如何在java中讀取文本文件的第一行並將其打印出來？

Question

我想通過寫入文件來保存狀態，然後用bufferedReader以後再讀取文件以再次獲取狀態。這裏是我的代碼：

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();   
} 

如果我按運行，我被打印出來null。我試過它：

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

但是從邏輯上講，它現在沒有打印出任何東西。我不知道我在這裏做錯了什麼。你能幫我嗎？

Answer 1

當寫一個的BufferedWriter文件，內容並未直接寫入到磁盤，因爲它顯然是緩衝。您應該在他的回答中將內容刷新爲@ludo_rj建議的內容。

更好的辦法是儘早關閉作者，這會自動刷新內容。順便說一下，關閉閱讀器也是必要的。

你應該用下面的機制去（我已分發的方式分成幾個方法，使之更加清楚）：

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(); 
     } 
    } 

} 

請注意，我用的嘗試，與資源的語句，只適用於Java 7（當然還有8）。如果你是在一箇舊版本的Java運行它，你必須寫的方法如下：

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

A BufferedWriter緩衝區輸出，直到您刷新您寫入的流爲止，因此您需要通過關閉文件或使用save.flush()來刷新流。

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(); 
    } 

Answer 3

// 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();