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float vs double data types in Java

By: Baski in Java Tutorials on 2007年09月07日 [フレーム]

Floating-point numbers, also known as real numbers, are used when evaluating expressions that require fractional precision. For example, calculations such as square root, or transcendentals such as sine and cosine, result in a value whose precision requires a floating-point type. Java implements the standard (IEEE-754) set of floating point types and operators. There are two kinds of floating-point types, float and double, which represent single- and double-precision numbers, respectively. Their width and ranges are shown here:

Name	Bits	Range
double	64	1.7e-308 to 1.7e+308
float	32	3.4e-038 to 3.4e+038

Each of these floating-point types is examined next.

float

The type float specifies a single-precision value that uses 32 bits of storage. Single precision is faster on some processors and takes half as much space as double precision, but will become imprecise when the values are either very large or very small. Variables of type float are useful when you need a fractional component, but don't require a large degree of precision. For example, float can be useful when representing dollars and cents.

Here are some example float variable declarations:

float hightemp, lowtemp;

double

Double precision, as denoted by the double keyword, uses 64 bits to store a value. Double precision is actually faster than single precision on some modern processors that have been optimized for high-speed mathematical calculations. All transcendental math functions, such as sin(), cos(), and sqrt(), return double values. When you need to maintain accuracy over many iterative calculations, or are manipulating large-valued numbers, double is the best choice.

Here is a short program that uses double variables to compute the area of a circle:

// Compute the area of a circle.
class Area {
 public static void main(String args[]) {
 double pi, r, a;
 r = 10.8; // radius of circle
 pi = 3.1416; // pi, approximately
 a = pi * r * r; // compute area
 System.out.println("Area of circle is " + a);
 }
}

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