9 Fractions

9.1 Overview

The fraction packages provides a fraction number type as well as fraction number formatting.

9.2 Fraction Numbers

Fraction and BigFraction provide fraction number type that forms the basis for the fraction functionality found in Commons-Math. The former one can be used for fractions whose numerators and denominators are small enough to fit in an int (taking care of intermediate values) while the second class should be used when there is a risk the numerator and denominator grow very large.

A fraction number, can be built from two integer arguments representing numerator and denominator or from a double which will be approximated:

Fraction f = new Fraction(1, 3); // 1 / 3
Fraction g = new Fraction(0.25); // 1 / 4

Of special note with fraction construction, when a fraction is created it is always reduced to lowest terms.

The Fraction class provides many unary and binary fraction operations. These operations provide the means to add, subtract, multiple and, divide fractions along with other functions similar to the real number functions found in java.math.BigDecimal:

Fraction lhs = new Fraction(1, 3);
Fraction rhs = new Fraction(2, 5);

Fraction answer = lhs.add(rhs);     // add two fractions
        answer = lhs.subtract(rhs); // subtract two fractions
        answer = lhs.abs();         // absolute value
        answer = lhs.reciprocal();  // reciprocal of lhs

Like fraction construction, for each of the fraction functions, the resulting fraction is reduced to lowest terms.

9.3 Fraction Formatting and Parsing

Fraction instances can be converted to and from strings using the FractionFormat class. FractionFormat is a java.text.Format extension and, as such, is used like other formatting objects (e.g. java.text.SimpleDateFormat):

FractionFormat format = new FractionFormat(); // default format
Fraction f = new Fraction(2, 4);
String s = format.format(f); // s contains "1 / 2", note the reduced fraction

To customize the formatting output, one or two java.text.NumberFormat instances can be used to construct a FractionFormat. These number formats control the formatting of the numerator and denominator of the fraction:

NumberFormat nf = NumberFormat.getInstance(Locale.FRANCE);
// create fraction format with custom number format
// when one number format is used, both numerator and
// denominator are formatted the same
FractionFormat format = new FractionFormat(nf);
Fraction f = new Fraction(2000, 3333);
String s = format.format(c); // s contains "2.000 / 3.333"

NumberFormat nf2 = NumberFormat.getInstance(Locale.US);
// create fraction format with custom number formats
format = new FractionFormat(nf, nf2);
s = format.format(f); // s contains "2.000 / 3,333"

Formatting's inverse operation, parsing, can also be performed by FractionFormat. To parse a fraction from a string, simply call the parse method:

FractionFormat ff = new FractionFormat();
Fraction f = ff.parse("-10 / 21");