Java can interfaces have static variables

An Interface is contract between two parties that is invariant, carved in the stone, hence final. See Design by Contract. In Java , interface doesn't allow you to declare any instance variables. Using a variable declared in an interface as an instance variable will return a compile time error.

You can declare a constant variable, using static final which is different from an instance variable. Interface can be implemented by any classes and what if that value got changed by one of there implementing class then there will be mislead for other implementing classes.

Interface is basically a reference to combine two corelated but different entity. Think of a web application where you have interface defined and other classes implement it. As you cannot create an instance of interface to access the variables you need to have a static keyword. Since its static any change in the value will reflect to other instances which has implemented it. So in order to prevent it we define them as final. Just tried in Eclipse, the variable in interface is default to be final, so you can't change it.

Compared with parent class, the variables are definitely changeable. From my point, variable in class is an attribute which will be inherited by children, and children can change it according to their actual need. On the contrary, interface only define behavior, not attribute. The only reason to put in variables in interface is to use them as consts which related to that interface. Though, this is not a good practice according to following excerpt:.

As well, the constants used by a class are typically an implementation detail, but placing them in an interface promotes them to the public API of the class. How are we doing?

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Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. 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? Improve this question. Jothi Jothi In an interface, variables are treated as static and final by default. But, for readability purpose in the project, we write complete declaration e.

Learn Interface variables in Java with example and Uses. Static Methods in Interface are those methods, which are defined in the interface with the keyword static. Unlike other methods in Interface, these static methods contain the complete definition of the function and since the definition is complete and the method is static, therefore these methods cannot be overridden or changed in the implementation class.

Similar to Default Method in Interface , the static method in an interface can be defined in the interface, but cannot be overridden in Implementation Classes. To use a static method, Interface name should be instantiated with it, as it is a part of the Interface only. Below programs illustrate static methods in interfaces: Program 1: To demonstrate use of Static method in Interface. In this program, a simple static method is defined and declared in an interface which is being called in the main method of the Implementation Class InterfaceDemo.

Unlike the default method, the static method defines in Interface hello , cannot be overridden in implementing the class. Therefore Bat can be sent to a method with a parameter Mammal and to another method with a parameter WingedAnimal parameter.

Java 8 Features From Java 8, Interface supports default and static methods. Default methods are already covered in this article , so the static methods are highly focused in this tutorial. In Java utility classes must define all their methods as static methods and the constructor must be private to avoid instantiation of that class. For example java. Arrays is an utility class where all the methods are static and the constructor is private.

Utility class developers take some extra effort to avoid the instantiation of the utility class. From Java 8 there are no need for a utility class with private constructor; it can be simply replaced by an interface with static methods.

Look at the example where a utility interface is provided to print any String arrays in a formatted manner. Do and Don't Whenever you need to provide a template, use the interface. Coding for interface is one of the best practice, so try to have an interface as the top most element in a class hierarchy. However you must be aware of what you are doing. Since Java 8 allows to have default and static methods in an interface, there is a high risk of polluting an interface by using default and static methods without any valid reasons.

Always remember these two rules when you are going to use default or static methods in an interface.