从文件中读取文本行

Question

I have a text file containing 5 customers (1 per line), Customer 1, Customer 2, Customer 3, Customer 4 and Customer 5. Using the following code, it reads the 5 lines of text perfectly;

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;
        }
          }

However when I change to the following the output is: 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;
        }
}

The main method is contained in the following runner.class file

public class Runner {

    public static void main(String[] args) {

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

Can you help me understand why this is happening? I want to read the customers into an arraylist when its reading in correctly rather than working with a String [].

Thanks

Answer 1

The problem is that you are calling readline twice for each loop.

Here is your buggy code:

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

A working solution could be:

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

Answer 2

The following code calls readLine() twice per iteration of the loop, which is why you only see every other line:

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

You need to assign the returned value to a variable, so you can check for null.

There are many ways to do that, so I'll show some, in decreasing order of recommendation.

// 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);
}