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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 (or base values in case the quantity cannot be treated as an SI unit). The method that converts a value in a given scale to the corresponding value in the base or SI scale is called toIdentityScale(value). The method that converts a value expressed in the SI or base scale to the given scale is called fromIdentityScale(value). Both methods return a double representation of the value in the target scale.

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

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

  • double toIdentityScale(double value) which takes a value in the user's unit and returns the corresponding base (usually SI) value.
  • double fromIdentityScale(double value) which takes a value in the base (usually SI) unit and returns the corresponding value in the user's unit.
  • boolean isIdentityScale() 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 to its corresponding 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.