strictfp is a keyword in the Java programming language

Java (programming language)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

 that restricts floating-point calculations to ensure portability. It was introduced into Java with the Java virtual machine

Java Virtual Machine

A Java virtual machine is a virtual machine capable of executing Java bytecode. It is the code execution component of the Java software platform. Sun Microsystems stated that there are over 4.5 billion JVM-enabled devices.-Overview:...

 (JVM) version 1.2.

Basis

The IEEE standard, IEEE 754, specifies a standard method for both floating-point calculations and storage of floating-point values in either single (32-bit, used in Java floats) or double (64-bit, used in Java doubles) precision. Prior to JVM 1.2, floating-point calculations were strict; that is, all intermediate floating-point results were represented as IEEE single or double precisions. As a consequence, errors of calculation, overflows

Arithmetic overflow

The term arithmetic overflow or simply overflow has the following meanings.# In a computer, the condition that occurs when a calculation produces a result that is greater in magnitude than that which a given register or storage location can store or represent.# In a computer, the amount by which a...

 and underflows

Arithmetic underflow

The term arithmetic underflow is a condition in a computer program that can occur when the true result of afloating point operation is smaller in magnitude...

, could occur. Whether or not an error had occurred, the calculation would always return a valid number; if an overflow or underflow had occurred, that number would be incorrect. Hence, whether an error had occurred was typically not obvious.
Since JVM 1.2, intermediate computations are not limited to the standard 32- and 64- bit precisions. On platforms that can handle other representations, those representations can be used, preventing overflows and underflows, thereby increasing precision

Accuracy and precision

In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

.
For some applications, a programmer might need every platform to have precisely the same floating-point behavior, even on platforms that could handle greater precision. The strictfp modifier accomplishes this by truncating all intermediate values to IEEE single- and double- precision, as occurred in earlier versions of the JVM.

Usage

Programmers can use the modifier strictfp to ensure that calculations are performed as in the earlier versions; that is, only with IEEE single and double precision types used. Using strictfp guarantees that results of floating-point calculations are identical on all platforms. This can be extremely useful when comparing floating-point numbers.

It can be used on classes

Class (computer science)

In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...

, interfaces

Interface (computer science)

In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...

 and non-abstract methods

Method (computer science)

In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...

. When applied to a method, it causes all calculations inside the method to use strict floating-point math. When applied to a class, all calculations inside the class use strict floating-point math. Compile-time constant expressions must always use strict floating-point behavior

Examples


public strictfp class MyFPclass {
// ... contents of class here ...
}

Ideas for further improvement of Java's floating-point performance

The introduction of the strictfp keyword allowed Java to take advantage of the speed and precision of the extended precision floating-point operations supported by x86 CPUs (for all code not explicitly using the strictfp keyword), while those wanting total platform-independency could still have it by using the strictfp keyword. There have been ideas that Java's floating-point behavior should be modified even more to allow faster or more precise calculations on platforms where this is possible.