Performance Tips

• Use ArrayLists, HashMap etc as opposed to Vector, Hashtable etc where possible. This is because the methods in ArrayList, HashMap etc are not synchronized.Even better is to use just arrays where possible.
• Set the initial capacity of a collection (e.g. ArrayList, HashMap etc) and StringBuffer/StringBuilder appropriately. This is because these classes must grow periodically to accommodate new elements. So, if you have a very large ArrayList or a StringBuffer, and you know the size in advance then you can speed things up by setting the initial size appropriately.
• Minimise the use of casting or runtime type checking like instanceof in frequently executed methods or in loops. The “casting” and “instanceof” checks for a class marked as final will be faster. Using “instanceof” construct is not only ugly but also unmaintainable. Look at using visitor pattern.
• Do not compute constants inside a large loop. Compute them outside the loop. For applets compute it in the init() method.
• Exception creation can be expensive because it has to create the full stack trace. The stack trace is obviously useful if you are planning to log or display the exception to the user. But if you are using your exception to just control the flow, which is not recommended, then throw an exception, which is precreated. An efficient way to do this is to declare a public static final Exception in your exception class itself.
• Avoid using System.out.println and use logging frameworks like Log4J etc, which uses I/O buffers
• Minimize calls to Date, Calendar, etc related classes.
• Minimize JNI calls in your code.