Just change PrintWriter out = new PrintWriter(log); to

PrintWriter out = new PrintWriter(new FileWriter(log, true));

TBH, just open the file and handle the FileNotFoundException as file systems can and tend to be volatile meaning in between the test a filecan be created or be destroyed just after the test. However, the issue is FileWriter is creating the file so the else will almost always be true...

PrintWriter out = null;
try {

out = new PrintWriter(new BufferedWriter(new FileWriter("outfilename", 

true)));
    out.println("the text");
} catch (IOException e) {
    e.printStackTrace ();
} 
finally {
  if (out != null) {
     out.close ();
  }
}

The second parameter to the FileWriter constructor will tell it to append to the file (as opposed to clearing the file).

You need use the boolean value true. From docs

public FileWriter(String fileName,
          boolean append)
           throws IOException

Constructs a FileWriter object given a file name with a boolean indicating whether or not to append the data written.

Parameters:

fileName - String The system-dependent filename.

append - boolean if true, then data will be written to the end of the file rather than the beginning.

First, you are calling createNewFile(). This indicates that you want to create a new file. If you do not want to create a new file, do not call createNewFile(). And, since the documentation for createNewFile() says "This method is not generally useful", you may wish to consider just getting rid of it.

Second, you need to indicate that you want to append to the existing data when you open and use that file. The standard recipe for this:

try {
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("outfilename", true)));
    out.println("the text");
    out.close();
} catch (IOException e) {
    //oh noes!
}

The true in the second parameter to the FileWriter constructor indicates that you want to append instead of overwrite.

Third, do not hardcode paths in Android. /sdcard/Output1.txt has been poor form for years and will not work on some Android devices.

Fourth, do not clutter up the root of external storage. Hence, instead of:

file = new File("/sdcard/Output1.txt");

use:

file = new File(getExtenalFilesDir(null), "Output1.txt");

to put the file in a subdirectory unique for your own app.

It is advised to use chain of BufferedWriter and FileWriter, and the key point is FileWriter will append String to current file when use the one of its constructor that lets appaneding by adding true to last paramter like

new FileWriter("login.txt", true)        

and when we surrounding it with BufferedWriter object in order to be more efficient if you are going to write in the file number of time, so it buffers the string in big chunk and write the big chunk into a file and clearly you can save a lot of time for writing into a file

Note :It is possible not to use BuffredWriter ,but it is advised because of better performance and ability to buffer the big chunk of Strings and write them once

Just change your

PrintWriter out = new PrintWriter("login.txt"); 

to

PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("login.txt", true)));

Example:

try(PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("login.txt", true)));) {
    String data = "This content will append to the end of the file";
    File file = new File("login.txt");
    out.println(data);
} catch(IOException e) {
}

It is possible to solve this issue without using BufferedWriter, yet the performance will be low as I mentioned.

Example:

try (PrintWriter out = new PrintWriter(new FileWriter("login.txt", true));) {
    String data = "This content will append to the end of the file";
    File file = new File("login.txt");
    out.println(data);
} catch (IOException e) {
}

You can use FileUtils CommonsIO 2.4 API

Example

package com.doc;

import java.io.File;
import java.io.IOException;

import org.apache.commons.io.FileUtils;

public class Test {

    public static void main(String[] args) throws IOException {

        File file = new File("test.txt");
        FileUtils.write(file, "\n my data", true);
        FileUtils.write(file, "\n my data", true);
        FileUtils.write(file, "\n my data", true);

    }
}

output

 my data
 my data
 my data