1. A user interface generation apparatus comprising: an application program execution unit, implemented by a processor, for implementing a variety of functions based on an application program; a user interface generation unit, implemented by the processor, for generating a user interface for instructing the application program execution unit to execute a predetermined function based on the application program; a memory for storing a user interface definition file including user interface object definition information for defining one or more input components of the user interface; and a selection screen generation unit, implemented by the processor, for generating a selection screen (i) for receiving a selection of one or more input components of at least a first user interface and (ii) for receiving a selection of one or more input components of a second user interface of a plurality of application programs based on the user interface object definition information, wherein each of the one or more input components of the first and second user interfaces corresponds to an user interface object that is displayed in the selection screen and available for selection, and wherein: the selection screen generation unit, if it is instructed to compound user interfaces of a plurality of application programs, generates the selection screen based on the user interface object definition information included in the user interface definition file stored in the memory corresponding to, at least, each of the first and second user interfaces of the plurality of application programs, the user interface generation unit generates a compound user interface definition file including user interface object definition information of the one or more input components that were selected from each of the first and second user interfaces of the plurality of application programs provided by the selection screen, and generates a compound user interface based on the compound user interface definition file, wherein the compound user interface comprises (i) the one or more input components selected from the first user interface and (ii) the one or more input components selected from the second user interface of the plurality of application programs selected in the selection screen, and a screen occupancy accumulation unit, implemented by the processor, that determines a total screen occupancy value by calculating a total of ratios of areas occupied by the user interface objects associated with each of the one or more input components selected in a predetermined user interface display area based on user interface object attribute information associated with each of the selected user interface objects, wherein a screen occupancy value of each of the selected user interface objects in the predetermined user interface display area is included in the user interface object attribute information in the user interface definition file for each user interface object, wherein the selection screen generation unit: (i) provides, during the selection of the interface objects, a visual display element representing the total screen occupancy value comprising a sum of the screen occupancy value of each of the selected user interface objects, (ii) updates the total screen occupancy value of the visual display element based on a selection of one of the user interface objects, (iii), if the total screen occupancy value of the compound user interface accumulated by the screen occupancy accumulation unit exceeds a predetermined value, displays the total screen occupancy value exceeding the predetermined value and cancels selection of the user interface object selected that caused the total screen occupancy value to exceed the predetermined value and informs that the user interface object cannot be selected.