>cannot convert from int to char. a+=b
This question already has an answer here:
The answer lies in binary numeric promotion. All operand are promoted to the type of the wider operand, and up to at least int . This is described by the JLS, Section 5.6.2:
2. Widening primitive conversion (§5.1.2) is applied to convert either or both operands as specified by the following rules:
If either operand is of type double, the other is converted to double.
Otherwise, if either operand is of type float, the other is converted to float.
Otherwise, if either operand is of type long, the other is converted to long.
Otherwise, both operands are converted to type int .
(emphasis mine)
Therefore a char ( checkSum ) plus a char ( charNum[i] ) is an int .
It is more simple than you think :).
If you add two char 's, it creates int . Always.
And you are not right in few things : The values of cells in array of char is char , not reference, therefore charNum[i] is char and it is only 2 bytes width.