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How to create a high score database in Java ME

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This article shows how to implement a high score database in Java ME.

Article Metadata
Code ExampleArticle
Created: firt (15 Apr 2007)
Last edited: hamishwillee (30 Jul 2013)

Contents

Introduction

The code example shows how to save and restore users' high scores using the Record Management System (RMS). It also provides an example of how to sort the scores by providing an implementation of the RecordComparator interface, which is passed as a parameter to the enumerateRecords method of the RecordStore class.

Source code

Adding a new high-score

// We open the recordstore
RecordStore highscore = RecordStore.openRecordStore("High Score", true);
 
// To add a new HiScore we use a quick string comma-separated
String str = new String("John,3400");
byte [] strb = str.getBytes();
int id = highscore.addRecord(strb, 0, strb.length);
 
highscore.closeRecordStore();

Retrieving top high-scores

RecordStore highscore = RecordStore.openRecordStore("High Score", true);
 
// To read all hiscores saved on the RMS
RecordEnumeration enum = highscore.enumerateRecords(null, null, false);
int id;
byte[] record;
String str;
while (enum.hasNextElement( )) {
id = enum.nextRecordId( );
record = highscore.getRecord(id);
str = new String(record);
// Operate with str to extract name and points
}
highscore.closeRecordStore();

How to retrieve correctly sorted scores

To sort the scores from the highest to the lowest one, an option is to implement the RecordComparator interface, and to use it when calling the RecordStore enumerateRecords() method.

A basic implementation of a RecordComparator that sorts the scores stored is the following one:

class ScoresComparator implements RecordComparator
{
public int compare(byte[] arg0, byte[] arg1)
{
int score0 = 0, score1 = 0;
 
for(int i = 0; i < arg0.length; i++)
{
if(arg0[i] == ',')
{
score0 = Integer.parseInt(new String(arg0, i + 1, arg0.length - i - 1));
}
}
for(int i = 0; i < arg1.length; i++)
{
if(arg1[i] == ',')
{
score1 = Integer.parseInt(new String(arg1, i + 1, arg1.length - i - 1));
}
}
 
if(score0 < score1)
return RecordComparator.FOLLOWS;
else if(score0 == score1)
return RecordComparator.EQUIVALENT;
else
return RecordComparator.PRECEDES;
}
}

So, the previous call to enumerateRecords() will accordingly change:

RecordEnumeration enumerator = highscore.enumerateRecords(null, new ScoresComparator(), false);

Download

You can download a sample MIDlet showing the code presented in this article here: Media:HighScoreDatabaseMIDlet.zip

Implementation notes

  • You should use try-catch sentences to catch exceptions on the I/O operation or make a throws.
  • When writing and reading data to and from RMS, you should use separate threads to avoid blocking the MIDlet main thread.

Related resources

This page was last modified on 30 July 2013, at 05:59.
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