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Scale

Introduction

A Scale takes care of converting values as displayed for the user to and from the internal SI values.

  • A LinearScale is used for most units. A linear scale is a scale that is both additive and multiplicative. I.e. toBaseValue(a * b + c) == a * toBaseValue(b) + toBaseValue(c).
  • The IdentityScale is a special case of a LinearScale where toBaseValue(a) == a.
  • The GradeScale is used to express grades and angles in percentages.

The Scale interface prescribes that a scale shall implement these three methods:

  • double toBaseValue(double value) which takes a value in the user's unit and returns the corresponding base (usually SI) value.
  • double fromBaseValue(double value) which takes a value in the base (usually SI) unit and returns the corresponding value in the user's unit.
  • boolean isBaseScale() which should return true for scales that need no conversion to the base value, e.g., the IdentityScale.

Optional methods to implement are:

  • String toString()
  • hashCode()
  • equals(Object object)

LinearScale

The LinearScale is the most used one. It just contains a multiplication factor to transform a value the a value on the base or SI scale. So, a Length.Unit in miles is defined with a scale factor to the SI-unit of Length: meter.

In the Length.Unit class, this is done as follows:

public static final double CONST_FT = 0.3048;
public static final double CONST_MI = 5280.0 * CONST_FT;
public static final Length.Unit mi =
   new Length.Unit("mi", "mi", "mile", new LinearScale(CONST_MI), UnitSystem.IMPERIAL);

The above example shows that the mile unit is defined as being linearly dependent to the default (meter) unit with a factor of 5280 ft, and the foot is defined as 0.3048 m.