What is the purpose of an interface in java

what is the purpose of an interface in java

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Jul 04, An interface in Java is a specification of method prototypes. Whenever you need to guide the programmer or, make a contract specifying how the methods and fields of a type should be you can define an interface. To create an object of this type you need to implement this interface, provide body for all the abstract methods of the interface and obtain the object of the implementing class. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.

Join Stack Overflow to learn, share knowledge, and build your career. Connect and share knowledge within a single location that is structured and easy to search. OK so I gather that Interfaces are a way to enforce that an object implements a certain amount of functionality, without having to use inheritance.

Kind of like a contract. And I semi see the point of them. If you are creating a number of classes all implementing such features and the implementation is only slightly different, this is going to be a lot of hard work.

Say you create a class Animal. And all animals, including humans extend that. And each of those animals inherits common methods like eat, breathe, etc. But now let's say you have a MathProblem class. And you want to have certain classes that can solve that problem by passing the problem to a solve MathProblem problem method. And you know that a Humanbut also a Computer might solve the math problem.

So they both need how to build a drafting table be able to solve that problem. You might be able to get the Computer to extend some MathSolver class that has the method, but Human already extends Animal, and can't extends anything else. So a better way is to make MathSolver an interface and have both HumanComputerand any other classes that need to solve problems implement that. Also note that a Human and a Computer might solve the problems in completely different ways, since their such different objects.

That's what interfaces are best for. Defining certain abilities that cut across multiple inheritance hierarchies, and can have very different implementations, but can all be passed to a method that accepts any of them. Think of the Comparable interface; it's not something a certain class of objects has, all sort of things can be compared, and usually in very different ways. But you can always call sort on a List of Comparable objects since you know they have a certain order, no matter if they're NumbersAnimalsComputers or anything else as long how to make a spinner for fishing they implement Comparable and define their ordering.

Prefer Composition over Inheritance. This way, you can implement say eat in a class that gets incorporated into all your animals as a data member. Write it once, reuse it, but in a way that doesn't bind one kind of functionality explicitly to another. You are confusing interfaces and inheritance. They are different concepts and can complement to each other. If all the eat methods are only slightly different, then you can create a base class which will contain the common code and will be invoked from the subclasses through overriden methods which add the different parts.

The base class what is a bilateral mastectomy still implement the interface. Hope it is clear. Right, you need to implement it ever time but you can implement it differently every time and any class that calls it doesn't need to worry about how it's implemented. For example, if you have a Zoo object with a bunch of animals new TigerLionBear then your zoo can do for each Animal a in some collection a.

The zoo doesn't care that there are three different types of animals that eat in totally different ways. In that case you are easily allowed to create another layer in you hierarchy of classes which implements Animal but is how to get everything in focus dslr ancestor class for all animals that eat in some way, for example. You should look forward code reuse and encapsulation of components at the same time.

You should think of an interface as an authoritative declaration of behaviourwhich has nothing to do with implementation issues in the first place.

If you want to avoid code duplication, then you use an abstract base class in combination with the interface. Here you then can implement all the stuff that might be repeated in all interface-implementing classes otherwise. Using interfaces is more about giving the consuming code a way to know what you expect from it, rather than you needing to be concerned about the details of the consuming code.

What happens if the DAL wants what is the purpose of an interface in java use objects rather than individual fields? You would have to define your own DAL objects, and hydrate them with the input you've just received. Basically, a lot more work, more resources consumed, and multiple objects that represent the same data which makes for a maintenance nightmare.

Then you can implement those interfaces in the BL and pass instances of the interfaces instead of BL objects. Interfaces are all about abstracting out the details of the implementation where they're not absolutely necessary.

And old thread, I know. But I just read "Interfaces are how to make changes in voter id only way to create multiple inheritance in Java". This is very wrong, because delegation or "compositon" as Carl said is the only way to get multiple inheritance remember: "delegation is inheritance", well, almost.

You only need interfaces to tell the developer "hey, don't forget to delegate this or that class"! Interfaces are only needed as reminder for a correct delegation or in general: implementationbut they can't inherite any code. With multiple inheritance interfaces wouldn't be needed at all. Actually, you don't really need interfaces to create a working program, they're just helpers without any function or functional code.

Btw Thomas was very right with the abstract classes, these are far more important than interfaces, because that's where you can get reusable code from. Normally, when I write a java application I only create interfaces in the very end, as a helper for future programmers. Or I don't create any interfaces at all ;D.

One major reason is that you can create an object using an interface reference, similar to an abstract method. When you do this, every object which implements the interface can be assigned to it.

For example, if Dog and Car both implement Washable, then you can do:. This also means that you can accept an interface as a parameter for a method meaning you don't have to add unecessary code to deal with every class which implements a certain interface. The generalization means here sub-classes having same behavior implemented in a different way. Interface allows to set standardization for all the sub-classes which implements it. It specifies "what" the sub-classes must have but doesn't enforce how it should have.

This isn't possible if we use abstract classes. While developing an application, the code which interacts with end users can be loosely coupled to the code running on the server[B L C] by using interfaces.

Each implementation of the interface can be different. The point is that you can use interface without knowing the implementation. What record label is paramore signed to example:. But it doesn't matter how logger is implemented - it can be FileSystemLoggeror DatabaseLoggeror any other implementation. So you will be able to substitute implementations at any time without changing all places in code where logging was mentioned.

You are thinking about it backwards. Instead of thinking about the implementation first, you think about behavior as described by the method signature first. Then you implement the behavior as appropriate in the base classes giving a much more flexible, extensible system.

You dismissed "design by contract' pretty quickly, but it's a key design strategy and the basis for web services, SOA, etc. The user can implement this however he or she wants. Or it can be based on age, etc. However the user wants. Implementing this interface is very useful, in that it allows the user to use things like Collections.

Stack Overflow for Teams Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. The purpose of interfaces continued Ask Question. Asked 10 years, 6 months ago. Active 4 years, 11 months ago. Viewed 15k times.

Any help to get my head around this is appreciated because I know it's really important. Improve this question. John Topley k 45 45 gold badges silver badges bronze badges. Julio Julio 1, 7 7 gold badges 19 19 silver badges 22 22 bronze badges. Sorry about my awful 'typos' Carl just clicked the button without really reading it thoroughly. Lot on my mind it seems.

Cringe-worthy stuff when you look at the mistakes. Add a comment. Active Oldest Votes. Interfaces are the only way to create multiple inheritance in Java. Improve this answer. Andrei Fierbinteanu Andrei Fierbinteanu 7, 3 3 gold badges 28 28 silver badges 44 44 bronze badges. Interfaces don't do anything useful toward the second purpose, but are very helpful toward the first. If you had two or ten different ways of eating, you could swap them out as needed. Carl Manaster Carl Manaster Although composition seems the best way to avoid code duplication in this particular case, this answer tends to make think that interfaces are useless in Java.

Don't forget that there's also another design what is financial hardship for 401k withdrawal saying "program to interfaces, not concrete classes". So interfaces in Java are definitely something to understand and use almost everywhere.

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1) To achieve security - hide certain details and only show the important details of an object (interface). 2) Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma (see example below). The interface in Java is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java. In other words, you can say that interfaces can have abstract methods and variables. Interface allows to set standardization for all the sub-classes which implements it. It specifies "what" the sub-classes must have but doesn't enforce how it should have. % Abstraction. Interface body provides % abstraction, so that the sub-class should not miss any implementation of abstract method.

An interface in Java is a blueprint of a class. It has static constants and abstract methods. The interface in Java is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java. In other words, you can say that interfaces can have abstract methods and variables. It cannot have a method body.

An interface is declared by using the interface keyword. It provides total abstraction; means all the methods in an interface are declared with the empty body, and all the fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.

Since Java 8 , interface can have default and static methods which is discussed later. In other words, Interface fields are public, static and final by default, and the methods are public and abstract. As shown in the figure given below, a class extends another class, an interface extends another interface, but a class implements an interface. In this example, the Printable interface has only one method, and its implementation is provided in the A6 class. In this example, the Drawable interface has only one method.

Its implementation is provided by Rectangle and Circle classes. In a real scenario, an interface is defined by someone else, but its implementation is provided by different implementation providers.

Moreover, it is used by someone else. The implementation part is hidden by the user who uses the interface. If a class implements multiple interfaces, or an interface extends multiple interfaces, it is known as multiple inheritance. As we have explained in the inheritance chapter, multiple inheritance is not supported in the case of class because of ambiguity. However, it is supported in case of an interface because there is no ambiguity.

It is because its implementation is provided by the implementation class. For example:. As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class TestTnterface1, so there is no ambiguity. Since Java 8, we can have method body in interface. But we need to make it default method. Let's see an example:.

An interface which has no member is known as a marker or tagged interface, for example, Serializable , Cloneable, Remote, etc. Note: An interface can have another interface which is known as a nested interface. We will learn it in detail in the nested classes chapter. JavaTpoint offers too many high quality services. Mail us on hr javatpoint. Please mail your requirement at hr javatpoint. Duration: 1 week to 2 week.

Java Training Basics of Java. Abstract class Interface Abstract vs Interface. Package Access Modifiers Encapsulation. Interface Example of Interface Multiple inheritance by Interface Why multiple inheritance is supported in Interface while it is not supported in case of class. Marker Interface Nested Interface. Next Topic Difference between Abstract class and interface. Manual T. Verbal A. Angular 7. Compiler D. Software E.

Web Tech. Cyber Sec. Control S. Data Mining. Javatpoint Services JavaTpoint offers too many high quality services. Java Interface also represents the IS-A relationship. It cannot be instantiated just like the abstract class. Since Java 8, we can have default and static methods in an interface. Since Java 9, we can have private methods in an interface. Why use Java interface?

There are mainly three reasons to use interface. They are given below. It is used to achieve abstraction. By interface, we can support the functionality of multiple inheritance. It can be used to achieve loose coupling. How to declare an interface? Internal addition by the compiler The Java compiler adds public and abstract keywords before the interface method. Moreover, it adds public, static and final keywords before data members. The relationship between classes and interfaces As shown in the figure given below, a class extends another class, an interface extends another interface, but a class implements an interface.

Java Interface Example In this example, the Printable interface has only one method, and its implementation is provided in the A6 class.

File: TestInterface1. File: TestInterface2. Interface inheritance A class implements an interface, but one interface extends another interface. Let's see an example: File: TestInterfaceDefault. Let's see an example: File: TestInterfaceStatic.

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