Why Can’t i++++ or (i++)++ Be Written in Java?

decrementincrementjavajls

When I try to write a postfix/prefix in/decrement, followed by a post/prefix in/decrement, I get the following error: Invalid argument to operation ++/–.

But, according to JLS:

PostIncrementExpression:
        PostfixExpression ++

and

PostfixExpression:
        Primary
        ExpressionName
        PostIncrementExpression
        PostDecrementExpression

so writing:

PostfixExpression ++ ++

should be possible… Any thoughts?

Best Answer

Note that the raw grammar lacks any semantics. It's just syntax, and not every syntactically valid program will generally be valid. For example, the requirement that variables have to be declared before usage is typically not covered by the grammar (you can, but it's cumbersome).

Postfix-increment yields an rvalue – and just as you cannot postfix-increment literals, you cannot postfix-increment the result of i++.

Quoting from the JLS (3^rd ed., page 486):

The result of the postfix increment expression is not a variable, but a value.

Related Solutions

Java – Difference Between x++ and ++x in Java

++x is called preincrement while x++ is called postincrement.

int x = 5, y = 5;

System.out.println(++x); // outputs 6
System.out.println(x); // outputs 6

System.out.println(y++); // outputs 5
System.out.println(y); // outputs 6

Difference Between n++ vs ++n in Java

++n increments the value and returns the new one.

n++ increments the value and returns the old one.

Thus, n++ requires extra storage, as it has to keep track of the old value so it can return it after doing the increment.

I would expect the actual difference between these two to be negligible these days. I know a lot of compilers will optimize it so they're identical if the return of n++ isn't actually used, though I don't know of Java does that.

Best Answer

Related Solutions

Java – Difference Between x++ and ++x in Java

Difference Between n++ vs ++n in Java

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