My name
is
Jon Skeet

Simple short addition yields warning

I found some questions concerning short arithmetic but none of them compared the following three cases. I wonder why these two pieces of code are ok

(a)

            short m = 1;
            m += m;

(b)

            short m = 1;
            m += m + m;

while this

(c)

            short m = 1;
            m = m + m;

leads to the error "Type mismatch: cannot convert from int to short" ?

It's not a warning - it's an error.

There are two facts at work here:

There's no short + short operator; the "smallest" addition is int + int, with a result type of int, and the operands are automatically promoted to int if necessary (see JLS 15.18.2)
Compound assignment operators always have an implicit cast (see JLS 15.26.2)

The second point is why the first two operations work. You've effectively got:

m = (short) (m + m);

And

m = (short) (m + m + m);

The first point is why the last operation doesn't work - the type of m + m is int, and you can't assign an int value to a short variable. (You need a cast...)

See more on this question at Stackoverflow