Why does + work with Strings in Java?

+ is not an example of operator overloading. + is built into the language as a concatentation operator and an arithmetic-addition operator.

What this means is that a person writing a program with Java cannot overload operators, but as far as the grammar of the Java language is concerned, + is defined as a concatenation and an addition operator.

EDIT

It works for other classes such as Integer and Double because of autoboxing.

If you take a look at the bytecode of a Java program that performs string concatenation, you'll see that it creates StringBuilder and uses the append() method. The Java compiler sees the + operator and realizes that the operands are strings and not primitive types (like int).

If you look at the bytecode of a program that does integer addition, you will see that it uses the iadd instruction to perform integer addition. This is because the compiler realizes that the operands to the + operation are integers.

As far as doing something like Integer i = 4, the bytecode will show that you're actually doing Integer i = Integer.valueOf(4). This is called autoboxing. Later on, when you do something like i + p, where both i and p are of type Integer, the generated bytecode will show that you're doing i.intValue() + p.intValue(), where the return types of both methods are int (the actual bytecode instruction again, is iadd).

This is why + works Integer even though they are not actual primitive types.

It works for primitive wrappers like Integer because of autoboxing.

It works for String because that's a special case for concatenating strings:

The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings. String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method. String conversions are implemented through the method toString, defined by Object and inherited by all classes in Java. For additional information on string concatenation and conversion, see Gosling, Joy, and Steele, The Java Language Specification.

+ is a built-in operation. It's an exception, not a rule.