Java. Arithmetic operations.tokenized. How?

Who can 开发者_如何转开发explain or give a good reference for understanding this example:

int a=1;
int b=2;
System.out.println(a---b); //correct
System.out.println(a- -b); //correct
System.out.println(a--b); //wrong

thanks.

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The expression a---b is not (as you perhaps expected) parsed as a-(-(-b)) but rather as (a--) - b.

This example illustrates it:

int a = 0;
int b = 0;
System.out.println(a---b);  // prints 0
System.out.println(a);      // prints -1

With this behaviour in mind, a--b is parsed as (a--)b which is obviously an error.

When you put a space between the minuses, a- -b it's no longer parsed as the -- operator, but as a binary and unary minus: a - (-b).

Note that you can write a- - -b which is interpreted as a-(-(-b)).

So why is it interpreted like this? Well @EJP gave an excellent comment on another answer. In the JLS, section 3.2 you can read the following:

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. Thus the input characters a--b are tokenized (§3.5) as a, --, b, which is not part of any grammatically correct program, even though the tokenization a, -, -, b could be part of a grammatically correct program.

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The Java Language Specification.

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- and -- are unary operators. Therefore can't be used with two operands. That's why

System.out.println(a--b);

is wrong. -- is applied to a, so the new value of a is 0. If you add one more -, then the value of a decremented by 1 will be subtracted by the value of b yielding -2