Java Performance Tuning

Caching Example I

When accessing elements from sets of data, it is often the case that some elements are accessed much more frequently than others. In these cases, it is possible to apply caching techniques to speed up access to these frequently accessed elements. This is best demonstrated with the following example.

Consider a CacheTest class that consists mainly of a Map populated with Integer objects. I use Integer objects for convenience to populate the Map with many elements, but the actual object type is of no significance since you use only the hashCode( ) and equals( ) methods, just as the Map does.

Basically, you provide two ways to access the elements of the Map. The first, plain_access(), just calls the Map.get( ) method as normal. The second method, cached_access( ), uses the lower bits of the hash code of the object to obtain an index value into an array. This index is then checked to see whether the object is there. If it is, the corresponding value in a parallel value array is returned. If it’s not, the object is placed there with the value in the corresponding value array.

This is about the simplest example of general cached access. It demonstrates the advantages and pitfalls of cached access. I have selected 10 integers that do not map to the same indexes for the example. Running the class gives a straightforward comparison between the two access methods, and I get the result that the cached access varies significantly depending on the VM used. The access speedups ...