Casting spells on numbers Download

Christina Cacioppo

int, double, casting Polish needed


In Java, every value is stored as a type; we've been doing htis already, though we haven't been calling that part out:

int i = 10;
double d = 24.4;

So far, we've kept these types constant: once we call something an int, we keep it an int, and don't try to make it a double.

Humans deal with types implicitly: we can tell 42 (an int or whole number) from 42.0 (a double or real number.)

Computers aren't that smart; they only see bytes, so to them, 42 and 42.0 aren't the same. In fact, to a computer, converting 42.0 to 42 is weird: it means we want it to store less information than it had been. (After all, 42.0 is more precise than 42.)

Since computers find dropping information so off-putting, we've got to tell them we really, really mean it. Casting is how we do that.


Run these math expressions, one at a time, in the Java REPL at the right.

  1. What does 5.0/2 return?

  2. What does (double) 5/2 return?

  3. What about 5/2?

  4. Why do the first two return the same answer and the third returns a different answer?

  5. How about 3+5.0/2+5*2?

  6. Let's change the place of the double. What does 3.0+5/2+5*2 return?

  7. Why did changing the place of the double change the answer?

  8. Now for an explicit cast: what does (int)(3.0 + 5)/(2 + 5 * 2) return?

  9. Why do these last three expressions, which look similar, return different values?

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