Java, operator overloading and “+” operator for String

问题

I have a doubt regarding operator overloading in Java. I know that Java does not support operator overloading but then what is the "+" operator doing in below valid Java program:

import java.util.*;
import java.lang.*;
import java.io.*;

class OperatorOverloadingTest
{
    public static void main (String[] args) throws java.lang.Exception
    {
        String str1 = "Operator ";
        String str2 = "overloading";
        String str3 = str1+str2;

        System.out.println(str3);
    }
}

Stdout:
Operator overloading

回答1:

This operator is not "overloaded", it is pre-defined operator, called String Concatenation Operator.

15.18.1 String Concatenation Operator +

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time. The result of string concatenation is a reference to a String object that is the concatenation of the two operand strings. The characters of the left-hand operand precede the characters of the right-hand operand in the newly created string.

In other words, when Java sees

String res = stringObj + someObj;

it replaces the expression with code that constructs the resulting string by concatenating an existing string value with someObj.toString().

来源：https://stackoverflow.com/questions/29084340/java-operator-overloading-and-operator-for-string