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The underlying theme of this course is that Java is a highly portable programming language and is able to run on a very wide range of platforms.
This means that Java software may be required to run on very small scale systems with few resources, such as very limited storage or very small displays, as well as on large networks of powerful servers with very extensive hardware resources. To cater for these very different environments, we have the concept of Java editions. So far, we have mostly been concerned with the Java 2 Standard Edition (J2SE). In this unit, we will look at Java 2 Micro Edition (J2ME), designed for use on smaller systems with limited resources, especially the concept of MIDlets, applications which run on so-called Mobile Information Devices. Large scale systems are catered for by the Java 2 Enterprise Edition (J2EE), which we will consider only briefly in this unit. Fortunately, many of the concepts we have met in the previous units apply to all Java editions.
On networked systems, including clients and servers, there is the additional question of where the Java software is located and where it is executed. In this unit, we will see that there are many possible answers to this – in a complex system, Java software may run on the client, on the server (possibly as a servlet), as an applet invoked from a web page, or even as a software agent (an aglet) roaming around the network. Typically, a system will use an appropriate combination of such Java software components.
In this unit, we aim to:
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show how to write applets and when to use them;
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demonstrate and apply the J2ME concept of MIDlets, to produce software for small scale devices;
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discuss the idea of servlets, running on a web server;
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briefly explain software agents, or aglets;
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show how Java is used in a variety of embedded devices, such as smart cards or printers.
First, we look at applets, which are Java programs invoked from a web page. |
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