In Java, you'll sometimes see people handing around instances of nearly (or entirely) empty classes with a single method, for use as callbacks. In languages wit

In Java, you'll sometimes see people handing around instances of nearly (or entirely) empty classes with a single method, for use as callbacks. In languages with first-class functions, you can just hand around function pointers directly, you don't need to dress them up with classes. In C, you'll sometimes see people make a struct containing only a collection of function pointers, and different parts of the code will produce instances of the struct with the function pointers populated differently. In languages with built-in polymorphism, this isn't necessary.