Persistent Storage I: MIDP Record Store - Beginning J2ME: From Novice to Professional - page 111

Performing RecordStore Queries
The real power of a database is being able to pull out just the record or records you want. In a
larger database, this is called performing a query . In the RecordStore world, you use the
enumerateRecords() method:
public RecordEnumeration enumerateRecords(RecordFilter filter,
RecordComparator comparator, boolean keepUpdated)
throws RecordStoreNotOpenException
This single method in RecordStore involves three different interfaces that you've never
seen before. Let's start with the big picture first, and then drill down into the new interfaces.
The enumerateRecords() method returns a sorted subset of the records in a RecordStore .
The RecordFilter determines which records will be included in the subset, while the
RecordComparator is used to sort them. The returned RecordEnumeration allows you to navigate
through the returned records.
RecordFilter
The simplest interface is RecordFilter . When you call enumerateRecords() on a RecordStore , each
record's data is retrieved. RecordFilter has a single method, matches() , which is called for
each record. A record filter should examine the record data and return true if the record should
be included in the results returned from enumerateRecords() .
Here's a simple RecordFilter implementation that will only select records whose first byte
of data is 7:
public class SevenFilter
implements javax.microedition.rms.RecordFilter {
public boolean matches(byte[] candidate) {
if (candidate.length == 0) return false;
return (candidate[0] == 7);
}
}
RecordComparator
The job of a RecordComparator implementation is to determine the order of two sets of record
data. RecordComparator is similar to the java.util.Comparator interface in J2SE.
To implement the RecordComparator interface, you just need to define one method:
public int compare(byte[] rec1, byte[] rec2)
This method examines the data contained in rec1 and rec2 and determines which of them
should come first in a sorted list. It should return one of the following constants defined in
RecordComparator :
• PRECEDES indicates that rec1 should come before rec2 .
• FOLLOWS indicates that rec1 should come after rec2 .
• EQUIVALENT signals that rec1 and rec2 are the same, at least as far as sorting is concerned.