Java中两个String对象之间使用+运算符是怎么实现的？

Question

Java does not support user defined overloaded operators. Java does allow the use of using the + operator between two String Objects. When this happens, is there some internal/built in overloading to the + happening or what happens in order for two String objects to be added with the use of +?

Answer 1

There is no overloading of operator. The behavior of the + operator is well-defined in the Java Language Specification , section 15.18. Additive Operators :

The operators + and - are called the additive operators .

If the type of either operand of a + operator is String , then the operation is string concatenation .

Section 15.18.1. String Concatenation Operator + then goes on saying:

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.

Answer 2

"+" operator (in this context) 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() .

Reference: https://stackoverflow.com/a/29084397/11504153