How does the String class override the + operator?

Question

Why in Java you're able to add Strings with the + operator, when String is a class? In theString.java code I did not find any implementation for this operator. Does this concept violate object orientation?

Answer 1

Let's look at the following simple expressions in Java

int x=15;
String temp="x = "+x;

The compiler converts "x = "+x; into a StringBuilder internally and uses .append(int) to "add" the integer to the string.

5.1.11. String Conversion

Any type may be converted to type String by string conversion.

A value x of primitive type T is first converted to a reference value as if by giving it as an argument to an appropriate class instance creation expression (§15.9):

• If T is boolean, then use new Boolean(x).
• If T is char, then use new Character(x).
• If T is byte, short, or int, then use new Integer(x).
• If T is long, then use new Long(x).
• If T is float, then use new Float(x).
• If T is double, then use new Double(x).

This reference value is then converted to type String by string conversion.

Now only reference values need to be considered:

• If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).
• Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.

^{The toString method is defined by the primordial class Object (§4.3.2). Many classes override it, notably Boolean, Character, Integer, Long, Float, Double, and String.}

See §5.4 for details of the string conversion context.