Few tips that will speed-up your java code
Finalizers: object that overwrites the finalize() method (“Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.”) is slower!(for both allocation and collection) if it’s not a must, do clean ups in other ways ( e.g. in JDBC close the connection using the try-catch-finally block instead)
New is expensive: creating new heavy object() on the heap is expensive!, it’s recommended to recycle old objects (by changing their fields) or use the flyweight design pattern.
Strings : (1) Strings are immutable which mean that upon usage of the + operator between 2 strings the JVM will generate a new String(s1+s2) on the heap (expensive as I just mentioned), in order to avoid that, it’s recommended to use the StringBuffer.(Update) since JDK 1.5 was introduced StringBuilder is a better option than Stringbuffer in a single-threaded environment.
(2) Don’t convert your strings to lower case in order to compare them, use String.equalIgnoreCase() instead.
(3) String.startswith() is more expensive than String.charat(0) for the first character.
Inline method: inline is a compiler feature, when you call a method from anywhere in your code, the compiler copies the content of the inline method and replace the line that calls the method with it.
Obviously,It saves runtime time: (1) there is no need to call a method (2) no dynamic dispatch.
In some languages you can annotate a method to be inline, yet in Java it’s impossible, it’s the compiler decision.
the compiler will consider inline a method if the method is private.
My recommendation is to search in your code for methods that are heavily used(mostly in loops) and annotate those method as private if possible.
Don’t invent the wheel: the java api is smart and sophisticated and in some cases use native implementation, code that you probably can’t compete with. unless you know what you are doing (performance wise) don’t rewrite methods that already exists in the java API. e.g. benchmarks showed that coping and array using a for loop is at least n/4 times slower than using System.arraycopy()
Reflection: reflection became much faster for those of you who use the most recent JVMs, yet using reflection is most certainly slower than not using it. which mean that you better avoid reflection if there is no need.
Synchronization: Some data structures auto-support concurrency, in case of a single thread application don’t use those to avoid overhead e.g. use ArrayList instead of a Vector
Multithreads: in case of a multi processor use threads, it will defiantly speed up your code, if you are not a thread expert some compilers know how to restructure your code to thread automatically for you. you can always read a java threads tutorial as well
Get familiar with the standard data structures: e.g. if you need a dast for puting and retriving objects use HashMap and not an ArrayList. (O(1))
Add an id field, for faster equals(): object that contains many fields are hard to compare ( equals() wise), to avoid that add an id(unique) field to your object and overwrite the equals() method to compare ids only.
Be careful, In case your code already works, optimizing it is a sure way to create new bugs and make your code less maintainable!
it’s highly recommended to time your method before and after an optimization.