Why aren't op-assign operators type safe in java?

Question

I'm not sure the question is clearly worded, but an example will be clearer.

I found out that will not work in Java:

int a = ...;
a = 5.0;

but this will:

int a = ...;
a += 5.0;

I.e., it seems that the = operator is type safe but += isn't. Is there any deep reason for this or is it just another arbitrary decision language designers must take.

Answer 1

The reason is that math operations do some implicit casting:

a += 5.0; is evaluated as follows:

a = (int) ((double) a + 5.0);

Assignment, however, requires an explicit cast.

(It might be float rather than double, I don't remember which Java treats as decimal literals.)