ADDITIONAL SYSTEM INFORMATION :
Mac OS

A DESCRIPTION OF THE PROBLEM :
The split function returns different type of Strings in case where the splitted has one or many chars.

REGRESSION : Last worked in version 8u381

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
String str1 = "A"; // String with one char
String str2 = "AB"; // or "ABCD..." String with at leas 2 chars
String s1 = str1.split("")[0];
String s2 = str2.split("")[0];
String strA = "A";
boolean compare1 = (strA==s1);
boolean compare2 = (strA==s2);
System.out.println(compare1);
System.out.println(compare2);

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
true
true
ACTUAL -
true
false

---------- BEGIN SOURCE ----------
public class Main {

public static void main(String[] args) {
String str1 = "A"; // String with one char
String str2 = "AB"; // or "ABCD..." String with at leas 2 chars
String s1 = str1.split("")[0];
String s2 = str2.split("")[0];
String strA = "A";
        boolean compare1 = (strA==s1);
        boolean compare2 = (strA==s2);
        System.out.println(compare1);
        System.out.println(compare2);
}

}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Never use == to compare two strings. Use only equals as follows:
boolean compare1 = (strA.equals(s1));
boolean compare2 = (strA.equals(s2));


FREQUENCY : always