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?

update:
Why does this work?

Integer i = 4;
Integer p = 5;

System.out.println(i*p); // prints 20

Answer 1

+ 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.