为什么用Java计算错误？

Question

I use this code to calculate something but in nearly every case the result is 0.0. Why? There are no warnings or errors in the compiler.

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    try {
        int potsum = Integer.valueOf(jTextField1.getText());
        int bla = Integer.valueOf(jTextField2.getText());
        float result = ((Integer.valueOf(bla)/Integer.valueOf(potsum)) * 100);
        jLabel3.setText(Float.toString(result));
    } catch (NumberFormatException e) {
        System.out.println("Error!");
    }
}

Answer 1

This is because this is a integer on integer division. Cast both of them to float before the division and you should get correct results.

Answer 2

float result = ((Integer.valueOf(bla)/Integer.valueOf(potsum)) * 100);

is performing integer division, which floors the value (1/2 => 0, 2/3 => 0, 3/3 => 1).

Consider typecasting or making the numerator/denominator of division a floating point value.

float result = Integer.valueOf(bla) * 100.0f / Integer.valueOf(potsum);

or

float result = (float)Integer.valueOf(bla) / Integer.valueOf(potsum) * 100.0f;

or

float result = Integer.valueOf(bla) / (float)Integer.valueOf(potsum) * 100.0f;

I'll add that I often see people place the division as the last operator in the expression, mostly because that clarifies intent and doesn't require people to understand order of operations + how evaluation is from left to right.

Answer 3

You need to convert to float, otherwise it looks like integer arithmetic to Java.

float result = bla * 100f/ potsum;

Notice the correct calculation for percentages.

Answer 4

float result = ((Float.valueOf(bla)/Float.valueOf(potsum)) * 100);