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Conversion from int or long to float or from long to double can lead to loss of precision (loss of least significant bits). In these cases, the resulting floating-point value is a rounded version of the integer value, using IEEE 754 round-to-nearest mode. Despite this loss of precision, The Java Language Specification (JLS) requires that the conversion and rounding occur silently, that is, without any runtime exception (see the JLS, §5.1.2, "Widening Primitive Conversion" [JLS 20052015], for more information). Conversions from integral types smaller than int to a floating-point type and conversions from int to double can never result in a loss of precision. Consequently, programs must ensure that conversions from an int or long to a floating-point type or from long to double do not result in a loss of required precision.

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Note that this compliant solution cannot be used when the primitive integers are of type long because Java lacks a primitive floating-point type whose mantissa can represent the full range of a long.

Exceptions

NUM13-J-EX0: Conversion from integral types to floating-point types without a range check is permitted when suitable numerical analysis demonstrates that the loss of the least significant bits of precision is acceptable.

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Converting integer values to floating-point types whose mantissa has fewer bits than the original integer value can result in a rounding error.

Automated Detection

Automatic detection of casts that can lose precision is straightforward. Sound determination of whether those casts correctly reflect the intent of the programmer is infeasible in the general case. Heuristic warnings could be useful.

Related Guidelines

Bibliography

[JLS

2005

2015]	§5.1.2, "Widening Primitive Conversion"
[Seacord 2015]	Image Modified NUM13-J. Avoid loss of precision when converting primitive integers to floating-point LiveLesson

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