Question

I often use the Scanner class to read files because it is so convenient.

      String inputFileName;
      Scanner fileScanner;

      inputFileName = "input.txt";
      fileScanner = new Scanner (new File(inputFileName));

My question is, does the above statement load the entire file into memory at once? Or do subsequent calls on the fileScanner like

      fileScanner.nextLine();

read from the file (i.e. from external storage and not from memory)? I ask because I am concerned about what might happen if the file is too huge to be read into memory all at once. Thanks.

Answer 1

If you read the source code you can answer the question yourself.

It appear that the implementation of the Scanner constructor in question shows:

public Scanner(File source) throws FileNotFoundException {
        this((ReadableByteChannel)(new FileInputStream(source).getChannel()));
}

Latter this is wrapped into a Reader:

private static Readable makeReadable(ReadableByteChannel source, CharsetDecoder dec) {
    return Channels.newReader(source, dec, -1);
}

And it is read using a buffer size

private static final int BUFFER_SIZE = 1024; // change to 1024;

As you can see in the final constructor in the construction chain:

private Scanner(Readable source, Pattern pattern) {
        assert source != null : "source should not be null";
        assert pattern != null : "pattern should not be null";
        this.source = source;
        delimPattern = pattern;
        buf = CharBuffer.allocate(BUFFER_SIZE);
        buf.limit(0);
        matcher = delimPattern.matcher(buf);
        matcher.useTransparentBounds(true);
        matcher.useAnchoringBounds(false);
        useLocale(Locale.getDefault(Locale.Category.FORMAT));
    }

So, it appears scanner does not read the entire file at once.