The fact that PrintWriter's method is called append() doesn't mean that it changes mode of the file being opened.

You need to open file in append mode as well:

PrintWriter pw = new PrintWriter(new FileOutputStream(
    new File("persons.txt"), 
    true /* append = true */)); 

Also note that file will be written in system default encoding. It's not always desired and may cause interoperability problems, you may want to specify file encoding explicitly.

Do this in order to create a PrinterWriter working with a FileWriter in append mode:

PrintWriter outFile = new PrintWriter(new FileWriter("planetaryData.txt", true)); 

Are you doing this for logging purposes? If so there are several libraries for this. Two of the most popular are Log4j and Logback.

Java 7+

For a one-time task, the Files class makes this easy:

try {
    Files.write(Paths.get("myfile.txt"), "the text".getBytes(), StandardOpenOption.APPEND);
}catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Careful: The above approach will throw a NoSuchFileException if the file does not already exist. It also does not append a newline automatically (which you often want when appending to a text file). Another approach is to pass both CREATE and APPEND options, which will create the file first if it doesn't already exist:

private void write(final String s) throws IOException {
    Files.writeString(
        Path.of(System.getProperty("java.io.tmpdir"), "filename.txt"),
        s + System.lineSeparator(),
        StandardOpenOption.CREATE, StandardOpenOption.APPEND
    );
}

However, if you will be writing to the same file many times, the above snippets must open and close the file on the disk many times, which is a slow operation. In this case, a BufferedWriter is faster:

try(FileWriter fw = new FileWriter("myfile.txt", true);
    BufferedWriter bw = new BufferedWriter(fw);
    PrintWriter out = new PrintWriter(bw))
{
    out.println("the text");
    //more code
    out.println("more text");
    //more code
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Notes:

• The second parameter to the FileWriter constructor will tell it to append to the file, rather than writing a new file. (If the file does not exist, it will be created.)
• Using a BufferedWriter is recommended for an expensive writer (such as FileWriter).
• Using a PrintWriter gives you access to println syntax that you're probably used to from System.out.
• But the BufferedWriter and PrintWriter wrappers are not strictly necessary.

Older Java

try {
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("myfile.txt", true)));
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Exception Handling

If you need robust exception handling for older Java, it gets very verbose:

FileWriter fw = null;
BufferedWriter bw = null;
PrintWriter out = null;
try {
    fw = new FileWriter("myfile.txt", true);
    bw = new BufferedWriter(fw);
    out = new PrintWriter(bw);
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}
finally {
    try {
        if(out != null)
            out.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(bw != null)
            bw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(fw != null)
            fw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
}

Once you call PrintWriter out = new PrintWriter(savestr); the file is created if doesn't exist hence first check for file existence then initialize it.

As mentioned in it's Constructor Docmentation as well that says:

If the file exists then it will be truncated to zero size; otherwise, a new file will be created.

Since file is created before calling f.exists() hence it will return true always and ìf block is never executed at all.

Sample code:

String savestr = "mysave.sav"; 
File f = new File(savestr);

PrintWriter out = null;
if ( f.exists() && !f.isDirectory() ) {
    out = new PrintWriter(new FileOutputStream(new File(savestr), true));
}
else {
    out = new PrintWriter(savestr);
}
out.append(mapstring);
out.close();

For better resource handling use Java 7 - The try-with-resources Statement

you have to open the file in append mode, which can be achieved by using the FileWriter(String fileName, boolean append) constructor.

output = new BufferedWriter(new FileWriter(my_file_name, true));

should do the trick