This is a String expression, using the string concatenation
operator, +.
The concatenation of 2 Strings is meant to be analogous to
the addition of 2 numbers, hence the use of the same
operator: +.
However, num1 and num2 are not Strings! They are
of type int.
When the Java compiler sees an expression using the + operator,
where at least 1 of its 2 operands is a String, it
converts the other operand (in this case, an int) to its
String representation.
In general, mixing types in an expression can be dangerously
unclear.