GENERAL GUIDELINES FOR OPTIMIZATION in Java

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The table shows how rapidly Java performance has improved, through faster clock rates and improved VM technology. Since the Nokia 9210, the embedded tests have improved by well over a factor of 10, and the graphics tests by a factor of ve. The Nokia 6600 onwards use Sun s CLDC HI VM. The Wireless Toolkit results are intriguing: a very good graphics performance but a very poor embedded performance. Benchmarks should always be viewed with caution: the only real test is running representative applications on representative hardware.
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7.4 General Guidelines for Optimization
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This section outlines some general principles for optimizing code. These do not attempt to say anything new; however, restating the obvious is not always a bad thing. get the design right The biggest gains generally come from getting the overall architecture and design right: how operations should be split between server and client, what technologies to use (e.g. messaging, RMI, object database or relational database), what hardware to use, even what languages are used. It is important to design to interfaces, not implementations. This makes it easier to slot in a different or improved algorithm: for example, depending on your data size and how it might already be sorted, there are times when a pigeon sorting algorithm will be the best choice, and times when a bubble sort will be appropriate. optimize late: optimizing too early in the process means that you will produce intricate code that gets in the way of good design optimize only where necessary: nd out where the bottlenecks are and concentrate on sorting them out; this requires access to suitable pro ling and heap analysis tools do not over-optimize. The more you optimize your code, the more highly tuned it becomes to the particular environment. If the environment changes or you want to use the code in a different application, it may run more slowly. Compiler technology in particular can have a profound effect on the bene ts or otherwise of a particular optimization. Optimization can often con ict with other goals for the code: clarity and maintainability: improving performance at the code level generally (though not always) means writing more, and often quite obscure, code (we shall see an example of this in the case study in Section 7.14)
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WRITING OPTIMIZED CODE
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reliability: the corollary of the previous point is that you run the risk of introducing bugs when you optimize fast startup time and fast execution We can frequently improve startup time by deferring a task until it is required during execution. This is worthwhile if the task may not always be required, and even then may still be worthwhile, especially if the task can be carried out by a background thread. reducing memory usage: many of the optimizations require extra code; caching is a vital tool in improving performance, but requires extra memory. Finally, the behavior of an optimization will vary with the platform. As a Java developer for Symbian OS phones you are likely to be working with three platforms: Java under Windows, the Emulator and a target device. The rst two platforms may give a rough idea of the bene ts or otherwise of an optimization; however, they cannot be used for a reliable analysis. The performance of the Emulator in particular is very different to that of target hardware, for reasons we shall discuss.
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7.5 Feedback and Responsiveness
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Performance is in the eye of the beholder, so as well as being fast as measured by a stopwatch, our application also needs to be responsive to the user and to provide feedback. The user should never be confronted with an unresponsive screen that shows no indication that something is happening. Large applications, in particular, can take a long time to initialize. Rather than leave the user with a blank screen, pop up a splash screen. Unlike on desktop computers, there is generally no wait icon on mobile phones. Therefore it is necessary to have a status area, animated icon or some other way of conveying progress to the user. Threads are an expensive resource and should therefore be used judiciously; this is why native Symbian OS applications tend to be singlethreaded and to rely on cooperative multitasking. You might, however, want to consider loading or saving data in a separate thread, which allows the user to carry on with other work. Windows applications often lock the user out while a le is being saved; this is frustrating and unnecessary. While saving a le, the user should still be able to read it or edit another le of the same type.
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