Can interfaces have variables

More importantly, the methods of an interface are always considered public, and you can optionally declare them as so. Why public? Driveable , Runnable , and Updateable are good interface names. Any class that implements all the methods can then declare it implements the interface by using a special implements clause in its class definition.

For example:. Here, the class Automobile implements the methods of the Driveable interface and declares itself Driveable using an implements clause. As shown in Figure 6. The figure illustrates the Driveable interface being implemented by two different classes. After declaring the interface, we have a new type, Driveable. We can declare variables of type Driveable and assign them any instance of a Driveable object:. Both Automobile and Lawnmower implement Driveable , so they can be considered of that type.

Interfaces can be used to implement callbacks in Java. An object can, in effect, pass one of its methods to another object. The callback occurs when the other object subsequently invokes the method. Consider two classes: a TickerTape class that displays data and a TextSource class that provides an information feed. We could have TextSource store a reference to a TickerTape object, but then we could never use our TextSource to send data to any other kind of object.

A more elegant solution is to have TextSource store a reference to an interface type, TextUpdateable :. The only thing the TextSource really cares about is finding the right method to invoke in order to output some text. Why are interface variables static and final by default? Ask Question. Asked 11 years, 8 months ago. Active 1 year, 9 months ago. Viewed k times.

Why are interface variables static and final by default in Java? Jothi Jothi You shouldn't put any variables inside Interfaces. Because interfaces define contracts which can be implemented in various ways. The value of a variable is implementation. We certainly can when we know all the classes implementing the interface have some constant variables Field names for instance.

Is it a good idea to make a variable in a class an instance of the interface that the class implements? I have heard this before. Answer by Arun Raaj answered Apr 24 '18 at and comment by denis Aug 17 '17 at correctly identify Multiple Inheritance as the main issue Show 2 more comments. Active Oldest Votes. From the Java interface design FAQ by Philip Shaw: Interface variables are static because Java interfaces cannot be instantiated in their own right; the value of the variable must be assigned in a static context in which no instance exists.

Dave Jarvis Note that abstract classes cannot be instantiated "in their own right" neither and those can have instance variables. This explanation for the static modifier is completely spurious. A class's public instance variables are part of its interface and there's no reason why they shouldn't be abstracted in a Java interface , just like instance methods. It doesn't matter that a Java interface can't be instantiated directly - you can still have instances of classes that implement the interface and it's sensible to require that they have a certain public instance variable.

As for the part about final , that doesn't offer an explanation at all - it just describes what final means. The quote above is better in context. The reason it gives is that "interface variables are intended to be Java constants".

The quote was just elaborating on why such a constant would be static and final. That's true, but the real question is: why aren't variables allowed as part of the actual interface ie. If they wanted special "interface constants", they could have used new syntax, or just decided that any variables actually defined in an interface are interface constants.

Interfaces cannot have instance variables in order to avoid multiple inheritance of state problems. See docs. A class cannot extend more than one class due to the same reason. How is default methods introduced and they do have instance, yet instance variable is not supported Show 3 more comments. Nutan Nutan 9 9 silver badges 13 13 bronze badges. Add a comment. Kasun Siyambalapitiya 3, 6 6 gold badges 30 30 silver badges 53 53 bronze badges. Of course an instance variable would be accessible if it was allowed in a Java interface.

A class would implement the interface, declaring the instance variable as required by the interface. Its constructor or other method sets the instance variable. When an instance of the class is instantiated, you will be able to access its instance variable.

Java allows static methods with bodies to exist in an interface. Those could access the static variables. They just cannot change them, which means the static functions cannot store any data — simpleuser. Malvon Malvon 1, 2 2 gold badges 17 17 silver badges 37 37 bronze badges. You list the names of the interfaces to implement after the implements keyword, separated by a comma. If the interfaces are not located in the same packages as the implementing class, you will also need to import the interfaces.

Java interfaces are imported using the import instruction just like Java classes. For instance:. As you can see, each interface contains one method. These methods are implemented by the class MyInterfaceImpl. Since a Java class can only implement at method with a given signature once, this could potentially lead to some problems. The Java specification does not give any solution to this problem.

It is up to you to decide what to do in that situation. A Java interface can contain constants. In some cases it can make sense to define constants in an interface.

Especially if those constants are to be used by the classes implementing the interface, e. However, my advice to you is to avoid placing variables in Java interfaces if you can.

All variables in an interface are implicitly public , static and final , even if you leave out these keywords in the variable declaration. A Java interface can contain one or more method declarations. As mentioned earlier, the interface cannot specify any implementation for these methods. It is up to the classes implementing the interface to specify an implementation.

All methods in an interface are public, even if you leave out the public keyword in the method declaration. Before Java 8 Java interfaces could not contain an implementation of the methods, but only contain the method signatures. However, this results in some problems when an API needs to add a method to one of its interfaces.

If the API just adds the method to the desired interface, all classes that implements the interface must implement that new method. That is fine if all implementing classes are located within the API. But if some implementing classes are located in client code of the API the code that uses the API , then that code breaks. Let me illustrate this with an example.

Look at this interface and imagine that it is part of e. Now imagine that a project uses this API and has implemented the ResourceLoader interface like this:. If the developer of the API wants to add one more method to the ResourceLoader interface, then the FileLoader class will be broken when that project upgrades to the new version of the API.

To alleviate this Java interface evolution problem, Java 8 has added the concept of interface default methods to Java interfaces. An interface default method can contain a default implementation of that method. Classes that implement the interface but which contain no implementation for the default interface will then automatically get the default method implementation. You mark a method in an interface as a default method using the default keyword.

Here is an example of adding a default method to the ResourceLoader interface:. This example adds the default method load Path. The example leaves out the actual implementation inside the method body because this is not really interesting. What matters is how you declare the interface default method. A class can override the implementation of a default method simply by implementing that method explicitly, as is done normally when implementing a Java interface.

Any implementation in a class takes precedence over interface default method implementations. A Java interface can have static methods. Static methods in a Java interface must have implementation. Here is an example of a static method in a Java interface:. Calling a static method in an interface looks and works just like calling a static method in a class. Here is an example of calling the static print method from the above MyInterface interface:.

Static methods in interfaces can be useful when you have some utility methods you would like to make available, which fit naturally into an interface related to the same responsibility. For instance, a Vehicle interface could have a printVehicle Vehicle v static method.

It is possible for a Java interface to inherit from another Java interface, just like classes can inherit from other classes. You specify inheritance using the extends keyword. Here is a simple interface inheritance example:. Unlike classes, interfaces can actually inherit from multiple superinterfaces. You specify that by listing the names of all interfaces to inherit from, separated by comma.

A class implementing an interface which inherits from multiple interfaces must implement all methods from the interface and its superinterfaces.