如何在java中读取文本文件的第一行并打印出来？

Question

我想通过写入文件来保存状态，然后稍后使用bufferedReader 读取文件以再次获取状态。这是我的代码：

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

   FileWriter fw = new FileWriter("C:\\Users\\Nicolas\\Desktop\\save.txt");
   BufferedWriter save = new BufferedWriter(fw);

   save.write("helloWorld");

   BufferedReader r = new BufferedReader(new FileReader("C:\\Users\\Nicolas\\Desktop\\save.txt"));

   System.out.println(r.readLine());

   save.close();        
}

如果我按下运行，我会打印出null。我试过了：

if(r.readLine() != null){
    System.out.println(r.readLine());
}

但从逻辑上讲，它现在什么也没打印出来。我不知道我在这里做错了什么。你能帮帮我吗？

Answer 1

当使用 BufferedWriter 写入文件时，内容不会直接写入磁盘，因为它显然是缓冲的。您应该按照@ludo_rj 在他的回答中建议的那样刷新内容。

更好的是尽早关闭编写器，这将自动刷新内容。顺便说一句，关闭阅读器也是必要的。

您应该使用以下机制（我已将该方法分为几种方法以使其更清晰）：

public class SaveStateTesing {

    private static final String FILE_NAME = "C:\\Users\\Nicolas\\Desktop\\save.txt";

    public static void main(String[] args) throws IOException {
        saveState("helloWorld", FILE_NAME);
        String state = readState(FILE_NAME);
        System.out.println(state);
    }

    private static void saveState(String state, String fileName) throws IOException {
        try(PrintWriter writer = new PrintWriter(new BufferedWriter(new FileWriter(fileName)))) {
            writer.println(state);
        }
    }

    private static String readState(String fileName) throws IOException {
        try(BufferedReader reader = new BufferedReader(new FileReader(fileName))) {
            return reader.readLine();
        }
    }

}

请注意，我使用了 try-with-resource 语句，该语句仅在 Java 7（当然还有 8）中可用。如果你在旧的 Java 版本上运行它，你必须编写如下方法：

    private static void saveState(String state, String fileName) throws IOException {
        PrintWriter writer = null;
        try {
            writer = new PrintWriter(new BufferedWriter(new FileWriter(fileName)));
            writer.println(state);
        } finally {
            if (writer != null)
                writer.close();
        }
    }

    private static String readState(String fileName) throws IOException {
        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new FileReader(fileName));
            return reader.readLine();
        } finally {
            if (reader != null)
                reader.close();
        }
    }

Answer 2

  // Write to file      
  FileWriter fw = new FileWriter("C:\\Users\\Nicolas\\Desktop\\save.txt");
  BufferedWriter save = new BufferedWriter(fw);

  save.write("example");
  save.flush()

  // read as stream
  BufferedReader r = new BufferedReader(new FileReader("C:\\Users\\Nicolas\\Desktop\\save.txt"));
  System.out.println(r.readLine());

  save.close();

Answer 3

BufferedWriter 会缓冲输出，直到您刷新写入的流，因此您需要通过关闭文件或使用 save.flush() 来刷新流。

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

        FileWriter fw = new FileWriter("C:\\Users\\Nicolas\\Desktop\\save.txt");
        BufferedWriter save = new BufferedWriter(fw);

        save.write("helloWorld");
        save.flush();  // this writes the bytes in the stream to the file.

        BufferedReader r = new BufferedReader(new FileReader("C:\\Users\\Nicolas\\Desktop\\save.txt"));

        System.out.println(r.readLine());

        save.close();
    }