Reading input fast in Java for competitive programming

Question

I have been solving problems on Timus Online Judge. In the beginning, I didn't worry much about I/O operations, so I just took some code from the internet and focused more on the logic. But after going through the helper class that I was using, I created my own class.

This is the original stuff:

import java.io.InputStream;

public class InputReader {

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int read() throws Exception {
        if (curChar >= numChars) {
            curChar = 0;
            numChars = stream.read(buf);
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public int readInt() throws Exception {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            res *= 10;
            res += c - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public String readString() throws Exception {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }
}

This is what I have written now:

import java.io.IOException;

public class IOUtils {

    public static String readString() throws Exception {
        int c = System.in.read();
        while (isSpaceChar(c))
            c = System.in.read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = System.in.read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public static int readInt() throws IOException {
        int i = 0;
        int c = System.in.read();
        while (isSpaceChar(c))
            c = System.in.read();
        int sign = 1;
        if (c == '-') {
            sign = -1;
            c = System.in.read();
        }
        do {
            i = i * 10 + c - '0';
            c = System.in.read();
        } while (!isSpaceChar(c));
        return i * sign;
    }

    public static int readPositiveInt() throws IOException {
        int i = 0;
        int c = System.in.read();
        while (isSpaceChar(c))
            c = System.in.read();
        do {
            i = i * 10 + c - '0';
            c = System.in.read();
        } while (!isSpaceChar(c));
        return i;
    }

    private static boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == -1;
    }
}

Is my code any better than the original? Could someone look into it and let me know about the performance and the pitfalls?

Answer 1

In my experience, non-buffered I/O based on plain System.in and System.out are not fast enough for some competitive programming problems. I wrote a template based on BufferedReader and BufferedWriter, which are much faster. They aren't necessary for every problem, but I haven't found any downside to using them by default.

Some example code:

BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in);
while (stdin.ready())
    String line = stdin.readLine();

BufferedWriter stdout = new BufferedWriter(new OutputStreamWriter(System.out));
stdout.write("test");
stdout.flush();
stdout.close();

I wrote a couple of blog posts with more details and some benchmark numbers:

• Solving UVa 11340 in Java
• Why is Java I/O Slow?