寫一個已經存在的文件

Question

如何編寫FileOutputStream存在的文件？ 當我運行兩次此程序時，第二次oos和fos為null

 public class ReadFile {
    static FileOutputStream fos = null;
    static ObjectOutputStream oos = null;
    public static void main(String[] args) throws IOException, ClassNotFoundException {

        File f = new File("file.tmp");
        if (f.exists()) {
            //How to retreive an old oos to can write on old file ?
            oos.writeObject("12345");
            oos.writeObject("Today");
        }
        else
        {
            fos = new FileOutputStream("file.tmp");
            oos = new ObjectOutputStream(fos);
        }
        oos.close();
    }
 }

Answer 1

ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(f,true));

如果要附加到文件

Answer 2

如果您不想覆蓋文件，則將true參數添加到File或File OutputStream構造函數中

new FileOutputStream( new File("Filename.txt"), true );

Parameters:
name - the system-dependent file name
append - if true, then bytes will be written to the end of the file rather than the beginning 

Answer 3

如果打算編寫純文本，請嘗試使用FileWriter而不是FileOutputStream 。

    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("outfilename", true)));
    out.println("the text");

第二個參數（ true ）告訴追加到文件。

Answer 4

ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(f, true));

在您的代碼中，您有ObjectOutputStream oos = null; ，因此oos為null 。 您需要初始化它。 像這樣：

public class ReadFile {
    static FileOutputStream fos = null;
    static ObjectOutputStream oos = null;
    public static void main(String[] args) throws IOException, ClassNotFoundException {

        File f = new File("file.tmp");
        oos = new ObjectOutputStream(new FileOutputStream(f, true));
        if (f.exists()) {
            //How to retreive an old oos to can write on old file ?
            oos.writeObject("12345");
            oos.writeObject("Today");
        }
        else
        {
            fos = new FileOutputStream(f, true);
            oos = new ObjectOutputStream(fos);
        }
        oos.close();
    }
 }

Answer 5

只需創建新的FileOutputStream並將第二個參數設置為true

FileOutputStream d = new FileOutputStream(file, append);