Question

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Why does + work with Strings in Java?

The following statements are valid in Java.

int a=50;
String tmp="a = ";
String b=tmp+a;

b of type String now contains a = 50 (as a String).

Although tmp is of type String and a is of type int, the concatenation is made (even though Java doesn’t support operator overloading).

One of the reasons why Java doesn’t support operator overloading (as with other languages. In fact, I have no depth knowledge of any language).

Java does not support operator overloading. Operator overloading is sometimes a source of
ambiguity in C++ program, and the Java design team felt that it causes more trouble than
benefit.

More about it.

How is this statement String b=tmp+a; evaluated? There must be some equivalent concept of Operator Overloading internally.

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Just one question : Can we see literally how it’s implemented or we should just believe “It’s just a feature of the language“?

I have heard that the Java compiler uses StringBuilder/StringBuffer (with the append() method) to achieve this but I’m not sure about that.

Answer 1

Technically this is just a different operator that happens to have the same symbol. It’s the “string concatenation operator“; see section 15.18.1 of the Java Language Specification.

Regarding implementation, the JLS has this to say:

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.