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Strict behavior cannot be inherited by a subclass that extends a strictfp superclass. An overriding method can independently choose to be strictfp when the overridden method is not or vice versa.

Noncompliant Code Example

This noncompliant code example does not mandate strictfp computation. Double.MAX_VALUE is multiplied by 1.1 and reduced back by dividing by 1.1, according to the evaluation order. If Double.MAX_VALUE is the maximum value permissible by the platform, the calculation will yield the result infinity.

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class Example {
  public static void main(String[] args) {
    double d = Double.MAX_VALUE;
    System.out.println("This value \"" + ((d * 1.1) / 1.1) + "\" cannot be represented as double.");
  }
}

Compliant Solution

For maximum portability, use the strictfp modifier within an expression (class, method, or interface) to guarantee that intermediate results do not vary because of implementation-defined compiler optimizations or by design. The calculation in this compliant solution is guaranteed to produce infinity because of the intermediate overflow condition, regardless of what floating-point support is provided by the platform.

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Note that this compliant solution also specifies that the floating-point literals (1.1) are of type double to clarify their expected type; this complies with rule "NUM18-J. Be aware of numeric promotion behavior."

Noncompliant Code Example

On platforms whose native floating-point hardware provides greater precision than double, the JIT is permitted to use floating-point registers to hold values of type float or type double (in the absence of the strictfp modifier), even though the registers support values with greater exponent range than that of the primitive types. Consequently, conversion from float to double can cause an effective loss of magnitude.

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The lost magnitude would also have been lost if the value were stored to memory, for example to a field of type float.

Compliant Solution

This compliant solution uses the strictfp keyword to require exact conformance with standard Java floating point. Consequently, the intermediate value of both computations of f * g will be identical to the value stored in this.d, even on platforms that support extended range exponents.

strictfp class Example {
  double d = 0.0;

  public void example() {
    float f = Float.MAX_VALUE;
    float g = Float.MAX_VALUE;
    this.d = f * g;
    System.out.println("d (" + this.d + ") might not be equal to " + (f * g));
  }

  public static void main(String[] args) {
    Example ex = new Example();
    ex.example();
  }
}

Exceptions

NUM09-EX1: This rule applies only to calculations that require consistent floating-point results on all platforms. Applications that lack this requirement need not comply.

NUM09-EX2: The strictfp modifier may be omitted when competent numerical analysis demonstrates that the computed values will meet all accuracy and behavioral requirements that are appropriate to the application. Note that "competent numerical analysis" generally requires a specialized professional numerical analyst; lesser levels of rigor fail to qualify for this exception.

Risk Assessment

Failure to use the strictfp modifier can result in implementation-defined behavior with respect to the behavior of floating-point operations.

Automated Detection

Sound automated detection of violations of this rule is not feasible in the general case.

Related Guidelines

Bibliography

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