Storing and Retrieving Mixed Types of Data
- It is common for records to consist of mixed data types such as string, boolean, and as shown below:
- Suppose we want to store the customer name, customer number, and gender. A string is used to store a customer name, an integer to store the customer number, and a boolean to indicate gender.
- Two streams are used to write the record to the record store.
- The first stream is an object of ByteArrayOutputStream
- The other stream is an object of type DataOutputStream, which is used to output the byte array output stream
- The DataOutputStream class has methods that write specific data types to a buffer. They are writeUTF() method, writeBoolean() method, and writeInt() method. Each is passed the appropriate data.
- The objective is to write data to a buffer, then write the buffered data to the stream. This is done by calling the flush() method.
- The data stream is then converted to a byte array. The byte array is then written to the record store. This process is done by calling the toByteArray() method, which returns a reference to the byte array of the stream.
Steps to Follow to Store Data in to the Store:
- Create an object of ByteArrayOutputStream by calling the ByteArrayOutputStream() constructor
- Create an object of DataOutputStream by calling the DataOutputStream() constructor and passing it reference to the byte array output stream.
- Call the write methods of the DataOutputStream object to store data in the buffer.
- Then the buffered data is placed in the data stream by calling the flush() method.
- The stream is converted to a byte array by calling the toByteArray() method, which returns a reference to the byte array of the stream.
- The reference to the byte array is passed to the addRecord() method, which saves the byte array as a new record in the record store.
- Call the reset() method of the ByteArrayOutputStream object to clear the internal store of the object
- Next, close the output stream and the data output stream objects.
- Any errors occurring while the data is being written to the record store are trapped by the catch {} block and displayed in an alert dialog box.
Steps to Read Data from a Data Store:
- Reading a mixed data type record from a record store is similar to the routine that writes mixed data types.
- First, you create an instance of the ByteArrayInputStream class.
- The constructor of this class is passed the byte array that was just created.
- Also create an instance of the DataInputStream class and pass reference to the ByteArrayInputStream class to the DataInputStream class constructor.
- The routine used to read records from a record store must assume that more than one record exists, and therefore you need to include a for loop so the MIDlet continues to read records from a record store until the last record is read.
MIDlet that Writes and Reads Mixed Types of Data:
import javax.microedition.rms.*;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.io.*;
class RecordStoreMidlet extends MIDlet implements CommandListener
{
private Display display;
private Alert alert;
private Form form;
private Command exit;
private Command openStore;
private Command newStore;
private Command clsStore;
private Command delStore;
private RecordStore recordstore = null;
private RecordEnumeration recordenumeration = null;
private StringItem st;
private TextField txtStoreName;
public RecordStoreMidlet ()
{
display = Display.getDisplay(this);
st = new StringItem("","");
txtStoreName = new TextField("Name of the Store", "", 31, 0);
openStore = new Command("Open Store", Command.SCREEN, 1);
newStore = new Command("Create Store", Command.SCREEN, 2);
exit = new Command("Exit App", Command.SCREEN, 5);
form = new Form("Record Store");
form.append(txtStoreName);
form.addCommand(openStore);
form.addCommand(newStore);
form.addCommand(exit);
form.setCommandListener(this);
}
public void startApp()
{
display.setCurrent(form);
}
public void pauseApp()
{
}
public void destroyApp( boolean unconditional )
{
}
public void commandAction(Command command, Displayable displayable)
{
if (command == exit)
{
destroyApp(true);
notifyDestroyed();
}
else if (command == newStore)
{
try
{
recordstore = RecordStore.openRecordStore(txtStoreName.getString(), true );
st.setText("The Record Store has been Created and Record has been Added");
byte[] outputRecord;
String outputString = "First Record";
int outputInteger = 15;
boolean outputBoolean = true;
ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
DataOutputStream outputDataStream =
new DataOutputStream(outputStream);
outputDataStream.writeUTF(outputString);
outputDataStream.writeBoolean(outputBoolean);
outputDataStream.writeInt(outputInteger);
outputDataStream.flush();
outputRecord = outputStream.toByteArray();
recordstore.addRecord(outputRecord, 0, outputRecord.length);
outputStream.reset();
outputStream.close();
outputDataStream.close();
form.append(st);
}
catch (Exception error)
{
alert = new Alert("File Status!", error.toString(), null, AlertType.WARNING);
alert.setTimeout(Alert.FOREVER);
display.setCurrent(alert);
}
}
else if (command == openStore)
{
try
{
recordstore = RecordStore.openRecordStore(txtStoreName.getString(), false );
st.setText("The Record Store Exists and Open for Storage and Retrieval");
//Code required for Retrieving Record from Record Store
String inputString = null;
int inputInteger = 0;
boolean inputBoolean = false;
byte[] byteInputData = new byte[100];
ByteArrayInputStream inputStream = new ByteArrayInputStream(byteInputData);
DataInputStream inputDataStream =
new DataInputStream(inputStream);
for (int x = 1; x <= recordstore.getNumRecords(); x++)
{
recordstore.getRecord(x, byteInputData, 0);
inputString = inputDataStream.readUTF();
inputBoolean = inputDataStream.readBoolean();
inputInteger = inputDataStream.readInt();
inputStream.reset();
}
inputStream.close();
inputDataStream.close();
alert = new Alert("Reading", inputString + " " + inputInteger + " " + inputBoolean, null, AlertType.WARNING);
alert.setTimeout(Alert.FOREVER);
display.setCurrent(alert);
}
catch (Exception error)
{
st.setText("The Record Store Does not Exist. Create a New One!");
alert = new Alert("File Status!", st.getText(), null, AlertType.WARNING);
alert.setTimeout(Alert.FOREVER);
display.setCurrent(alert);
form.append(st);
}
}
}
}
