When you work with a buffer, always use flush(), as it forces the data from the buffer to be written to the final destination. Not using flush on a buffer can cause the data not to be written to your file.

public void writeToFile(String fullpath, String contents) {

    // Paths AṔI
    Path filePath = Paths.get(fullpath, "contents.txt");

    try {
        // Files AṔI    
        Files.createDirectories(filePath.getParent());
    } catch (IOException e) {
        e.printStackTrace();
        return;
    }

    // Files AṔI    
     try (BufferedWriter bw = Files.newBufferedWriter(filePath)) {
        bw.write(contents);
        bw.flush(); // <<--- Flush force write in file
    } catch (IOException e) {
        e.printStackTrace();
    }
}

When working with files in Java, use the Paths and Files APIs.

• Path abstracts the path and location of files/directories in the file system.
• Files abstracts the operations and attributes of files (create, read, write, delete, check permissions, etc.).

This way, you don't need to worry about operating system issues.

• flush(): It is crucial for buffered streams (like BufferedWriter) because it ensures that the data in memory is actually written to the file.
• Path: An interface that represents a path in the file system, independent of the OS (Windows, Linux, macOS).
• Files: A utility class with static methods for common file operations.

Reference:

• Flush: What is the purpose of flush() in Java streams?