If even one method is declared as abstract then the class must be abstract and cannot be directly instantiated. There may be a lot of other functionality in the class, but declaring a method abstract means "this behavior must be specified by a subclass". To instantiate, you must at the very least implement the methods that are declared as abstract in a subclass and thus provide that "missing" behavior of the method. Abstract classes let you code a lot of common functionality into a base class and leave only the specific differences in implementation to be supplied.