1.6 Casting and Ranges of Values
In Java, type casting is used to convert values from one type to another. By casting we don't mean something to do with fishing, but it is a similar idea to casting a bronze, without needing to heat anything to 913 degrees Celsius. But like molten bronze is reshaped by melting it and pouring it into a mold, our data is reshaped via a cast operator. In Java when you cast you are changing the 'shape' (or type) of the value.
The cast operator, which looks like (int) and (double) placed before any expression (a literal, a number, a variable, or more complex expression in parentheses) produces a value of the given type by converting the value of the original expression to the new type.
Integer Division and Casting
When Java divides two ints, it produces an int result by truncating the actual mathematical result, removing anything after the decimal point. Thus 9 / 10 evaluates to 0, not 0.9. It also does not evaluate to 1; truncating is not the same as rounding!
But in any expression involving a double, the double is 'contagious' and will cause the value of that expression to also be a double. Thus the expression 9.0 / 10 is evaluated as if it had been written 9.0 / 10.0 and produces the double value 0.9.
Widening Conversion
A conversion from int to double is called a widening conversion because a double can represent any int value but not vice versa; thus a double is considered a wider data type than an int.
Rounding Doubles to Nearest Integer
Values of type double in the range that can be represented by an int can be rounded to the nearest int by adding or subtracting 0.5 and then casting the result with (int):
double number; // positive value from somewhere
double negNumber; // negative value from somewhere
int nearestInt = (int)(number + 0.5);
int nearestNegInt = (int)(negNumber – 0.5);
Integer Overflow
ints in Java are always 32-bit signed values which mean they can represent values from -2,147,483,648 to 2,147,483,647, inclusive. You can refer to the minimum and maximum int values with the constants Integer.MIN_VALUE and Integer.MAX_VALUE.
If you try to store any number larger or smaller than these numbers in an int variable, it will result in an integer overflow where an incorrect value could be stored.
Type in three made up int grades and then sum and average them. Use type casting to report the result as a double.