In Java only operator overloaded is " + ".
U cannot append character with a string since + is used only with Strings.
in the second line s+=c,
Everything that gets appended to String will be converted to String.So it works in the second case.
For ur clarification,
Outputs will be:
(1) abc 2 4-----> If anything appended after a string, entire is a string.
(2) 6 abc
Hope this shuld help...