Occasionally, the term internal API is used to indicate the opposite: classes and members that you do not allow your clients to view or use. It is the sum of everything that you (the developer) allow your clients to view and use. It is typically used to mean public API, which is synonymous with exported API. The term API is not a Java keyword, so at least we have no language-specific ambiguity to contend with. Note that in Java 9+ modules can be used to control access, so even public classes may not be exported and thus may not be public in the general sense. Effective Java uses the term exported to combine public and protected members. Since clients may view and use protected members as well, these may also be considered public in the general context of programming. In Java, the public keyword indicates a specific access level between classes and members. It also allows us to declare method signatures. Coming on to your question of attribute sharing, the variables declared in a interface are static and more. In general, the term public indicates any part of code that is visible to and used by clients outside of the development team. In general, both of these are abstractions and thus both are programming interfaces. Abstract class can be extended and only one abstract class can use for. In interface you can have only static final variable ( all the values declared in intrface is automatically constant and static I.e immutable variables ) but in abstract class you can have mutable variables. An interface is generally used to specify a contract by which the users of the interface sees how to use it whereas an abstract class may be partial implementation of an interface. In Java, the interface keyword indicates a specific type of stateless abstraction, which is contrasted with abstract class. Abstract class can have implementation in it but interface can't. If you would like some classes to have a default behaviour for a price, then use the abstract class design over the interface. Interfaces only allow you to define the signature of the methods, so each implementation will be different. An abstract class is nothing but a class that is declared using the abstract keyword. Interfaces and abstract class serve different purpose. Abstract classes allow you to define the default behaviour of certain methods which the subclasses can override. However, starting with Java 9, we can also add private methods in interfaces. In general, the term public indicates any part of code that is visible to and used by clients outside of the development team. Methods in an interface are implicitly abstract if they are not static or default and all are public. In Java, the interface keyword indicates a specific type of stateless abstraction, which is contrasted with abstract class. When you read, "Program to an interface" it means program to an abstraction. In general, the term interface is synonymous with abstraction. What the terms mean in the general context of programming is slightly different from what the specific keywords mean in Java. So your answer would surely be: "it's the choice of the developer !".Īnd it's really not an always valid answer.Ī Human can eat, but obviously the Robot cannot and even doesn't need.A big reason these terms are confusing is because they are keywords in Java and they have meaning in a general, language-agnostic context. You think."oh I know so: in this case, having one abstract class with an abstract method walk(), then is clearly the same than declaring an interface with the walk() method." So based on this case, he asks you: should I extract the walking feature in an abstract base class or in an interface, knowing that the implementations have nothing in common? I have a Robot that can walk and a Human that can walk too. In our codebase Ive seen both interface s and abstract class es being used for such BO definitions. That's the answer we give to Junior since it may be too complex to clearly figure out the difference between the essence of an abstract class and the essence of an interface without much architecture experience and without reading a lot of classic books about.Īny abstract class with public methods acts as a contract, as well as an interface.Īn abstract class that doesn't provide any implementation is in 99% of cases a representation of an object's Role.Įach object might have several different roles that shouldn't be tied together but composed by the concerned object. At my org we extensively use Value.Immutable annotation to generate Immutable classes (usually with builder s) for our internal business objects (BOs). The answer stating that an interface represents a contract is not acceptable.
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