8/29/2023 0 Comments Polymorphism java declaration![]() If you inherit from an abstract class and you want to make objects of the new type, you must provide method definitions for all the abstract methods in the base class. This way, the compiler ensures the purity of the abstract class, and you don’t need to worry about misusing it. If an abstract class is incomplete, what is the compiler supposed to do when someone tries to make an object of that class? It cannot safely create an object of an abstract class, so you get an error message from the compiler. (Otherwise, the compiler gives you an error message.) If a class contains one or more abstract methods, the class itself must be qualified as abstract. Here is the syntax for an abstract method declaration:Ī class containing abstract methods is called an abstract class. This is a method that is incomplete it has only a declaration and no method body. Java provides a mechanism for doing this called the abstract method. It’s better to catch problems at compile time. This can be accomplished by making all the methods in Instrument print error messages, but that delays the information until run time and requires reliable exhaustive testing on the user’s part. That is, Instrument is meant to express only the interface, and not a particular implementation, so creating an Instrument object makes no sense, and you’ll probably want to prevent the user from doing it. If you have an abstract class like Instrument, objects of that class almost always have no meaning. ![]() (However, as seen in the last section, if the method’s name is the same as the base class but the arguments are different, you’ve got overloading, which probably isn’t what you want.) All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism. You create an abstract class when you want to manipulate a set of classes through this common interface. ![]() Another way of saying this is to call Instrument an abstract base class (or simply an abstract class). ![]() It establishes a basic form, so you can say what’s in common with all the derived classes. The only reason to establish this common interface is so it can be expressed differently for each different subtype. That’s because the intent of Instrument is to create a common interface for all the classes derived from it. If these methods are ever called, you’ve done something wrong. In all the instrument examples, the methods in the base class Instrument were always “dummy” methods. ![]()
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