Automatically close & flush

The Answer by Arnaud is correct and good. Here is a variation, shorter and more modern.

Here we:

• Use the Path, File, and Files classes offered by modern Java NIO.2 to make easier work of file-handling.
• Use a BufferedWriter for better performance with large amounts of data.
• Specify the character encoding to be used. Usually UTF-8 is the best. If you do not understand, read this.
• Include the necessary try-catches for file-related exceptions.
• Add try-with-resources syntax to auto-close the file.
• Skip the explicit flushing, as the buffered writer will be flushed automatically as part of auto-closing the BufferedWriter and CSVPrinter. To quote the Javadoc, calling java.io.Writer::close “Closes the stream, flushing it first.”.

Code:

CSVFormat format = CSVFormat.EXCEL.withHeader();
Path path = Paths.get( savePath + ".csv" );
try 
(
    BufferedWriter writer = Files.newBufferedWriter( path , StandardCharsets.UTF_8 ) ;
    CSVPrinter printer = new CSVPrinter( writer , format ) ;
)
{
    printer.printRecords( excelParser.getRecords() );
} 
catch ( IOException e )
{
    e.printStackTrace();
}
// At this point, the `CSVPrinter` is automatically closed. 
// And, ➡️ the `BufferedWriter` is automatically closed and flushed. 

Here is one more easy way. After checking file existence only you should instantiate the Writer object else while instantiating file will get created and every time you will get file.exists() as true. If file exists you need to create CSVPrinter with withSkipHeaderRecord() else go with any implementation of header() method. FileWriter constructor takes appendable argument with File parameter. If file is there you have to make the appendable parameter as true.

File file = new File(filePath.concat("/").concat(fileName));
        if(file.exists()) {
            fileWriter = new FileWriter(file, true);
            csvPrinter = new CSVPrinter(fileWriter, CSVFormat.DEFAULT.withSkipHeaderRecord());
        }
        else {
            fileWriter = new FileWriter(file);
            csvPrinter = new CSVPrinter(fileWriter, CSVFormat.DEFAULT.withHeader("FirstName", "LastName", "DOB"));
            
        }