How to output the contents of an array into a text file in Java?

You have to close() the file (closing the PrintWriter will close the FileWriter which will close the file). You could use a try-with-resources to do it for you

public static void printToFile(double[] gravity) throws IOException
{
    try (PrintWriter outputFile = new PrintWriter(
            new FileWriter("gravitydata.txt", true))) {
        for(int a = 0; a < gravity.length; a++){
            outputFile.println(gravity[a]);
        }
    }
}

or the older finally block and something like

public static void printToFile(double[] gravity) throws IOException
{
    PrintWriter outputFile = new PrintWriter(
            new FileWriter("gravitydata.txt", true))
    try {
        for(int a = 0; a < gravity.length; a++){
            outputFile.println(gravity[a]);
        }
    } finally {
        outputFile.close();
    }
}

in either case you should use the array length property instead of hard-coding 9.

{
    PrintWriter outputFile = new PrintWriter (new FileWriter("gravitydata.txt", true));
    for(int a = 0; a < 9; a++)
    {
        outputFile.println(gravity[a]);
    }
    outputFile.close();

}

The following approach can be good if you wish to utilize the built-in functions from Java 8 (to avoid writing your own file handling and looping).

public static void printToFile(double[] gravity) throws IOException {
    // First, convert the double[] to a list of Strings
    final List<String> doublesAsStrings = Arrays.stream(gravity)
            .boxed() // Box it to Doubles
            .map(g -> String.valueOf(g)) // Convert each Double to a String
            .collect(Collectors.toList()); // Create a List<String>

    // Then, write the list to the file
    Files.write(new File("gravitydata.txt").toPath(), doublesAsStrings);
}

There are a few differences compared to the previous answers:

• The Files class is used to write the data which is convenient since it handles try/catch etc
• No external looping is required since the streaming API:s are used to create a sequence of strings that will be printed
• It is short and compact ;)