Can we declare final variable in interface?

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.

Can a interface have complete method?

Interfaces are declared using the interface keyword, and may only contain method signature and constant declarations (variable declarations that are declared to be both static and final ). All methods of an Interface do not contain implementation (method bodies) as of all versions below Java 8.

Are interface methods final by default?

That means all the methods in an interface are declared with an empty body and are public and all fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.

Can functional interface have final methods?

A functional interface can contain default and static methods which do have an implementation, in addition to the single unimplemented method. The above interface still counts as a functional interface in Java, since it only contains a single non-implemented method.

Can we declare final inside abstract class interface?

No, you cannot make an abstract class or method final in Java because the abstract and final are mutually exclusive concepts. This is why making an abstract method final in Java will result in a compile-time error. In short, the use of keywords abstract and final together is illegal in Java.

Can we declare final inside abstract class and interface both?

If you declare a class abstract, to use it, you must extend it and if you declare a class final you cannot extend it, since both contradict with each other you cannot declare a class both abstract and final if you do so a compile time error will be generated.

CAN interfaces have code?

Since Java 8, methods can be implemented ( can have a code body ) in an interface if only if it is declared static or default. Abstract methods cannot have a body; all they can have is a method signature as shown in the example above. Variables are not allowed in interface.

CAN interface have no method in it?

Yes, you can write an interface without any methods. A marker interface i.e. it does not contain any methods or fields by implementing these interfaces a class will exhibit a special behavior with respect to the interface implemented.

Why interface methods are static and final?

Interface variables are static because java interfaces cannot be instantiated on their own. The value of the variable must be assigned in a static context in which no instance exists. The final modifier ensures the value assigned to the interface variable is a true constant that cannot be re-assigned.

Can we declare final method in abstract class?

Yes, it can. But the final method cannot be abstract itself (other non-final methods in the same class can be). Yes, there may be “final” methods in “abstract” class. But, any “abstract” method in the class can’t be declared final.

Is it allowed to mark a method abstract as well as final?

Can we mark abstract class as final?

No, An abstract class can’t be final because the final and abstract are opposite terms in JAVA. Reason: An abstract class must be inherited by any derived class because a derived class is responsible to provide the implementation of abstract methods of an abstract class.

Is it possible to override a final method in an interface?

A final method can’t be overridden. That defies the purpose of having an interface if you cannot actually implement the method. For the static part, see this question. You got it wrong. All variables are implicitly public static and final in interfaces. Prior to Java 8, you can’t create static methods in interfaces.

Can interface methods be declared as final in Java?

An interface is a pure abstract class. Hence, all methods in an interface are abtract, and must be implemented in the child classes. So, by extension, none of them can be declared as final. Why Interface methods cannot be “static” & “final”?

Which variables are implicitly public static and final in interfaces?

All variables are implicitly public static and final in interfaces. Prior to Java 8, you can’t create static methods in interfaces. All methods are instance methods.

What is interface in Java with example?

Interface in Java is similar to class but, it contains only abstract methods and fields which are final and static. Since all the methods are abstract you cannot instantiate it. To use it, you need to implement this interface using a class and provide body to all the abstract methods int it. Making an interface final.