Avoid Floats and Doubles

Summary

Avoid using floats and doubles (both the primitives and their wrappers).

Detail

Floats and doubles introduce a minefield of rounding and comparison issues. While they are a sensible choice for some domains where you do not care about rounding errors integers or BigDecimal are usually a better choice for server-side business code.

The core issue is that floating point numbers are not able to represent many numbers (e.g. 0.1).

This leads to unexpected results that may not be caught by simple test cases

    double balance = 2.00;
    double transationCost = 0.10;
    int numberTransactions = 6;

    System.out.printf("After %s transactions balance is %s"
                    , numberTransactions
                    , balance - (transationCost * numberTransactions));
    // Gives After 6 transactions balance is 1.4 :-)

But

    double balance = 2.00;
    double transationCost = 0.10;
    int numberTransactions = 7;

    System.out.printf("After %s transactions balance is %s"
                     , numberTransactions
                     , balance - (transationCost * numberTransactions));
    // Gives After 7 transactions balance is 1.2999999999999998 :-(

The simplest solution in this case would be to replace the floats with integer values (i.e. track the balance in units of cents rather than dollars).

In situations where floats can't be replaced by integers code can be re-written to use BigDecimal.

    BigDecimal balance = new BigDecimal("2.00");
    BigDecimal transationCost = new BigDecimal("0.10");

    BigDecimal numberTransactions = BigDecimal.valueOf(7);

    System.out.printf("After %s transactions balance is %s"
                     , numberTransactions
                     , balance.subtract(transationCost.multiply(numberTransactions)));

   // Gives After 7 transactions balance is 1.30 :-)

Note that although BigDecimal can be constructed from a float this would take us back to where we started.

    BigDecimal balance = new BigDecimal("2.00");
    BigDecimal transationCost = new BigDecimal(0.10); // <- float used to construct

    BigDecimal numberTransactions = BigDecimal.valueOf(7);

    System.out.printf("After %s transactions balance is %s"
                     , numberTransactions
                     , balance.subtract(transationCost.multiply(numberTransactions)));

   // Gives After 7 transactions
   // balance is 1.2999999999999999611421941381195210851728916168212890625

When to use floats and doubles

Floats and doubles clearly can't be all bad or it is unlikely that they would have been included in the Java language.

The primitive floating point types have performance advantages over BigDecimal that can be significant in highly numerical domains such as machine learning, physics engines, scientific applications etc. In these domains the performance benefit may greatly out-weigh the additional risk of error.

Code using BigDecimal is also inherently more verbose and clumsy than code that uses primitives. If you are working in a domain where the imprecision of floating point types is acceptable you might prefer the cleaner code they allow, but be sure you are making this choice consciously with an understanding of the pitfalls involved.