An interface doesn’t provide method implementations. Before Java 8, if you had a class implementing an interface but not using.
Another welcome benefit of Java 8 Interfaces is the ability to add new concrete methods to an existing interface, which has already been implemented, without breaking the program. All the fields in the interface are constants. The most obvious benefit of using a Java 8 interface is its new concrete method capability. In other words, all the methods in the interface are abstract.
Java 8 and 9 introduced some new features in interfaces like default. An interface body has only public method signatures and initialized final fields. Before java 8 an interface can have only constant variables and abstract methods. It only contains the methods without implementations.įor example, a client using the application class, calling application code, etc. On the client side, a Slice interface maps to a Java interface with methods that correspond to the operations on that interface. An interface defines the class’s behavior and interactions. These methods define the object’s interface. For this reason, programs that contain many user-defined types will often define those types as interfaces wherever possible, and only use classes for types that must be directly instantiated.Java objects define the behavior and interactions with the outside world through the methods. This conceptual view of classes and interfaces highlights the fact that declaring something to be an instance of an interface is in many ways a less restrictive, more flexible, declaration than declaring it to be an instance of a class. The object stored in s could really be any kind of thing that acts in this way - it could be a fish, it could be a boat, it could be a human athlete, etc. Says that s merely acts like something that swims - perhaps by handling a swim message. For example, if Fish is a class, and Swimmer is an interface, then the declaration Fish f Ĭonceptually says that f is a fish, whereas the declaration Swimmer s On the other hand, if you only need to say that a variable or parameter stores objects that act in certain ways (i.e., by handling certain messages), without concern for what they really are, create an interface to use in the declarations. Thus, if you want to declare a variable or parameter as storing objects that are a certain type of thing, create a class to use in these declarations. If you want other classes to be derived from multiple types, then those types have to be interfaces (since a class can have at most one superclass, but can implement many interfaces).Īt a conceptual level, it may help you choose between making something a class versus making it an interface if you think of a class as defining what an object is, whereas an interface defines what an object acts like. If member variables are an important part of a type, then that type has to be a class (interfaces can't contain member variables). For example, if you want to instantiate a type, then that type has to be a class (since you can't instantiate an interface). Sometimes, the choice between class and interface will be dictated by the limitations of each. These similarities between classes and interfaces can make it hard to decide whether to define a type as a class or as an interface. Implementing an interface is in many ways analogous to subclassing a class. Both define a set of messages that certain objects will handle. There are close similarities between classes and interfaces. For example Counter c = new UnitCounter() Classes vs Interfaces However, you can assign them any instance of any class that implements the interface. The syntax for declaring an interface is interface name Īs mentioned earlier, the only thing you cannot do is use new with the interface name to create a value to assign to one of these variables or parameters. Certain advanced features of interfaces are not covered here, but can be found in a Java reference manual. This document describes the basics of declaring and using interfaces, and says a little bit about when to use interfaces and when to use classes. Because an interface defines no methods, it does exactly what the informal term "interface" implies: define the interface between certain objects and their clients, but without defining any of the objects' implementations. The big difference between interfaces and classes is that an interface is solely a specification of the messages that an object responds to, without any methods for handling those messages, whereas a class typically provides methods for handling the messages it declares. For instance, if ExampleInterface is the name of an interface, then you could declare a variable v as ExampleInterface v SUNY Geneseo, Department of Computer ScienceĪn interface in Java is similar to a class, in that, like a class, an interface is a type that can be used to declare variables and parameters.
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