+ operator for String in Java [duplicate]

Answer 1

String is defined as a standard type just like int, double, float, etc. on compiler level. Essentially, all compilers have operator overloading. Operator overloading is not defined for Developers (unlike in C++).

Interestingly enough: This question was logged as a bug: http://bugs.sun.com/view_bug.do?bug_id=4905919

Answer 2

It is handled by the compiler.

Answer 3

It's done at the language level. The Java Language Specification is very specific about what string addition must do.

Answer 4

Most of the answers here are correct (it's handled by the compiler, + is converted to .append()...)

I wanted to add that everyone should take a look at the source code for String and append at some point, it's pretty impressive.

I believe it came down to something like:

"a"+"b"+"c"

=

new String().append("a").append("b").append("c")

But then some magic happens. This turns into:

• Create a string array of length 3
• copy a into the first position.
• copy b into the second
• copy c into the third

Whereas most people believe that it will create "ab", then throw it away when it creates "abc". It actually understands that it's being chained and does some manipulation.

There is also a trick where if you have the string "abc" and you ask for a substring that turns out to be "bc", they CAN share the exact same underlying array. You'll notice that there is a start position, end position and "shared" flag.

In fact, if it's not shared, it's possible for it to extend the length of a string and copy the others in.

Now I'm just being confusing. Read the source code--it's fairly cool.

Answer 5

From the Fine Manual:

The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings. String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method. String conversions are implemented through the method toString, defined by Object and inherited by all classes in Java. For additional information on string concatenation and conversion, see Gosling, Joy, and Steele, The Java Language Specification.

See String Concatenation in the JLS.

Answer 6

The compiler treats your code as if you had written something like:

String both = new StringBuilder().append(ab).append(cd).toString();

Edit: Any reference? Well, if I compile and decompile the OP's code, I get this:

0:  ldc #2; //String ab
2:  astore_1
3:  ldc #3; //String cd
5:  astore_2
6:  new #4; //class java/lang/StringBuilder
9:  dup
10: invokespecial   #5; //Method java/lang/StringBuilder."<init>":()V
13: aload_1
14: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
17: aload_2
18: invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
21: invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
24: astore_3
25: return

So, it's like I said.