optimization - What's the fastest way to read from System.in in Java?

Question

I am reading bunch of integers separated by space or newlines from the standard in using Scanner(System.in).

Is there any faster way of doing this in Java?

Answer 1

Is there any faster way of doing this in Java?

Yes. Scanner is fairly slow (at least according to my experience).

If you don't need to validate the input, I suggest you just wrap the stream in a BufferedInputStream and use something like String.split / Integer.parseInt.

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A small comparison:

Reading 17 megabytes (4233600 numbers) using this code

Scanner scanner = new Scanner(System.in);
while (scanner.hasNext())
    sum += scanner.nextInt();

took on my machine 3.3 seconds. while this snippet

BufferedReader bi = new BufferedReader(new InputStreamReader(System.in));
String line;
while ((line = bi.readLine()) != null)
    for (String numStr: line.split("\s"))
        sum += Integer.parseInt(numStr);

took 0.7 seconds.

By messing up the code further (iterating over line with String.indexOf / String.substring) you can get it down to about 0.1 seconds quite easily, but I think I've answered your question and I don't want to turn this into some code golf.