Storing Data Using the Record Store - Beginning Java ME Platform - page 138

• RecordFilter : An instance is responsible for selecting which records should be
included in the enumeration.
• RecordComparator : An instance is responsible for comparing two records; the record
store enumerator uses this to determine the sort order of the returned records.
• boolean : When true , boolean indicates that the enumerator should remain current
during enumeration to any changes in the record store.
The method returns an instance of RecordEnumeration , upon which you can invoke
nextRecord repeatedly to obtain subsequent records sorted in the order dictated by the
RecordComparator you provided (if any). You can also invoke previousRecord to obtain the
record previously returned by nextRecord (or the last record of the enumeration if you
have not obtained any records using nextRecord ).
The RecordFilter interface your record filter must match has a single method, match ,
which takes an array of bytes (a record from the record store in its serialized form) and
returns true if the record matches the criteria you define. The RecordComparator interface
defines the compare method, which takes two arrays of bytes (each a record from the
record store in its serialized form) and returns one of RecordComparator.EQUIVALENT ,
RecordComparator.FOLLOWS , or RecordComparator.PRECEDES , depending on whether the first
record is equivalent to, comes after, or comes before the second interface. Listing 6-4
shows a purely fictitious example.
Listing 6-4. Filtering Record Store Entries with an Enumeration
RecordStore rs = RecordStore.openRecordStore( store, true );
RecordEnumeration re = rs.enumerateRecords(
new RecordFilter() {
public boolean match(byte[] r) {
return true;
}
},
new RecordComparator() {
public int compare( byte[] r1. byte[] r2) {
return RecordComparator.EQUIVALENT;
}, false);
As intuition would dictate, this isn't the best way to traverse all records if order is
unimportant. Instead, the most efficient use of enumerateRecords is to invoke it without a
filter or comparator (just pass null ). This returns an enumeration that returns each ele-
ment of the store in an undefined sequence. If you need to visit all records of the store in
a specific order, specify a comparison function but no filter; if you'd like to filter out some
criteria but order is unimportant, specify a record filter but no record comparator.