Mechanisms for instantiating an interface or abstract class in application code are provided. An object-oriented programming language is extended such that interfaces and abstract classes can be instantiated in application code without a concrete class specified. Metadata is defined which maps each interface or abstract class instantiation in the application code to a concrete class to be used either by the compiler at build time, or the virtual machine at runtime. Once the appropriate concrete class is determined from the metadata, the class is instantiated with a conventional class loader. The metadata may be provided, for example, as a separate file, such as a markup language file, defined with a virtual machine switch, as annotations in the application code, or the like.