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Operations that could suffer from integer overflow or loss of a fractional remainder should be performed on floating-point values rather than integral values.
Noncompliant Code Example
In this noncompliant code example, the division and multiplication operations are performed on integral values; the results of these operations are then converted to floating point:
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Note that the calculation for c violates NUM00-J. Detect or prevent integer overflow.
Compliant Solution (Floating-Point Literal)
This compliant solution performs the multiplication and division operations on floating-point values, avoiding both the truncation and the overflow seen in the noncompliant code example. In every operation, at least one of the operands is of a floating-point type, thereby forcing floating-point multiplication and division and avoiding truncation and overflow.
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Note that the original value of c cannot be represented exactly as a double. The representation of type double has only 48 mantissa bits, but a precise representation of the value of c would require 56 mantissa bits. Consequently, the value of c is rounded to the nearest value that can be represented by type double, and the computed value of f (9.328764743180247E18) differs from the exact mathematical result (9328564743180246912). This loss of precision is one of the many reasons correct programming of expressions that mix integer and floating-point operations or values requires careful consideration. See NUM13-J. Avoid loss of precision when converting primitive integers to floating-point for more information about integer-to-floating-point conversion. Even with this loss of precision, however, the computed value of f is far more accurate than that produced in the noncompliant code example.
Compliant Solution
This compliant solution eliminates the truncation and overflow errors by storing the integers in the floating-point variables before performing the arithmetic operations:
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As in the first compliant example, the value of c cannot be represented exactly as a double. Consequently, the value of f is imprecise both before and after the multiplication. The explanation and consequences are identical to those stated previously.
Noncompliant Code Example
This noncompliant code example attempts to compute the whole number greater than the ratio of two integers. The result of the computation is 1.0 rather than the intended 2.0.
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As a consequence of Java's numeric promotion rules, the division operation performed is an integer division whose result is truncated to 1. This result is then promoted to double before being passed to the Math.ceil function.
Compliant Solution
This compliant solution casts the divisor to double before the division is performed:
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Consequently, the numerator is automatically promoted to double. The division operation becomes a double divide, and value is assigned the correct result. As in the previous compliant solution, this practice ensures that at least one of the operands of each operation is a floating-point number
Applicability
Improper conversions between integers and floating-point values can yield unexpected results, especially from precision loss. In some cases, these unexpected results can involve overflow or other exceptional conditions.
It is acceptable to perform operations using a mix of integer and floating-point values when deliberately exploiting the properties of integer arithmetic before conversion to floating point. For example, use of integer arithmetic eliminates the need to use the floor() method. Any such code must be clearly documented to help future maintainers understand that this behavior is intentional.
Bibliography
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