What does compareTo() actually return?

Question

The compareTo() method in Java returns a value greater/equal/less than 0 and i know that. However, the value itself is my question. What is the difference between 2 or 4 when compareTo() returns. Look at the code below

String s1="hello";  
String s2="hello";  
String s3="meklo";  
String s4="hemlo";  
System.out.println(s1.compareTo(s2));     // 0
System.out.println(s1.compareTo(s3));     // -5
System.out.println(s1.compareTo(s4));     // -1

Why the last two commands are -5 and -1?

Answer 1

https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#compareTo(java.lang.String)

This is the definition of lexicographic ordering. If two strings are different, then either they have different characters at some index that is a valid index for both strings, or their lengths are different, or both. If they have different characters at one or more index positions, let k be the smallest such index; then the string whose character at position k has the smaller value, as determined by using the < operator, lexicographically precedes the other string. In this case, compareTo returns the difference of the two character values at position k in the two string -- that is, the value:

this.charAt(k)-anotherString.charAt(k)

If there is no index position at which they differ, then the shorter string lexicographically precedes the longer string. In this case, compareTo returns the difference of the lengths of the strings -- that is, the value:

this.length()-anotherString.length()

Answer 2

compareTo() returns the difference of first unmatched character in the two compared strings. If no unmatch is found, and one string comes out as shorter than other one, then the length difference is returned.

"hello".compareTo("meklo") = 'h' - 'm'  = -5
 ^                 ^
and 

"hello".compareTo("hemlo") = 'l' - 'm'  = -1
   ^                 ^

As a side note: Non-zero values are mostly considered as true inside conditional statements. So, compareTo can simply return these non-zero values instead of processing them into 1(small optimisation).