Why does out.write write null?

Question

Java

The main() method takes two parameters as input.
The first one is filePath - the path to the directory, the second one - resultFileAbsolutePath - the name (full path) of an existing file that will contain the result. Rename resultFileAbsolutePath to allFilesContent.txt . For each file in the filePath directory and all its nested subdirectories, do the following:
If the file has a length in bytes NOT greater than 50, then for all such files:
- Sort them by filename in ascending order, ignoring the path when sorting.
- In allFilesContent.txt, sequentially write the contents of all files of the sorted files. Write "\n" after each file body.
All files have a .txt extension.

I just can't figure out why null is written to allFilesContent.txt, all text files from the test directory are < 50 bytes.

File filePath = new File(args[0]);
       
        File resultAbsolutePath = new File(args[1]);
        File destination = new File(resultAbsolutePath.getParent() + "/allFilesContent.txt");
        FileUtils.renameFile(resultAbsolutePath, destination);
      
        ArrayList<File> fileList = getFileList(filePath);

        FileOutputStream out = new FileOutputStream(destination);
        
        Collections.sort(fileList);

        for(int i = 0; i < fileList.size(); i++){
            
            FileInputStream stream = new FileInputStream(fileList.get(i));
            BufferedReader reader = new BufferedReader(new InputStreamReader(stream));
            String str;
            
            while((str = reader.readLine()) != null){
                str += str;
            }
            stream.close();
            reader.close();
           
            out.write((str + "\n").getBytes());
            out.flush();
        }
        out.close();

    }

    public static ArrayList<File> getFileList(File path) {
        ArrayList<File> list = new ArrayList<>();
        for (File file : path.listFiles()) {
            if (file.isDirectory()) {
                ArrayList<File> proxyFileList = getFileList(file);
                list.addAll(proxyFileList);
            } else if (file.length() <= 50) {
                list.add(file);
            }
        }

Answer 1

In your case, str is constantly overwritten and null is written to str in the last loop

String str;
            
            while((str = reader.readLine()) != null){
                str += str;
            }

This code can be rewritten to

String resultStr;
String line;
            
            while((line = reader.readLine()) != null){
                resultStr += line;
            }

or in the case of using a java version of 8 and higher, the entire mechanism for reading from a file can be replaced by

String resultStr = String.join("\n", Files.readAllLines(file.toPath()));