The Java language allows platforms to make use of available floating point hardware that may can provide floating point support with mantissas and/or exponents that contain more bits than the standard Java primitive type double, thus enabling those platforms to represent a superset of the values that can be represented by the standard floating point types. Floating point computations on such platforms may can produce different results than would be obtained if the floating point computations were restricted to the standard representations of float and double. According to the JLS, Section 15.4, "FP-Strict Expressions": "
the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.
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When it is important to obtain consistent results from floating point operations across different JVMs and platforms, use the strictfp modifier. This modifier requires the JVM and platform to behave as though all floating point computations were performed using values limited to those representable by a standard Java float or double, thus guaranteeing that the result of the computations will match exactly across all JVMs and platforms.
Use of the strictfp modifier lacks impact on platforms that lack platform-specific floating point behavior. It can have substantial impact, however, on both the efficiency and the result values of floating point computations when executing on platforms that implement platform-specific floating point behavior. On these platforms, use of the strictfp modifier increases the likelihood that intermediate operations will overflow or underflow because it restricts the representable range and precision of intermediate values; it may can also reduce computational efficiency. These issues are unavoidable when portability is the main concern.
The strictfp modifier can be used with a class, method, or interface:
Usage | Strictness Behavior |
|---|---|
Class | All code in the class including (instance, variable, static initializers), code in nested classes |
Method | All code within the method is subject to strictness constraints |
Interface | All code in the class that implements the interface is also strict |
An expression is strict when any of the containing classes, methods, or interfaces is declared to be a strictfp. Constant expressions containing floating point operations are also evaluated strictly. All compile-time constant expressions are by default, strictfp.
The strict behavior cannot be inherited by a subclass that extends a strictfp superclass. An overriding method may can independently choose to be strictfp when the overridden method is not or vice versa.
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This noncompliant code example does not mandate strictfp computation. Double.MAX_VALUE is being multiplied by 1.1 and reduced back by dividing by 1.1, according to the evaluation order. JVM implementations are not required to report an overflow resulting from the initial multiplication, although they may can choose to treat this case as strictfp. The ability to use extended exponent ranges to represent intermediate values is implementation defined.
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To be compliant, 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. This code snippet is guaranteed to return positive INFINITY because of the intermediate overflow condition.
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Failure to use the strictfp modifier can result in platform-defined behavior, with respect to the accuracy of floating point operations.
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