The present invention for reducing memory usage of an object oriented program. The object oriented program comprises class definitions used to create objects, each class definition providing at least one function that can be run in respect of objects created from that class definition, and identifying a number of storage items to be provided in memory for each object to store data values associated with that object. The method involves determining an original size associated with each storage item, and analysing the class definition to identify any numeric ranges associated with the storage items. For each storage item for which a numeric range is identified, it is determined if the original size can be reduced whilst still supporting the numeric range, and if so a modified size less than the original size is allocated to that storage item. Thereafter, the class definition is altered having regard to any storage element that has been allocated a modified size.