What happens if you remove the space between the +

问题:

EDIT 1

DISCLAIMER: I know that +++ is not really an operator but the + and ++ operators without a space. I also know that there's no reason to use this; this question is just out of curiosity.

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So, I'm interested to see if the space between + and ++var is required in Java.

Here is my test code:

int i = 0;
System.out.println(i);
i = i +++i;
System.out.println(i);  

This prints out:

0
1

which works as I would expect, just as if there were a space between the first and second +.

Then, I tried it with string concatenation:

String s1 = "s " + ++i;
System.out.println(s1);
// String s2 = "s " +++i;

This prints out:

s 2

But if the third line is uncommented, the code does not compile, with the error:

Problem3.java:13: unexpected type
required: variable
found   : value
    String s2 = "s " +++i;
                ^
Problem3.java:13: operator + cannot be applied to <any>,int
    String s2 = "s " +++i;
                     ^

What's causing the difference in behavior between string concatenation and integer addition?

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EDIT 2

As discussed in Abhijit's follow-up question, the rule that people have mentioned (the larger token ++ be parsed first, before the shorter token ++) is discussed in this presentation where it appears to be called the Munchy Munchy rule.

回答1:

Compiler generates longest possible tokens when parsing source, so when it encounters +++, it takes it as ++ +.

So the code of

a +++ b

Will always be same as

(a++) + b

回答2:

There is no +++ operator. What you have there is a postfix ++ operator followed by an infix + operator. That is a compilation error because postfix ++ can only be applied to a variable, and "s " isn't a variable.

Since you really mean an infix + operator followed by a prefix ++ operator, you need to put the space in between the operators.

Actually, you should do it ANYWAY. +++ is a crime against readability!!!

回答3:

The triple plus is not an operator itself, it is two operators combined:

What the triple plus acutually does is:

a+++1 == a++ + 1;

What you are trying to do is ++ a String, which is undefined.

Never ever use +++ without spaces in your code; hardly anyone will know what it does (without consulting web). Moreover, after a week or so, you will not know what it actually does yourself.

回答4:

+++ isn't an operator by itself.

i = i +++i; results in a pre-increment value of i, then adding it to the value of i and storing it in i.

With the String, + doesn't mean addition, so you're attempting to concatenate the string and the integer together.