We describe a framework that is used to build and test software for ubiquitous and mobile computing. The approach involves software-level emulators for computing devices. Since each emulator is implemented as a mobile agent, it can dynamically carry its target software to each of the sub-networks that its device is connected to, on behalf of the device, and it permits the software to interact with other servers in its current sub-network. The framework can simulate the mobility and reconnection of mobile or ubiquitous computing devices by using the logical mobility of mobile agent-based emulators. That is, it can test software designed to run on a mobile or ubiquitous device in the same way as if the software were disconnected from the network, moved with the device, and reconnected to and executed on another network. This paper describes the lessons we learned from exploiting the framework in developing typical application software for mobile and ubiquitous computing devices.