Arrays java.util collections class - binarySearch() method

[Ted North]

You who read me now: I am having great problems understanding binarySearch(). I have no idea how to predict the output from this method.
Below, one will find problem number 16 from the Chapter 7 self exam in the most excellent Sierra and Bates (2008) SCJP book.

The output should be: pen marble map key -1

I have no clue how the output is -1 for the binarySearch().

import java.util.*;
 public class GeoCache {
 public static void main(String[] args) {
 String[] s = {"map", "pen", "marble", "key"};
 Othello o = new Othello();
 Arrays.sort(s,o);
for(String s2: s) System.out.print(s2 + " ");
 System.out.println(Arrays.binarySearch(s, "map"));
 }
 static class Othello implements Comparator<String> {
 public int compare(String a, String b) { return b.compareTo(a); }
 }

[Sebanti Sanyal]

The comparator object that is used for sorting is not used in binary search. Hence the the key won't be found. Negative values indicate "key not found" in binary search. You will get the correct result with the following modification:

System.out.println(Arrays.binarySearch(s, "map",o));

[Ted North]

Hello Sebanti,

I input your code like so:

import java.util.*;

 public class GeoCache {
	public static void main(String[] args) {

		String[] s = {"map", "pen", "marble", "key"};
		Othello o = new Othello();
		Arrays.sort(s,o);
		
		
		
		for(String s2: s) System.out.print(s2 + " ");
			System.out.println(Arrays.binarySearch(s, "map"));
			
			// java ranch codes
		System.out.println(Arrays.binarySearch(s, "map",o));
		}
		
		
		
		static class Othello implements Comparator<String> {


		public int compare(String a, String b) { return b.compareTo(a); } // this reverses the normal sort
	}
 }

This output gives: pen marble map key -1
2

Why is the output for new code 2?

Also when I put "marble" as the key in the binary search I receive positive integer, 1, as a result. When I put "map" or "key" in I receive the -1 output that still does not make any sense to me. I found that "pen" yields -5 as output...I am not sure if these results are consistent with each run though.

Thanks for the input though - crowd-source your problems,

Ted North

[Sebanti Sanyal]

Your comparator does a reverse sorting. When you use the same for binary search (new code), you get the correct index in the sorted array. Here, the sorted order is "pen marble map key" and so the index of "map" is 2.
If the same Comparator is not used in both sorting and binary search, result is unpredictable. In this case, however, I expect binary search to give negative results. I also checked 'System.out.println(Arrays.binarySearch(s, "marble"));' prints 1,which is the correct index. But for all other keys, result is negative as I expected. I have no clues.