java - 如何检查是否读取最后一行并且如果在最后一行，如何在JAVA中的文件末尾添加新行？

Question

我对编程相对陌生，尤其是 Java，所以在回答时请记住这一点。

我正在编写一个简单的集换式纸牌游戏套牌构建程序，但文件读取/写入被证明是有问题的。

这是我正在尝试使用的“addDeck”方法的代码:

/**
 * Adds a deckid and a deckname to decks.dat file.
 */
public static void AddDeck() throws IOException {
    // Opens the decks.dat file.
    File file = new File("./files/decks.dat");
    BufferedReader read = null;
    BufferedWriter write = null;

    try {
        read = new BufferedReader(new FileReader(file));
        write = new BufferedWriter(new FileWriter(file));
        String line = read.readLine();
        String nextLine = read.readLine();

        String s = null; // What will be written to the end of the file as a new line. 
        String newDeck = "Deck ";
        int newInd = 00; // Counter index to indicate the new deckid number.
                         // If there are already existing deckids in the file,
                         // this will be the biggest existing deckid number + 1.

        // If the first line (i.e. the whole file) is initially empty,
        // the following line will be created: "01|Deck 01", where the
        // number before the '|' sign is deckid, and the rest is the deckname.
        if (line == null) {
            s = "01" + '|' + newDeck + "01";
            write.write(s);
        }

        // If the first line of the file isn't empty, the following happens:
        else {

            // A loop to find the last line and the biggest existing deckid of the file.
            while (line != null) {

                // The following if clause should determine whether or not the next
                // line is the last line of the file.
                if ((nextLine = read.readLine()) == null) {

                    // Now the reader should be at the last line of the file.
                    for (int i = 0; Character.isDigit(line.charAt(i)); i++) {

                        // Checks the deckid number of the last line and stores it.
                        s += line.charAt(i);    
                    }

                    // The value of the last existing deckid +1 will be stored to newInd.
                    // Also, the divider sign '|' and the new deckname will be added.
                    // e.g. If the last existing deckid of decks.dat file is "12",
                    // the new line to be added would read "13|Deck 13".

                    newInd = (Integer.parseInt(s)) + 1;
                    s += '|' + newDeck + newInd;
                    write.newLine();
                    write.write(s);
                }

                else {

                    // If the current line isn't the last line of the file:
                    line = nextLine;
                    nextLine = read.readLine();
                }
            }
        }       

    } finally {
            read.close();
            write.close();
    }
}

每次调用addDeck方法时，decks.dat文件都会加长一行。但无论我调用这个方法多少次，Decks.dat 只有一行“01|Deck 01”。

另外，我需要创建一个方法removeDeck，它从decks.dat文件中删除一整行，而我对此更加不知所措。

如果有任何帮助，我将非常感激!

Answer 1

对于初学者来说，每次程序运行时，此行都会创建一个名为 Decks.dat 的新文件。也就是说，它总是会覆盖文件的内容。

文件 file = new File("./files/decks.dat");

因此，if (line == null) { 始终计算为 true，并且文件中始终会出现“01|Deck 01”。

要解决此问题，请删除上面的行并打开 BufferedReader，如下所示:read = new BufferedReader(new FileReader("./files/decks.dat"));

第二个问题是，你不能真正打开同一个文件来同时读写，所以你不应该像你那样打开write。我建议你将更新后的版本收集到一个变量中(我建议StringBuilder)，最后将该变量的内容写入decks.dat文件中。

一旦解决了这些问题，您就应该能够在您想做的事情上取得进展。