How work end() method from class Matcher?

[Sergej Smoljanov]

  Pattern pattern = Pattern.compile("a?");  
        Matcher matcher = pattern.matcher("babba");  
        System.out.println("b a b b a ");  
        System.out.println("0 1 2 3 4 ");  
        System.out.println();  
        while (matcher.find()) {  
        System.out.println(matcher.start() + " " + matcher.end() + " " + matcher.group());  

result:
0 0
1 2 a
2 2
3 3
4 5 a
5 5
Can anyone explain how matcher.end() work, what it find?
from
https://docs.oracle.com/javase/7/docs/api/java/util/regex/Matcher.html#end%28%29
Returns the offset after the last character matched.
but i shill not understand
0 0 - there is no matched, but there is zero matched
1 2 a - there second char from start, definitely there is matched
2 2 - there is no matched, but there is zero matched, last matched was a and use previous matched offset
3 3 - but here again there is no matched, but there is zero matched why result is 3 not 2
4 5 a
5 5

[Jeanne Boyarsky]

a? matches either zero characters or a single 'a' character. Since each character is either a or not a (zero characters), every index will match.

0 0 prints on the match of zero characters. Note this is different than "no match". There is a match. It is of length zero which means the start and end indexes will be the same. This logic applies to 3 3 as well. The