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 Ja

Answer 1

Reading from disk, again and again, makes the Scanner slow. I like to use the combination of BufferedReader and Scanner to get the best of both worlds. i.e. speed of BufferredReader and rich and easy API of the scanner.

Scanner scanner = new Scanner(new BufferedReader(new InputStreamReader(System.in)));

Answer 2

You can use BufferedReader for reading data

BufferedReader inp = new BufferedReader(new InputStreamReader(System.in));
  int t = Integer.parseInt(inp.readLine());
  while(t-->0){
    int n = Integer.parseInt(inp.readLine());
    int[] arr = new int[n];
    String line = inp.readLine();
    String[] str = line.trim().split("\\s+");
    for(int i=0;i<n;i++){
      arr[i] = Integer.parseInt(str[i]);
    }

And for printing use StringBuffer

    StringBuffer sb = new StringBuffer();
    for(int i=0;i<n;i++){
              sb.append(arr[i]+" "); 
            }
    System.out.println(sb);

Answer 3

You can read from System.in in a digit by digit way. Look at this answer: https://stackoverflow.com/a/2698772/3307066.

I copy the code here (barely modified). Basically, it reads integers, separated by anything that is not a digit. (Credits to the original author.)

private static int readInt() throws IOException {
    int ret = 0;
    boolean dig = false;
    for (int c = 0; (c = System.in.read()) != -1; ) {
        if (c >= '0' && c <= '9') {
            dig = true;
            ret = ret * 10 + c - '0';
        } else if (dig) break;
    }
    return ret;
}

In my problem, this code was approx. 2 times faster than using StringTokenizer, which was already faster than String.split(" "). (The problem involved reading 1 million integers of up to 1 million each.)