One plus plus two compiles unexpectedly

Question

So, I expect this not to compile, and it doesn't:

// the two is inc'd, so reduces symbolically to println(int int)
// which is a compile error
System.out.println(1 ++ 2);

But this does:

System.out.println(1 + + 2);   // returns three

What gives? Shouldn't it also not compile?

Also, this question is very hard to search for because of the operators..

++ is an increment operator so the compiler may be expecting a variable name either before or after ++ — ryekayo, Dec 23 '14 at 17:37
In the second case `+` is a "syntactic sugar" operator (mirroring `-`) that does nothing to the operand. — Hot Licks, Dec 23 '14 at 17:38
*"Also, this question is very hard to search for because of the operators.."* Why not search for 'Java operators'? You will get a list of operators and these operators are listed. — Radiodef, Dec 23 '14 at 18:59
@Radiodef so once you search for "java operators" and you get hundreds of unrelated questions with the dozens of operator combinations Java uses, is there a way to further refine the search? — jabgibson, Dec 23 '14 at 19:32
To search for symbols, use http://symbolhound.com/ (not sure it would solve this problem, but it's a useful resource) — Tim S., Dec 23 '14 at 20:44
[Analogous question in Java](http://stackoverflow.com/q/24279192/1157100) and [C#](http://stackoverflow.com/q/2528543/1157100). You can go even further, with `1 + + + + + 2`, in C [as in Python](http://stackoverflow.com/q/470139/1157100). — 200_success, Dec 24 '14 at 04:51

score 66 · Accepted Answer · edited Dec 23 '14 at 19:30

66

Java is interpreting the working 1 + + 2 as 1 plus positive 2. See the Unary operator section.

edited Dec 23 '14 at 19:30

David says Reinstate Monica

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answered Dec 23 '14 at 17:38

ryanyuyu

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Though not as detailed as the other correct answers, this one was fairly quick to parse through and understand. – jabgibson Dec 23 '14 at 17:50
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Sometimes a detailed breakdown of the specification is not needed. Sometimes, the best answer is the simplest one. :) – Niet the Dark Absol Dec 24 '14 at 12:54

Sotirios Delimanolis · Answer 2 · 2014-12-23T18:48:49.750

From the Specification, on Lexical Translations

The longest possible translation is used at each step, even if the result does not ultimately make a correct program while another lexical translation would. There is one exception: if lexical translation occurs in a type context (§4.11) and the input stream has two or more consecutive > characters that are followed by a non-> character, then each > character must be translated to the token for the numerical comparison operator >.

(Also known as maximal munch.)

The ++ is interpreted as a postfix increment operator which cannot be applied to an integer literal, thus the compiler error.

While

1 + + 2

each character is interpreted separately. 1 is an integer literal, + is the additive operator, + is the unary plus operator, and 2 is an integer literal. The whole expression is equivalent to

1 + (+2)

which is easier to read.

fastcodejava · Answer 3 · 2014-12-26T22:52:37.300

In Java/C++/C ++ is not same as + +. ++/-- are the Increment/Decrement operator. The first case does not work because it does not apply to literals (1 or 2). Even then it would not be a valid statement, Neither 1++ 2 nor 1 ++2 are valid statement in Java. The second example works because it is interpreted as 1 + (+2). The Java lexer ignores white space. In the same way this is valid :

1 + + + 2   --> 1 + (+ (+2))

Or

1 + + + + + + + + + + + + + + + + + + + + + + + + + + + 2

It works only because + is a unary operator. It does not work for strings as below :

"a" + + "b"   // does not work because +"b" is not valid.

Similarly it is not valid with multiplication

1 * * 2       // does not work because *2 is not valid.

It isn't quite right to say the lexer ignores whitespace, eh? After all, it doesnt interpret `1++2` as `int biplus unplus int`, nor `1+ +2` as `int incr int` — djeikyb, Dec 24 '14 at 14:35

Salman A · Answer 4 · 2014-12-24T17:11:46.453

The message is:

Main.java:14: error: ')' expected
        System.out.println(1 ++ 2);
                               ^

The 1 ++ 2 statement is parsed as 1 followed by ++ followed by 2. This is interpreted as 1++ and 2 creating a syntax error (and not an unexpected type error; in fact, you will get the same error if you used variables e.g. i ++ j).

The 1 + + 2 statement on the other hand is parsed as 1 followed by + followed by +2 which compiles as expected. The space between the two operators separates the two operators.

score 3 · Answer 5 · answered Dec 24 '14 at 13:33

Sometimes it's easier to see a problem using variables.

Your snippet could be rewritten as:

int a = 1;
int b = +2;
System.out.println(a + b);

Now, you can easily see that the second + operator is used to indicate a positive value. You could also have written +1 + +2.

The - operator could be used to negate an expression.

+ and - are unary operators.

One plus plus two compiles unexpectedly

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