binarySearch与自定义Comparator-object不检查第一个元素

Question

I am trying to make a custom Comparator-object to use with java.util.Arrays.binarySearch .

I want search the array for a String whose reverse value is equal to the reverse value of the target String element (dumb, I know, but it's just for learning).

However, when I attempt to run binarySearch on the array, all the elements match properly, except array element 0.

Comparator:

class SearchComparator<T extends Comparable<T>> implements Comparator<T> {
    public int compare(T s1, T s2) {

        String reverse1 = new StringBuilder((String)s1).reverse().toString();
        String reverse2 = new StringBuilder((String)s2).reverse().toString();

        return reverse1.equals(reverse2) ? 0 : -1;
    }
}

Main function:

String[] words = new String[]{"One", "Two", "Three", "Four"};
SearchComparator scmp = new SearchComparator();

System.out.println(Arrays.deepToString(words));

System.out.println(Arrays.binarySearch(words, "One", scmp));
System.out.println(Arrays.binarySearch(words, "Two", scmp));
System.out.println(Arrays.binarySearch(words, "Three", scmp));
System.out.println(Arrays.binarySearch(words, "Four", scmp));

Output:

[One, Two, Three, Four]
-5
1
2
3

What is up with -5 ? I don't know man.

Answer 1

The int compare(T s1, T s2) from the Comparator<T> interface returns a number that indicates whether s1 is less, equal to, or greater than s2 ; your code compares s1 and s2 for equality, which is incorrect.

You need to change the return line of your code to call the method for lexicographic comparison of String objects, like this:

return reverse1.compareTo(reverse2);

Another problem is that your array is not sorted in accordance with the same comparator. That is why you get negative indexes when you search in it. Add this line before making calls to binarySearch to fix the problem:

Arrays.sort(words, scmp);