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

org.apache.commons.math.fraction.Fraction provides a fraction number type that forms the basis for the fraction functionality found in commons-math.

To create a fraction number, simply call the constructor passing in two integer arguments, the first being the numerator of the fraction and the second being the denominator: Fraction f = new Fraction(1, 3); // 1 / 3

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.

Fraction instances can be converted to and from strings using the org.apache.commons.math.fraction.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");