How does StdIn/Out in Java work?

Question

Im new to Java and am trying to make a QuickUnion algorithm run. I have these text files on my desktop and want to program to read the integers in them. This is the end of the QuickUnion class.

 public static void main(String[] args) {

    int N = StdIn.readInt();    // Read number of sites
    QuickUnionUF quickunion = new QuickUnionUF(N);
    while (!StdIn.isEmpty()) {
        int p = StdIn.readInt();
        int q = StdIn.readInt();                     // Read pair to connect
        if (quickunion.connected(p, q)) continue;     // Ignore if connected
        quickunion.union(p, q);                       // Combine components
        StdOut.println(p + " " + q);                 // and print connection
    }
    StdOut.println(quickunion.count() + " components");
}

My question is: how does StdIn work? How do I read the text file? the first test file contains two columns of numbers.

Are you reading from files, or from the user's input to the console window? — Andrew Williamson, May 14 '16 at 07:51
StdIn.readInt() seems to be supposed to read integers. However, I don't see how it would read anything. "N" stays empty. — augusti, May 14 '16 at 17:48

score 2 · Answer 1 · answered Sep 02 '19 at 03:31

The Std library does not provide the functionality that is needed to read from an arbitrary file. Its purpose is to provide an easy way for beginners (students) to write simple programs that read and write to standard input / standard output. It has limited functionality¹.

By contrast, production code uses the standard System.in and System.out, either directly or (in the case of System.in) via the Scanner class. So to read text from a file (or standard input) in production code, you would typically do something like this:

Scanner in = new Scanner(new File("/some/path/to/file.txt"));

or

Scanner in = new Scanner(System.in);

and then use the various Scanner methods to either read lines or individual items.

(You could also use the Reader or BufferedReader APIs, and parse the input lines yourself in various ways. Or use an existing parser library to read CSV files, JSON files, XML files and so on.)

Your example would look something like this:

import java.util.Scanner;
...
public static void main(String[] args) {
    Scanner in = new Scanner(System.in);  // replace as required!
    int n = in.nextInt();    // Read number of sites
    QuickUnionUF quickUnion = new QuickUnionUF(n);
    while (in.hasNextInt()) {
        int p = in.nextInt();
        int q = in.nextInt();                     // Read pair to connect
        if (quickUnion.connected(p, q)) continue; // Ignore if connected
        quickUnion.union(p, q);                   // Combine components
        System.out.println(p + " " + q);          // and print connection
    }
    System.out.println(quickUnion.count() + " components");
}

Note that new Scanner(new File(someString)) is declared as throwing FileNotFoundException which your code must deal with.

^{1 - My advice would be to stop using StdIn, StdOut and the rest as soon as you can. Learn to use the standard APIs, and switch.}

score 1 · Answer 2 · answered May 14 '16 at 14:56

Standard library (Std) is often provided for students who are taking their first course in programming in Java. Std library is not part of "installed java libraries" therefore in order to use it you have to download the Std library and declare it in your path. It works identical to Java Scanner class. Consider

public static void main(String[] args) {
        StdOut.print("Type an int: ");
        int a = StdIn.readInt();
        StdOut.println("Your int was: " + a);
        StdOut.println();
    }
}

Which takes in only ONE integer, a , and prints it out. How ever as I see you're using isEmpty()which returns true if standard input is empty (except possibly for whitespace). Therefore you can use the isEmpty() to take in as many input as you want. Here is an example code of this usage

public static void main(String[] args) {
        double sum = 0.0; // cumulative total
        int n = 0; // number of values
        while (!StdIn.isEmpty()) {
            double x = StdIn.readDouble();
            sum = sum + x;
            n++;
        }
        StdOut.println(sum / n);
}

Which calculates instantly the average of your inputs and prints it out as you type in new input.

Ok, Now I think I got it. Thanx to you and thanks to the others who pointed out the Scanner class. It least I can experiment with this. — augusti, May 14 '16 at 17:59

How does StdIn/Out in Java work?

2 Answers2