Why it is not possible to increment and then pass the value in set method by ++

Question

I have a line of code that works like this,

mrq.setId((mrq.getId()+1));

But, When I tried to write it like this,

mrq.setId((mrq.getId()++));

It doesn't work, The error is, Invalid argument ot the operation ++/--

What is the technical reason behind it?

Uh, because "mrq.getId()" isn't a primitive that you can assign to? You can't say "mrq.getId() = 1", either ;) — paulsm4, Jul 29 '12 at 03:10
Related: http://stackoverflow.com/questions/8536223/why-doesnt-integer-parseint1-work-in-java — James P., Jul 29 '12 at 03:54
Related: http://stackoverflow.com/questions/4651615/in-java-why-cant-i-write-i-or-i — James P., Jul 29 '12 at 03:55

score 4 · Accepted Answer · answered Jul 29 '12 at 03:16

The increment operator requires a field or variable. Evaluating getId() doesn't result in an id field; it returns a copy of the value getId() returns (by "copy" I mean a literal copy for primitive types and a new reference for reference types). getId() might be implemented as return id; internally, but you don't get back the field id, only a copy of its value.

The closest equivalent would be int i = getId(); setId( getId() + 1 ); return i;, but you're asking a lot to allow getId()++ as syntactic sugar for such an expression.

score 3 · Answer 2 · answered Jul 29 '12 at 03:12

3

x++ is essentially equivalent to x = x + 1, which doesn't make sense in your case:

mrq.getId() = mrq.getId() + 1

answered Jul 29 '12 at 03:12

blahdiblah

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score 1 · Answer 3 · answered Jul 29 '12 at 08:52

++ operator is used to increment the value of variable by 1. so a++ is treated as a=a+1. That means the operand on which it is used will be where the assignment will be done.

When you write mrq.getId()++ it doesn't provide you an operand variable which it can increment and hence this doesn't work.

Why it is not possible to increment and then pass the value in set method by ++

3 Answers3