從文件中讀取文本行

Question

我有一個包含 5 個客戶（每行 1 個）的文本文件，客戶 1、客戶 2、客戶 3、客戶 4 和客戶 5。使用以下代碼，它完美地讀取了 5 行文本；

import java.io.*;

public class CustomerIO  {

public void method () {

        try (BufferedReader br = new BufferedReader(new FileReader(new File ("Customers.txt")))) {
            int numberOfLines = readLines();
            String [] text = new String [numberOfLines];

            for (int i = 0; i < numberOfLines; i++) {
                text[i] = br.readLine();        
            }

            for (int i = 0; i < numberOfLines; i++) {
                System.out.println(text[i]);        
            }
        } catch (Exception e) {
            e.printStackTrace();
        }


    private int readLines() {

        try (BufferedReader br = new BufferedReader(new FileReader(new File ("Customers.txt")))) {

            while ((line = br.readLine()) != null) {
                numberOfLines++;
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
        return numberOfLines;
        }
          }

但是，當我更改為以下輸出時：Customer 2, Customer 4, null

import java.io.*;

public class CustomerIO  {

    String line;
    int numberOfLines = 0;

    public void method () {

        try (BufferedReader br = new BufferedReader(new FileReader(new File ("Customers.txt")))) {

            while (br.readLine() != null) {
                System.out.println(br.readLine());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }


    }

    private int readLines() {

        try (BufferedReader br = new BufferedReader(new FileReader(new File ("Customers.txt")))) {

            while ((line = br.readLine()) != null) {
                numberOfLines++;
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
        return numberOfLines;
        }
}

main 方法包含在以下 runner.class 文件中

public class Runner {

    public static void main(String[] args) {

        CustomerIO cus = new CustomerIO ();
        cus.method();
    }
}

你能幫我理解為什么會這樣嗎？ 我想在正確讀取時將客戶讀入數組列表，而不是使用字符串 []。

謝謝

Answer 1

問題是您為每個循環調用 readline 兩次。

這是您的錯誤代碼：

while (br.readLine() != null) {
    System.out.println(br.readLine());

一個可行的解決方案可能是：

String line = br.readline();
while (line != null) {
    System.out.println(line);
    line = br.readline()
}

Answer 2

以下代碼在每次循環迭代中調用readLine()兩次，這就是您只能看到每隔一行的原因：

while (br.readLine() != null) {
    System.out.println(br.readLine());
}

您需要將返回值分配給一個變量，以便您可以檢查是否為空。

有很多方法可以做到這一點，所以我將按推薦的降序展示一些。

// Using for loop (recommended)
//   Pro: Keeps the loop logic with the loop
//        Limits the scope of the variable
//        Smallest amount of code (lines of loop code: 2)
//   Con: Unusual construct
//        Inline assignment
for (String line; (line = br.readLine()) != null; ) {
    System.out.println(line);
}

// Using while loop with inline assignment
//   Pro: Common construct
//        Keeps the loop logic with the loop
//        Small amount of code (lines of loop code: 3)
//   Con: Variable scope not limited to the loop
//        Inline assignment
String line;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}

// Using while loop without inline assignment
//   Pro: Common construct
//   Con: Loop logic both before and at end of loop
//        Variable scope not limited to the loop
//        More code (lines of loop code: 4)
//        Calling the readLine() method in two places
String line = br.readLine();
while (line != null) {
    System.out.println(line);
    line = br.readLine();
}

// Using break to exit forever loop
//   Pro: Limits the scope of the variable
//   Con: Loop logic both before and at end of loop
//        Using infinite loop and the break statement
//        More code (lines of loop code: 6)
for (;;) { // or: while (true)
    String line = br.readLine();
    if (line == null) {
        break;
    }
    System.out.println(line);
}