java - 逐行读取文件 - 到达最后一行后终止 while 循环

Question

读取文件并打印所有字母字符的程序，当到达最后一行时抛出 NullPointerException。

import java.io.*;

public class Foo {

    public static void main(String[] args) throws IOException {

        FileReader file = new FileReader(new File("source.txt"));

        BufferedReader read = new BufferedReader(file);

        String line = read.readLine();

        while (line != null) {
            for (int i = 0; i < line.length(); i++) {
                line = read.readLine(); // this is where the problem is. When it reaches the last line, line = null and the while loop should terminate!
                if (Character.isLetter(line.charAt(i))) {
                    System.out.print(line.charAt(i));
                }
            }
        }
    }

}

Answer 1

While 循环不像您在评论中解释的那样工作:

// this is where the problem is. When it reaches the last line, line = null and the while loop should terminate!

While 循环仅检查每次迭代开始时的条件。它们不会仅仅因为下一次迭代开始时条件为假而终止中循环。

因此，您在 while (line != null) 开始时进行的空检查只会且始终在每次迭代的开始时发生，即使 line 在迭代中设置为 null

正如其他人所表明的，您可以将 while 循环构造为:

String line = null;

while ((line = read.readLine()) != null) 
{
    for (int i = 0; i < line.length(); i++) 
    {
        if (Character.isLetter(line.charAt(i))) 
        {
            System.out.print(line.charAt(i));
        }
    }
}

并从代码中删除所有其他read.readLine()语句。 (这是最短的代码行)。

或者，如果您想更明确地提高可读性，您可以保留初始的 read.readLine() ，但移动迭代的 read.readLine() 完成所有对 line 的使用后:

String line = read.readLine();

while (line != null) 
{
    for (int i = 0; i < line.length(); i++) 
    {
        if (Character.isLetter(line.charAt(i))) 
        {
            System.out.print(line.charAt(i));
        }
    }
    line = read.readLine();
    //line is never used after this so an NPE is not possible
}