04 J2ME Persistent Storage

String brand = "Honda"; byte bytes[] = brand.getBytes(); int recID = rs.addRecord(bytes,0,bytes.length);
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Retrieving Records
There are two versions to retrieve a record:
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Records
RMS is a system for managing records. A record is an individual data item. No data type. A record is represented by an array of bytes. A record can contain a number, a string, an array, an image -- anything that a sequence of bytes can represent. Manipulating byte array is difficult. A simple way is to use String as it has a rich API to manipulate and convert it to and from byte array is simple.
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Adding Records
The MIDlet invokes the addRecord() method of RecordStore class to insert a new record into the record store.
– public int addRecord(byte[] data, int offset, int numBytes) inserts a record represented by an array of bytes data with offset as its starting index and numBytes as its length.
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Where are the fields?
Don't be confused by the term record. The answer is simple: – In RMS a record doesn't have any fields. A record consists of an array of single binary field of variable size identified by a record Id. This keeps RMS small and flexible -- important attributes for a MIDP subsystem. The programmer has to handle type conversion, comparison, etc.
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Record Stores
A record store is an ordered collection of records. Each record must belong to a record store, and all record access occurs through the record store. In fact, the record store guarantees that records are read and written atomically, with no possibility of data corruption.
String brand = "Toyota"; byte data[] = newappt.getBytes(); rs.setRecord(recID, data, 0, data.length());
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Deleting Records
The MIDlet invokes the deleteRecord() method to delete a record from the record store. public void deleteRecord(int recordId) – deletes the record represented by recordId. The recordId is not reused.
byte[] retrieved = new byte[rs.getRecordSize(recID)]; rs.getRecord(id, retrieved, 0); String retrievedString = new String(retrieved); byte[] retrieved = rs.getRecord(recID); String retrievedString = new String(retrieved);
public int getRecord(int recordId, byte[] buffer, int offset) – copies the data stored in the given record to the byte array represented by buffer. public byte[] getRecord(int recordId) – returns a new copy of the data represented by recordId.
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Record Stores
When a record is created, the record store assigns it a unique identifier, an integer called the record ID.
The first record added to a record store has a record ID of 1, the second a record ID of 2, and so on.
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Managing Record Stores
To open aຫໍສະໝຸດ record store, you simply need to call the openRecordStore method.
– public static RecordStore openRecordStore( String recordStoreName, boolean createIfNecessary) throws RecordStoreException, RecordStoreFullException, RecordStoreNotFoundException
– sets new information, a stream of bytes (newData) with offset as its starting index and numBytes as its length, at the record location represented by recordId.
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Example - Simple Password Records
import javax.microedition.midlet.*; import javax.microedition.lcdui.*; import javax.microedition.rms.*; public class SimpleDBDemo extends MIDlet implements CommandListener { private Display display; private Form fmMain; private Command cmExit, cmAdd, cmUpdate, cmDelete, cmRetrieve; private TextField tfRecID; private TextField tfName; private TextField tfPwd; private RecordStore rs; public SimpleDBDemo() throws RecordStoreException, RecordStoreFullException, RecordStoreNotFoundException { display = Display.getDisplay(this);
rs.deleteRecord(1);
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Example - Simple Password Records
A simple program that allows a user to add, update, delete and retrieve records. The user needs to remember the record IDs.
Question: Which method is easier? Which method is faster?
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Updating Records
To update a record use the method setRecord: public void setRecord(int recordId, byte[] newData, int offset, int numBytes)