java - compile error: cannot find symbol: In, StdIn and StdOut

Question

The code is from http://algs4.cs.princeton.edu/11model/BinarySearch.java.html for Algorithms textbook.

import java.util.Arrays;

public class BinarySearch {

    // precondition: array a[] is sorted
    public static int rank(int key, int[] a) {
        int lo = 0;
        int hi = a.length - 1;
        while (lo <= hi) {
            // Key is in a[lo..hi] or not present.
            int mid = lo + (hi - lo) / 2;
            if      (key < a[mid]) hi = mid - 1;
            else if (key > a[mid]) lo = mid + 1;
            else return mid;
        }
        return -1;
    }

    public static void main(String[] args) {
        int[] whitelist = In.readInts(args[0]);

        Arrays.sort(whitelist);

        // read key; print if not in whitelist
        while (!StdIn.isEmpty()) {
            int key = StdIn.readInt();
            if (rank(key, whitelist) == -1)
                StdOut.println(key);
        }
    }
}

I get this error

$ javac BinarySearch.java 
BinarySearch.java:44: cannot find symbol
symbol  : variable In
location: class BinarySearch
        int[] whitelist = In.readInts(args[0]);
                          ^
BinarySearch.java:49: cannot find symbol
symbol  : variable StdIn
location: class BinarySearch
        while (!StdIn.isEmpty()) {
                ^
BinarySearch.java:50: cannot find symbol
symbol  : variable StdIn
location: class BinarySearch
            int key = StdIn.readInt();
                      ^
BinarySearch.java:52: cannot find symbol
symbol  : variable StdOut
location: class BinarySearch
                StdOut.println(key);
                ^
4 errors

Answer 1

Classes StdIn, StdOut and In aren't part of the standard Java libraries. They're support classes provided to go along with the Princeton course.

From the 1.1 Programming Model page linked in the source code:

Standard input and standard output. StdIn.java and StdOut.java are libraries for reading in numbers and text from standard input and printing out numbers and text to standard output. Our versions have a simpler interface than the corresponding Java ones (and provide a few tecnical improvements).

...

In.java and Out.java are object-oriented versions that support multiple input and output streams, including reading from a file or URL and writing to a file.

So if you want to use the binary search code as-is, you'll need to download those files.