Java += operator on String with charAt() methods results in char addition

Question

I'm working through the problems on Codingbat.com and bumped into the fact that for the += operator, a += b isn't necessarily exactly equal to a = a + b. This is well known and has been discussed on SO before, but I bumped into something strange happening when using the charAt() method combined with the above syntax that I can't make sense of.

Say I have two variables:

String str    = "The";
String result = "";

I want to add the first letter in "str" two times to "result". One way of doing this is to:

result = result + str.charAt(0) + str.charAt(0);

which results in result = "TT".

However, if I use the += operator, e.g.:

result += str.charAt(0) + str.charAt(0);

I get result = "168". Clearly character addition has taken place (ASCII code for 'T' is 84, 84*2 = 168).

I'm wondering what is actually going on in the first case using the += operator. According to the docs on assignment operators: E1 op= E2 is equivalent to E1 = (T)((E1) op (E2)). So I would expect the latter expression to output "168" just like using the += operator did. But the following outputs "TT" correctly, and not "168":

result = (String)(result + str.charAt(0) + str.charAt(0));

Have I just misunderstood the documentation? I also found an answer on SO that suggests that str1 += str2 is equivalent to:

str1 = new StringBuilder().append(str1).append(str2).toString();

But evaluating the following:

result = new StringBuilder().append(str.charAt(0)).append(str.charAt(0)).toString();

Still results in "TT", not "168".

Sorry for the long post! I'm just curious at what actually is happening when using charAt() in combination with a String and +=, because the two "equivalents" I've found (if I've translated them from two to three terms correctly) does not produce the same result.

Answer 1

result = result + str.charAt(0) + str.charAt(0);

result is a string, so the first + performs string concatenation, producing another string. The second +, by the same logic, will also perform string concatenation. This gives the desired result.

result += str.charAt(0) + str.charAt(0);

The right-hand side, str.charAt(0) + str.charAt(0), is evaluated first. Now, you're adding two characters, which works like simple integer addition, adding the ASCII values. We then append this result (an int -- 84 in this case) to result, which is what you see.

char c = 'T';
System.out.println(c + c);

168

The underlying difference here is just the order in which things are evaluated.