Type mismatch: cannot convert from long to int

Question

I had the following lines of code

long longnum = 555L;
int intnum = 5;
intnum+=longnum;
intnum= intnum+longnum; //Type mismatch: cannot convert from long to          


        

Answer 1

That's because the compound assignment operator does implicit casting.

From JLS Compound Assignment Operator:

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.

While in case of binary + operator, you have to do casting explicitly. Make your 4th assignment:

intnum = (int)(intnum+longnum);

and it would work. This is what your compound assignment expression is evaluated to.