J2ME Programming/Connected Limited Device Configuration

Introduction

Many people confuse the role of CLDC in J2ME application development. It alone is not a framework for application development because you need more than a configuration to produce J2ME applications("J2ME". The J2ME Platform.). You can think of CLDC as the spec that helps define the subset of the J2ME language items such as input/output, math, and etc that the Java Virtual Machine uses as its set of rules in actually runing MIDlets under the Mobile Information Device Profile(MIDP).

Package Structure

We can get a clearer picture of this by observing what each package in the CLDC defines as the J2ME java subset. Although its not clear from the javadocs, Sun Microsystems does not separate MIDP and CLDC javadocs, these are the actual packages of CLDC("cldcapi" (PDF). CLDC API Documentation, V1.0.):

CLDC1.0 list of Packages under java.*

• java.lang.*
• java.io.*
• java.util.*

Thus, we have classes that deinfe the java language subset attributes used. How the java language subset definitions for input/output are used. Also, a language definition of the subset of java.util.* from the full java language.

Math Classes(CLDC1.0) in java.lang.* package from J2SE1.3

• java.lang.Long
• java.lang.Short
• java.lang.Integer

Notice when compared to J2SE math we have no floating point(Float class) and no double(Double class).

Utility Classes(CLDC1.0) from J2SE1.3

• java.lang.Math
• java.util.Random

System Classes(CLDC1.0) in java.lang.* package from J2SE1.3

• java.lang.System
• java.lang.Object
• java.lang.Runtime
• java.lang.Thread
• java.lang.Throwable
• java.lang.String
• java.lang.StringBuffer

Data Type Classes(CLDC1.0) in java.lang.* package from J2SE1.3 excluding math

• java.lang.Boolean
• java.lang.Byte
• java.lang.Character

Collection Classes(CLDC1.0) in java.util.* package from J2SE1.3

• java.util.Enumeration
• java.util.Hashtable(see note in conclusion, from jdk1.1.8 not jse1.3)
• java.util.Stack
• java.util.Vector(see note in conclusion, from jdk1.1.8 not jse1.3)

Calendar and Time Classes(CLDC1.0) in java.util.* package from J2SE1.3

• java.util.Calendar
• java.util.Date
• java.util.TimeZone

IO Classes(CLDC1.1) excluding GCF in java.io.* package from J2SE1.3

• java.io.Writer
• java.io.Reader
• java.io.ByteArrayInputStream
• java.io.ByteArrayOutputStream
• java.io.DataInputStream
• java.io.DataOutputStream
• java.io.InputStream
• java.io.OutputStream
• java.io.InputStreamWriter
• java.io.OuputStreamWriter
• java.io.PrintStream

Exception Classes(CLDC1.0) in java.util.* package from J2SE1.3

• java.util.EmptyStackException
• java.util.NoSuchElementException

Exception Classes(CLDC1.0) in java.io.* package from J2SE1.3

• java.io.EOFException
• java.io.IOException
• java.io.InterruptedException
• java.io.UnsupportedEncodingException
• java.io.UTFDataFormatException

Error Classes(CLDC1.0) in java.lang.* package from J2SE1.3

• java.lang.Error
• java.lang.VirtualMachineError
• java.lang.OutOfMemoryError

The other clases are mixed in with MIDP in the javadocs, thus I will list the full list of CLDC classes under the javax.microedition.io.* package. The class is the capitalized name at the right sight of the dot ending the package.

CLDC1.0 list of classes under javax.microedition.io.*

• javax.microedition.io.InputConnection
• javax.microedition.io.Connection
• javax.microedition.io.StreamConnection
• javax.microedition.io.OutputConnection
• javax.microedition.io.Datagram
• javax.microedition.io.DatagramConnection
• javax.microedition.io.StreamConnectionNotifier
• javax.microedition.io.ContentConnection
• javax.microedition.io.ConnectionNotFoundException
• javax.microedition.io.Connector

The common aspect that sets these classes apart is that they are interfaces defining the capabilities of the input/output connections. The classes in java.io.* define the java language attributes that input/output classes in javax.microedition.io.* use as their rule set.

CLDC1.1 additional classes

• java.lang.Float
• java.lang.Double
• java.lang.Math

GCF, General Connection Framework

In the J2ME platform the J2SE IO is replaced with a smaller General Connection Framework(GCF).

javax.microedition.io GCF Classes from CLDC1.0

• javax.microedition.io.InputConnection
• javax.microedition.io.Connection
• javax.microedition.io.StreamConnection
• javax.microedition.io.OutputConnection
• javax.microedition.io.DatagramConnection
• javax.microedition.io.StreamConnectionNotifier
• javax.microedition.io.ContentConnection
• javax.microedition.io.HttpConnection

Conclusion

In conclusion, under the subset of java language for CLDC we do not have floating point math, no double, no class loaders, no sockets and no serialization. A smaller IO framework known as GCF is used.

While the javadocs read MID Profile in top right corner the actual Configuration or Profile they belong to is noted down before the class method summary is listed as:

since CLDC1.0 or since MIDP1.0

Note, that not all classes are based on JDK1.3

Classes(CLDC1.0) Based on JDK1.1.8 instead of JDK1.3

• java.util.Vector
• java.util.Hashtable

These classes were based on JDK1.1.8 to conserve space.

CLDC1.0 Limitations

• Unicode Translation to and from reader/writer bytes is limited
• Does not support java.util.Properties from J2SE but limited stuff can be acquire as System.getProperty(String key) that starts with the keyword microedition

References

• "cldcapi" (PDF). CLDC API Documentation, V1.0.
• "KVM White paper" (PDF). J2ME Building Blocks for Mobile Devices White Paper on KVM and the Connected Limited Device Configuration (CLDC).

Trademark Notices

J2ME, Java and all Java-based marks are trademarks or registered trademarks of Sun Microsystems, Inc. in the U.S. and other countries.