 # NUM04-J. Do not use floating-point numbers if precise computation is required

Deprecated

This rule may be deprecated and replaced by a similar guideline.

06/28/2014 -- Version 1.0

The Java language provides two primitive floating-point types, `float` and `double`, which are associated with the single-precision 32-bit and double-precision 64-bit format values and operations specified by IEEE 754 [IEEE 754]. Each of the floating-point types has a fixed, limited number of mantissa bits. Consequently, it is impossible to precisely represent any irrational number (for example, pi). Further, because these types use a binary mantissa, they cannot precisely represent many finite decimal numbers, such as 0.1, because these numbers have repeating binary representations.

When precise computation is necessary, such as when performing currency calculations, floating-point types must not be used. Instead, use an alternative representation that can completely represent the necessary values.

When precise computation is unnecessary, floating-point representations may be used. In these cases, you must carefully and methodically estimate the maximum cumulative error of the computations to ensure that the resulting error is within acceptable tolerances. Consider using numerical analysis to properly understand the problem. See Goldberg's work for an introduction to this topic [Goldberg 1991].

## Noncompliant Code Example

This noncompliant code example performs some basic currency calculations:

```double dollar = 1.00;
double dime = 0.10;
int number = 7;
System.out.println(
"A dollar less " + number + " dimes is \$" + (dollar - number * dime)
);
```

Because the value 0.10 lacks an exact representation in Java floating-point type (or any floating-point format that uses a binary mantissa), on most platforms, this program prints the following:

```A dollar less 7 dimes is \$0.29999999999999993
```

## Compliant Solution

This compliant solution uses an integer type (such as `int`) and works with cents rather than dollars:

```int dollar = 100;
int dime = 10;
int number = 7;
System.out.println(
"A dollar less " + number + " dimes is \$0." + (dollar - number * dime)
);
```

This code correctly outputs the following:

```A dollar less 7 dimes is \$0.30
```

## Compliant Solution

This compliant solution uses the `BigDecimal` type, which provides exact representation of decimal values. Note that on most platforms, computations performed using `BigDecimal` are less efficient than those performed using primitive types.

```import java.math.BigDecimal;

BigDecimal dollar = new BigDecimal("1.0");
BigDecimal dime = new BigDecimal("0.1");
int number = 7;
System.out.println ("A dollar less " + number + " dimes is \$" +
(dollar.subtract(new BigDecimal(number).multiply(dime) )) );
```

This code outputs the following:

```A dollar less 7 dimes is \$0.3
```

## Risk Assessment

Using floating-point representations when precise computation is required can result in a loss of precision and incorrect values.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

NUM04-J

Low

Probable

High

P2

L3

## Automated Detection

Automated detection of floating-point arithmetic is straightforward. However, determining which code suffers from insufficient precision is not feasible in the general case. Heuristic checks, such as flagging floating-point literals that cannot be represented precisely, could be useful.

ToolVersionCheckerDescription
Parasoft Jtest
2023.1
CERT.NUM04.UBDDo not use "float" and "double" if exact answers are required

## Android Implementation Details

The use of floating-point on Android is not recommended for performance reasons.

## Bibliography

 Item 48, "Avoid `float` and `double` If Exact Answers Are Required" Puzzle 2, "Time for a Change" [IEEE 754] [JLS 2015] §4.2.3, Floating-Point Types, Formats, and Values [Seacord 2015] NUM04-J. Do not use floating-point numbers if precise computation is required LiveLesson

1. This guideline has been marked deprecated since June of 2014 (maybe right after publishing the book).

> This rule may be deprecated and replaced by a similar guideline.

Any plan to merge the contents to some other guidelines?

With quick looking other guidelines, I feel the contents are related to NUM13-J and NUM52-J, but restructuring those guidelines (to a new set of guidelines) sounds a non-trivial work...

1. Well, given how long ago that sign has existed, I'd say that any such plans are long dead My offhand suggestion is to demote this rule to a recommendation, but otherwise leave it unchanged. That would mirror C, which has a similar recommendation. What do you think?

1. I would keep this rule such as.

In particular, I think of the equality test between floatting point number with may cause bad flow.