java - Floating point arithmetic not producing exact results

I need to do some floating point arithmetic in Java as shown in the code below:

public class TestMain {
    private static Map ccc = new HashMap() {
      { put(1, 0.01); put(2, 0.02); put(3, 0.05); put(4, 0.1); put(6, 0.2);
        put(10, 0.5); put(20, 1.0); put(30, 2.0); put(50, 5.0); put(100, 10.0);
      }
    };

    Double increment(Double i, boolean up) {
        Double inc = null;


        while (inc == null) {
            inc = ccc.get(i.intValue());

            if (up)
                --i;
            else
                ++i;
        }
        return inc;

    }

    public static void main(String[] args) {
        TestMain tt = new TestMain();

        for (double i = 1; i < 1000; i += tt.increment(i, true)) {
            System.out.print(i + ",");
        }
    }
}

This is to simulate the range of values given as output by the Betfair spinner widget.

Floating point arithmetic in Java seems to introduce some unexpected errors. For example, I get 2.180000000000001 instead of 2.18. What use are floating point numbers is you can't trust the results of arithmetic performed on them? How can I get around this issue?

Answer

If you need exact decimal values, you should use java.math.BigDecimal. Then read "What Every Computer Scientist Should Know About Floating-Point Arithmetic" for the background of why you're getting those results.

(I have a .NET-centric article which you may find easier to read - and certainly shorter. The differences between Java and .NET are mostly irrelevant for the purposes of understanding this issue.)