写一个已经存在的文件

Question

如何编写FileOutputStream存在的文件？当我运行两次此程序时，第二次oos和fos为空

 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));

如果你想附加到文件

Answer 2

如果您不想覆盖该文件，请将true参数添加到File或Fileoutputstream构造函数

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

如果您打算撰写纯文本，请尝试使用FileWriter代替FileOutputStream。

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

第二个参数（true）将告诉追加文件。

Answer 4

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

在您的代码中，您有ObjectOutputStream oos = null;，因此oos为null。你需要初始化它。像这样：

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

只需创建新的FileOutputStream，将第二个参数设置为true

FileOutputStream d = new FileOutputStream(file, append);