How does the String class override the + operator?

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.

15.18.1.

Optimization of String Concatenation :
An implementation may choose to perform conversion and concatenation
in one step to avoid creating and then discarding an intermediate
String object. To increase the performance of repeated string
concatenation, a Java compiler may use the StringBuffer class or a
similar technique to reduce the number of intermediate String objects
that are created by evaluation of an expression.

For primitive types, an implementation may also optimize away the
creation of a wrapper object by converting directly from a primitive
type to a string.

The optimized version will not actually do a full wrapped String conversion first.

This is a good illustration of an optimized version used by the compiler, albeit without the conversion of a primitive, where you can see the compiler changing things into a StringBuilder in the background:

http://caprazzi.net/posts/java-bytecode-string-concatenation-and-stringbuilder/

---

This java code:

public static void main(String[] args) {
    String cip = "cip";
    String ciop = "ciop";
    String plus = cip + ciop;
    String build = new StringBuilder(cip).append(ciop).toString();
}

Generates this – see how the two concatenation styles lead to the very same bytecode:

 L0
    LINENUMBER 23 L0
    LDC "cip"
    ASTORE 1
   L1
    LINENUMBER 24 L1
    LDC "ciop"
    ASTORE 2

   // cip + ciop

   L2
    LINENUMBER 25 L2

    NEW java/lang/StringBuilder
    DUP
    ALOAD 1
    INVOKESTATIC java/lang/String.valueOf(Ljava/lang/Object;)Ljava/lang/String;
    INVOKESPECIAL java/lang/StringBuilder.<init>(Ljava/lang/String;)V
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append(Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString()Ljava/lang/String;

    ASTORE 3

    // new StringBuilder(cip).append(ciop).toString()

   L3
    LINENUMBER 26 L3

    NEW java/lang/StringBuilder
    DUP
    ALOAD 1
    INVOKESPECIAL java/lang/StringBuilder.<init>(Ljava/lang/String;)V
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append(Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString()Ljava/lang/String;

    ASTORE 4
   L4
    LINENUMBER 27 L4
    RETURN

Looking at the example above and how the byte code based on the source code in the given example is generated, you will be able to notice that the compiler has internally transformed the following statement

cip+ciop; 

into

new StringBuilder(cip).append(ciop).toString();

In other words, the operator + in string concatenation is effectively a shorthand for the more verbose StringBuilder idiom.