java - 写入一个已经存在的文件

Question

如何写入 FileOutputStream 中存在的文件？当我运行该程序两次时，第二次 oos 和 fos 为 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));

如果你想追加到文件