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Sec. 4.4 Dictionaries 133not get at a record of the dictionary that he didn’t already know the key value for.With the removeAny method, the user can process all records in the dictionary asshown in the following code fragment.while (dict.size() > 0) {it = dict.removeAny();doSomething(it);}There are other approaches that might seem more natural for iterating though adictionary, such as using a “first” and a “next” function. But not all data structuresthat we want to use to implement a dictionary are able to do “first” efficiently. Forexample, a hash table implementation cannot efficiently locate the record in thetable with the smallest key value. By using RemoveAny, we have a mechanismthat provides generic access.Given a database storing records of a particular type, we might want to searchfor records in multiple ways. For example, we might want to store payroll recordsin one dictionary that allows us to search by ID, and also store those same recordsin a second dictionary that allows us to search by name.Figure 4.30 shows an implementation for a payroll record. Class Payroll hasmultiple fields, each of which might be used as a search key. Simply by varyingthe type for the key, and using the appropriate field in each record as the key value,we can define a dictionary whose search key is the ID field, another whose searchkey is the name field, and a third whose search key is the address field. Figure 4.31shows an example where Payroll objects are stored in two separate dictionaries,one using the ID field as the key and the other using the name field as the key.The fundamental operation for a dictionary is finding a record that matches agiven key. This raises the issue of how to extract the key from a record. We wouldlike any given dictionary implementation to support arbitrary record types, so weneed some mechanism for extracting keys that is sufficiently general. One approachis to require all record types to support some particular method that returns the keyvalue. For example, in Java the Comparable interface can be used to provide thiseffect. Unfortunately, this approach does not work when the same record type ismeant to be stored in multiple dictionaries, each keyed by a different field of therecord. This is typical in database applications. Another, more general approachis to supply a class whose job is to extract the key from the record. Unfortunately,this solution also does not work in all situations, because there are record types forwhich it is not possible to write a key extraction method. 22 One example of such a situation occurs when we have a collection of records that describe booksin a library. One of the fields for such a record might be a list of subject keywords, where the typicalrecord stores a few keywords. Our dictionary might be implemented as a list of records sorted bykeyword. If a book contains three keywords, it would appear three times on the list, once for eachassociated keyword. However, given the record, there is no simple way to determine which keyword