Data Storage, Continued

PROG 38448

Mobile Java Application Development

PROG 38448

Mobile Java Application Development

Data Storage, Continued

04/21/23 Wendi Jollymore, ACES 2

Review of FileConnectionReview of FileConnection

Can read data that other apps have written to file systemA traditional file systemjavax.microedition.io.file packageUse Connector class to create the actual connection


Review Connector ClassReview Connector Class

javax.microedition.io.ConnectorA factory class used to create connectionsFile connections, network connections, etc..

Connector.open(string) opens a connection and returns a Connection object{scheme}:[{target}][{parms}] (see docs for a list of schemes){scheme} - name of a protocol, such as http:// or file://{target} - normally some kind of network address or resourceAny {parms} are formed as a series of equates of the form ";x=y". i.e. ";type=a“


Review File SystemReview File System

It’s important to know the locations of the various storage locations:

store/device memory home - internal flash memory

SDCard/ sd card home

system/ internal sd card on Bold and Storm (transient data objects and runtime processes - 860MB)


Review Creating ConnectionReview Creating Connection

Exception is thrown if path is invalidDoesn’t have to exist - *could* existOnly the last item can be non-existentThis allows you to create new files/directories

try { FileConnection storeDirectory = FileConnection)Connector.open("file:///store/");} catch (IOException e) { Dialog.alert(e.getMessage());}


Review FileConnection MethodsReview FileConnection Methods

FileConnection.close()From parent interface ConnectionCloses file connection

FileConnection.isDirectory()True if the object points to a directory

FileConnection.exists() True of the directory/file exists


FileConnection MethodsFileConnection Methods

FileConnection.create()Creates a file corresponding to the string provided in the Connection.open() methodCreated with 0 lengthDoes not create directories in the path

Throws IOExceptionUse output streams to write data to the file


FileConnection MethodsFileConnection Methods

FileConnection.mkdir()Creates a directory corresponding to the string provided in the Connection.open() methodNot recursive – you must create all new dirs in a path if they don’t exist

throws IOException


Working with Dirs and FilesWorking with Dirs and Files

If you have a file that doesn’t exist in a directory that doesn’t exist

You have to create the directory first:

FileConnection fileConn = (FileConnection) Connector.open(aPath);if (!fileConn.exists()) { fileConn.mkdir();}



Then you have to reset the file connection:

Then you have to create a connection to the file and create it:

fileConn.close();fileConn = null;

fileConn = (FileConnection) Connection.open(fileName);if (!fileConn.exists()) { fileConn.create();}



If your directory and file already exist, there’s a shorter way:

This still allows you to use the same FileConnection object

But it only works when every part of the path/file exists

Throws an IOException

fileConn.setFileConnection(fileName);


Working with StreamsWorking with Streams

To read and write data, you need an input or output streamWe use binary steams only

more efficientcan use regular InputStream or OutputStreamDataInputStream or DataOutputStream a bit more convenient

you can read/write data types and strings (sort of)



FileConnection.openInputStream();Opens and returns an InputStream object Connected to the file specified when the connection was createdThe file must exist, must be accessible

FileConnection.openDataInputStream();Opens and returns a DataInputStream objectSame as above

Both throw IOException



FileConnection.openOutputStream();Opens and returns an OutputStream object File pointer positioned at start of fileThe file must exist, must be accessibleopenOutputStream(long offset) will start at byte offset

FileConnection.openDataOutputStream();Opens and returns a DataOutputTream objectSame as above but can’t use offset

All throw IOException



Opening streams from the file connecton object:

When you are done with a stream:Close it using close() methodAssign null to stream object

DataInputStream fileIn = fileConn.openDataInputStream();

DataOutputStream fileOut = fileConn.openDataOutputStream();



Look up DataInputStream and DataOutputStream in the api docsNote the methods to read and write data to the fileEverything must be a fixed size

E.g. ints are always 4 bytes, doubles are 8 bytes, Strings are 2 bytes per character

Remind you of Random Access Files?



Strings are the ones you need to pay most attention to.When storing data as “records”, all records must be exact same size

So you know where to find record x

All strings must be fixed widthDefine how many characters your string fields are

Make sure they’re saved as that exact size



Example: Field size is 25

Field is either truncated or padded to be the exact size of 25 chars

int currSize = str.length();if (currSize > size)

str = str.substring(0, size);else if (currSize < size) {

String temp = str;for (int i=currSize; i<=size; i++)

str += “ “;}



Example: reading back the same data

Read individual chars until you build the string field

Must be the correct number of chars!

String temp = “”;for (int i=0; i<size; i++) { String c = String.valueOf(fileIn.readChar()); temp += c;}

ExerciseExercise

New ProgramAdd three EditFields

One to enter an int, one for a double, and one for a String

Add buttons Save, Clear, ReadAdd event handling stuff for buttonsAdd a display label

Save: saves the 3 valuesInfo: clears txt fieldsRead: shows some saved values


ExerciseExercise

We’ll set the string to 25 charsA whole record will be 62 bytes

4 for the int, 8 for the double25 * 2 for the string

We’ll need methods:Prep the string for writingRead the string from a stream

Add the same loadFile() methodDisplay file size in label, too


ExerciseExercise

Saving Data method: fileDataOut()Use the file connection to create a DataOutputStreamwriteInt(), writeDouble()Prep the string value and writeChars()Close stream and set to null


ExerciseExercise

Reading Data method: fileDataIn()Calc number of records

File size / rec sizeIf the file connection is readable and we have records:

Create a DataInputStreamFor each record:

readInt(), readDouble(), your readString() methodBuild an output string

Close stream and set to nullDisplay output from file in label



ExerciseExercise

When program exits, you must:Close file connectionAssign null to file connection object

Override the close() method:if (fileConn != null && fileConn.isOpen()) { fileConn.close(); fileConn = null;}


Simulator NotesSimulator Notes

If you want to start from scratch each time you debug:

Erase File System optionBe careful doing this if you’re using SDCard option!!


File IssuesFile Issues

What happens when you run the app multiple times?Add new data each timeWhat happens to your old data?

You can’t append with DataOutputStream

You must use OutputStreamSet the offset to the file size



Example:long size = fileConn.fileSize();OutputStream fileOut = fileConn.openOutputStream(size);

Also:OutputStream works with data differentlyThere are no methods like writeInt()You have to convert ints and doubles to bytes!



Converting ints and doubles to bytes:I gave you some code in the notes

Note that strings are no longer 2 bytes per character

They’re one byte eachYou’ll need to adjust your read methodYou’ll need to adjust any size constantsString.getBytes() returns byte[]


ExerciseExercise

Edit your program so that you can append records to the end of the file

String.getBytes() will give you the bytes for the String valueoutputStream.write(byte[]) will write an array of bytesdataInputStream.readFully(byte[]) can be used to read your string data back

Then String s = new String(bytes);

Data Storage, Continued

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