我正在开发一个基于位置的 J2ME 应用程序，并且我正在使用 RMS 来存储数据。
在RecordStore我删除任何记录时，基础记录不会被重新索引。例如，如果我有 5 条记录并且我删除了第 2 条记录，那么记录 ID 将为 {1、3、4、5}。但我希望删除后的记录 ID 为 {1、2、3、4}。我该怎么做？？？因为 recordId 在我的应用程序中扮演着重要的角色来检索和更新记录。
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2 回答
2
因此，既然您已经说过您的记录存储基本上很小（没有那么多数据），我建议您只需将您自己的自定义id字段添加到每条记录。 正如 Meier 所说，一旦创建了记录，RMS 记录 ID 并不是真的要重新计算和更改。所以，我会用你自己的。
如果您的每条记录都包含：
boolean isMale
int age
String firstName
然后，我只需在每条记录的开头添加另一个字段：
int id
它使您的记录更大一点，但不多（4 字节/记录）。如果您的记录少于 64k，那么您也可以将 ashort用于id, 并保存几个字节。
这是一个使用这种记录读取、写入和删除的示例（改编自此 IBM 教程）：
private RecordStore _rs;
// these next two methods are just small optimizations, to allow reading and
// updating the ID field in a record without the overhead of creating a new
// stream to call readInt() on. this assumes the id is a 4 byte int, written
// as the first field in each record.
/** Update one record with a new id field */
private static final void putIdIntoRecord(int id, byte[] record) {
// we assume the first 4 bytes are the id (int)
record[0] = (byte)(id >> 24);
record[1] = (byte)(id >> 16);
record[2] = (byte)(id >> 8);
record[3] = (byte)id;
}
/** Get the id field from one record */
private static final int getIdFromRecord(byte[] record) {
// we assume the first 4 bytes are the id (int)
return ((0xFF & record[0]) << 24) |
((0xFF & record[1]) << 16) |
((0xFF & record[2]) << 8) |
(0xFF & record[3]);
}
/** delete a record with the given (custom) id, re-indexing records afterwards */
private void delete(int idToDelete) {
try {
RecordEnumeration enumerator = _rs.enumerateRecords(new IdEqualToFilter(idToDelete),
null, false);
_rs.deleteRecord(enumerator.nextRecordId());
// now, re-index records after 'idToDelete'
enumerator = _rs.enumerateRecords(new IdGreaterThanFilter(idToDelete), null, true);
while (enumerator.hasNextElement()) {
int recordIdToUpdate = enumerator.nextRecordId();
byte[] record = _rs.getRecord(recordIdToUpdate);
// decrement the id by 1
int newId = getIdFromRecord(record) - 1;
// copy the new id back into the record
putIdIntoRecord(newId, record);
// update the record, which now has a lower id, in the store
_rs.setRecord(recordIdToUpdate, record, 0, record.length);
}
} catch (RecordStoreNotOpenException e) {
e.printStackTrace();
} catch (InvalidRecordIDException e) {
e.printStackTrace();
} catch (RecordStoreException e) {
e.printStackTrace();
}
}
/** generate some record store data ... example of writing to store */
public void writeTestData()
{
// just put 20 random records into the record store
boolean[] booleans = new boolean[20];
int[] integers = new int[20];
String[] strings = new String[20];
for (int i = 0; i < 20; i++) {
booleans[i] = (i % 2 == 1);
integers[i] = i * 2;
strings[i] = "string-" + i;
}
writeRecords(booleans, integers, strings);
}
/** take the supplied arrays of data, and save a record for each array index */
public void writeRecords(boolean[] bData, int[] iData, String[] sData)
{
try
{
// Write data into an internal byte array
ByteArrayOutputStream strmBytes = new ByteArrayOutputStream();
// Write Java data types into the above byte array
DataOutputStream strmDataType = new DataOutputStream(strmBytes);
byte[] record;
for (int i = 0; i < sData.length; i++)
{
// Write Java data types
strmDataType.writeInt(i); // this will be the ID field!
strmDataType.writeBoolean(bData[i]);
strmDataType.writeInt(iData[i]);
strmDataType.writeUTF(sData[i]);
// Clear any buffered data
strmDataType.flush();
// Get stream data into byte array and write record
record = strmBytes.toByteArray();
_rs.addRecord(record, 0, record.length);
// Toss any data in the internal array so writes
// starts at beginning (of the internal array)
strmBytes.reset();
}
strmBytes.close();
strmDataType.close();
}
catch (Exception e)
{
e.printStackTrace();
}
}
/** read in all the records, and print them out */
public void readRecords()
{
try
{
RecordEnumeration re = _rs.enumerateRecords(null, null, false);
while (re.hasNextElement())
{
// Get next record
byte[] recData = re.nextRecord();
// Read from the specified byte array
ByteArrayInputStream strmBytes = new ByteArrayInputStream(recData);
// Read Java data types from the above byte array
DataInputStream strmDataType = new DataInputStream(strmBytes);
// Read back the data types
System.out.println("Record ID=" + strmDataType.readInt());
System.out.println("Boolean: " + strmDataType.readBoolean());
System.out.println("Integer: " + strmDataType.readInt());
System.out.println("String: " + strmDataType.readUTF());
System.out.println("--------------------");
strmBytes.close();
strmDataType.close();
}
}
catch (Exception e)
{
e.printStackTrace();
}
}
在这里，我使用了几个小类RecordFilter，在搜索记录存储时使用：
/** helps filter out records greater than a certain id */
private class IdGreaterThanFilter implements RecordFilter {
private int _minimumId;
public IdGreaterThanFilter(int value) {
_minimumId = value;
}
public boolean matches(byte[] candidate) {
// return true if candidate record's id is greater than minimum value
return (getIdFromRecord(candidate) > _minimumId);
}
}
/** helps filter out records by id field (not "recordId"!) */
private class IdEqualToFilter implements RecordFilter {
private int _id;
public IdEqualToFilter(int value) {
_id = value;
}
public boolean matches(byte[] candidate) {
// return true if candidate record's id matches
return (getIdFromRecord(candidate) == _id);
}
}
于 2013-03-10T04:52:15.663 回答
2
You need to change your application logic. ID is just for identification, and not for sorting. Because it is for identification, it must remains the same.
Very often the easiest thing to do is to read and write the whole recordstore at once.
于 2013-03-09T23:14:52.400 回答