Why does + work with Strings in Java?

Question

Java can\'t do operator overloading, but + works okay for String and Integer and some other classes. How is this possible?

Answer 1

As @yan said, this is the exception, not the rule. Strings have a special status in Java. There's a whole subsection of the Java Language Specification devoted to + in its role as the string concatenation operator: §15.18.1.

Regarding your update, that's another special case. Java is sometimes, depending on the case, smart enough to convert things that are not Strings into Strings when Strings are needed. One of these special cases is the one you described, where primitives are showing up in a place that needs a String. The primitives are first converted to their reference types — Integer, Double, &c. — and then into Strings via the toString() method.

Another special case is when one String and one non-String are being combined with the string concatenation operator +, as described in JLS §5.4 — String Conversion.

For completeness: + in its more common "adding numbers together" role is described in the other part of of §15.18, §15.18.2 — Additive Operators (+ and -) for Numeric Types.