Why I have to cast this statement in order to get a result?

Question

private final static int L1_MAX_SCORE = 30;
private final static int L2_MAX_SCORE = 150;

public void UpdateLevel(int score) {
   double progress;

   //returns 0.0
   progress = score / (L2_MAX_SCORE  - L1_MAX_SCORE) * 100;   

   //works correctly.
   progress = (double) score / (L2_MAX_SCORE  - L1_MAX_SCORE) * 100;  

Thanks.

Answer 1

Arithmetic operations in Java whose operands are all ints will result in ints, so you're actually assigning an integer result to a double variable. Thus you must cast at least one of them to a double so that the calculations are performed based on doubles, because of the higher precision.

Answer 2

Dividing an integer with an integer is defined to do integer division, just like in most (all?) other C-like languages.

By casting score to a double, you are dividing a floating-point value with an integer, and you get a floating-point value back.

Answer 3

L2_MAX_SCORE - L1_MAX_SCORE = 150 - 30 = 120

Let's assume score is 30

Using floating point, the answer should be 30/120 = 0.4 With integer division, this will get rounded to 0 hence why it 'doesn't work' - it is actually working, but doing the wrong kind of division for your needs

You can either

• a) Cast either numerator or denominator as double
• b) Rearrange the formula to be (score * 100) / (L2_MAX_SCORE - L1_MAX_SCORE)
  • with this, it would become (30 * 100) / 120 = 3000 / 120 = 40
  • it still does integer division, so if the score is 31 your answer (3100 / 120) is not 25.8333 but 25