Gets longest substrings formed by two unique characters

LongestSubstring.java

import java.util.Arrays.*;

public class LongestSubstring {

  public String getLongestSubstring(String s) {
    String[] charArray = s.split("(?!^)");    
    int resStart = 0, resEnd = 0;
    String firstChar = charArray[0], secondChar = null;

    for (int i = 0, j = 0;i < charArray.length; i++) {
      if (secondChar == null && !charArray[i].equals(firstChar)) {
        secondChar = charArray[i];
      }

      if (!charArray[i].equals(firstChar) && !charArray[i].equals(secondChar)) {
        if (i-j > resEnd-resStart) {
          resStart = j;
          resEnd = i;
        }
        j = i-1;
        firstChar = charArray[j];
        secondChar = charArray[i];
      }
    }
    return s.substring(resStart, resEnd);
  }
}