This class models an exchange rate, which defines the factor the numeric value of a base amount in some currency 'A' must be multiplied to get the corresponding amount in the terminating currency 'B'. Hereby

• an exchange rate always models one rate from a base (source) to a term (target) CurrencyUnit.
• an exchange rate is always bound to a rate type, which typically matches the data source of the conversion data, e.g. different credit card providers may use different rates for the same conversion.
• an exchange rate may restrict its validity. In most of the use cases a rates' validity will be well defined, but it is also possible that the data provider is not able to support the rate's validity, leaving it undefined-
• an exchange rate has a provider, which is responsible for defining the rate. A provider hereby may be, but must not be the same as the rate's data source.
• an exchange rate can be a direct rate, where its factor is represented by a single conversion step. Or it can model a derived rate, where multiple conversion steps are required to define the overall base/term conversion. In case of derived rates the chained rates define the overall factor, by multiplying the individual chain rate factors. Of course, this also requires that each subsequent rate's base currency in the chain does match the previous term currency (and vice versa):
• Whereas the factor should be directly implied by the format rate chain for derived rates, this is obviously not the case for the validity range, since rates can have a undefined validity range. Nevertheless in many cases also the validity range can (but must not) be derived from the rate chain.
• Finally a conversion rate is always unidirectional. There might be cases where the reciprocal value of getFactor()} matches the correct reverse rate. But in most use cases the reverse rate either has a different rate (not equal to the reciprocal value), or might not be defined at all. Therefore for reversing a ExchangeRate one must access an ExchangeRateProvider and query for the reverse rate.

The class also implements Comparable to allow sorting of multiple exchange rates using the following sorting order;

• Exchange rate type
• Exchange rate provider
• base currency
• term currency

Finally ExchangeRate is modeled as an immutable and thread safe type. Also exchange rates are Serializable, hereby serializing in the following form and order:

• The base CurrencyUnit
• The target CurrencyUnit
• The factor (NumberValue)
• The ConversionContext
• The rate chain

Implementation Specification

Implementations of this interface

• must be Comparable(with ExchangeRate)
• must implement equals/hashCode considering #getBaseCurrency, #getCurrency, #getFactor and #getContext.
• should be thread-safe
• should be serializable
• should provide a fluent builder API for constructing new rate instances easily.