A method of loading an object-oriented computer program comprises translating the program into compiled code and holding that code, ready for execution, in an execution environment which does not necessarily also hold the original program code. In a preferred embodiment, the original class file is discarded from memory prior to execution of the program. The invention is particularly applicable to wireless communications networks consisting of a plurality of client devices, such as mobile phones, each of which typically has only a small amount of memory available. Using the present invention, the original class file is discarded after compilation, and does not need to be retained in the client device memories.