我的回答:

JFileChooser chooser= new JFileChooser();
chooser.showOpenDialog(chooser);
File file = chooser.getSelectedFile();
String Content = "What you want to append to file";

try 
{
    RandomAccessFile random = new RandomAccessFile(file, "rw");
    long length = random.length();
    random.setLength(length + 1);
    random.seek(random.length());
    random.writeBytes(Content);
    random.close();
} 
catch (Exception exception) {
    //exception handling
}

在Java-7中也可以这样做:

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

//---------------------

Path filePath = Paths.get("someFile.txt");
if (!Files.exists(filePath)) {
    Files.createFile(filePath);
}
Files.write(filePath, "Text to be added".getBytes(), StandardOpenOption.APPEND);

1.7的方法:

void appendToFile(String filePath, String content) throws IOException{

    Path path = Paths.get(filePath);

    try (BufferedWriter writer = 
            Files.newBufferedWriter(path, 
                    StandardOpenOption.APPEND)) {
        writer.newLine();
        writer.append(content);
    }

    /*
    //Alternative:
    try (BufferedWriter bWriter = 
            Files.newBufferedWriter(path, 
                    StandardOpenOption.WRITE, StandardOpenOption.APPEND);
            PrintWriter pWriter = new PrintWriter(bWriter)
            ) {
        pWriter.println();//to have println() style instead of newLine();   
        pWriter.append(content);//Also, bWriter.append(content);
    }*/
}

    String str;
    String path = "C:/Users/...the path..../iin.txt"; // you can input also..i created this way :P

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    PrintWriter pw = new PrintWriter(new FileWriter(path, true));

    try 
    {
       while(true)
        {
            System.out.println("Enter the text : ");
            str = br.readLine();
            if(str.equalsIgnoreCase("exit"))
                break;
            else
                pw.println(str);
        }
    } 
    catch (Exception e) 
    {
        //oh noes!
    }
    finally
    {
        pw.close();         
    }

这将达到你的目的。

FileOutputStream fos = new FileOutputStream("File_Name", true);
fos.write(data);

true允许在现有文件中追加数据。如果我们写

FileOutputStream fos = new FileOutputStream("File_Name");

它将覆盖现有文件。所以选择第一种方法。

您可以使用fileWriter将标志设置为true，用于追加。

try
{
    String filename= "MyFile.txt";
    FileWriter fw = new FileWriter(filename,true); //the true will append the new data
    fw.write("add a line\n");//appends the string to the file
    fw.close();
}
catch(IOException ioe)
{
    System.err.println("IOException: " + ioe.getMessage());
}