Introduce several important concepts of J2ME

There are several important concepts in the conceptual J2ME platform, such as memory, CLDC, MIDP, and so on. Beginner J2ME often don't understand these concepts deeply, and even have deviations. The purpose of this paper is to expound the relevant important concepts in J2ME.

1. Memory
We have been emphasizing that the Mobile information device memory is very small, use should be doubly cherished, but we rarely know how these memory is classified, the following will do a detailed introduction. In fact, MIDP device memory is divided into three kinds, programme Memory, Heap, persistent Storage.
Programme memory is the space for mobile information devices to be assigned to the MIDlet suite, because the MIDlet suite is published as a jar file, so the size of the file can be considered programme memory. Some vendors have limits on the maximum value of programme memory, such as my Nokia 6108 maximum of 64k, which will not be installed. Reducing the size of the MIDlet suite is important, and an easy way is to confuse the application with obfuscation, which reduces the size of the jar file. In a future article I'll talk about how to use Proguard.
Heap is the storage space in which the application stores the objects created during the run, and local and member variables are placed on the heap, and the heap space provided in the MIDP device is approximately dozens of k to hundreds of K.
Persistent storage space is used to implement the local data persistence store for MIDP applications, and I have made a detailed introduction to the record Management system from getting started to mastering. Here is no more to say.

2. Connected Limited Device Configuration

CLDC includes a Java Virtual machine and a series of basic classes, J2ME's team of experts after the hardware abstraction of Mobile information devices to get their characteristics, and then design and implement a Mobile information device running Java Virtual machine, usually we call it KVM. The CLDC1.0 also provides a base class consisting of Java.io, Java.lang, Javax.microediton.io, and Java.util. Java.lang.ref was added to the CLDC1.1.

3. Mobile Infomation Device Profile

MIDP is run on the basis of CLDC, in MIDP defined the application lifecycle, user graphical interface, data management system and so on subset, thus constructs the J2ME platform. Typically, the J2ME platform consists of a CLDC and one or more profiles.