Can an interface be instantiated?

An interface can't be instantiated directly. Its members are implemented by any class or struct that implements the interface. A class or struct can implement multiple interfaces.

Why can't we instantiate an interface?

You can't instantiate an interface or an abstract class because it would defy the object oriented model. Interfaces represent contracts - the promise that the implementer of an interface will be able to do all these things, fulfill the contract.

Can u instantiate an interface in Java?

Interfaces cannot be instantiated, but rather are implemented. A class that implements an interface must implement all of the non-default methods described in the interface, or be an abstract class.

Can Java 8 interface be instantiated?

We can't create an instance(interface can't be instantiated) of the interface but we can make the reference of it that refers to the Object of its implementing class. A class can implement more than one interface. An interface can extend to another interface or interface (more than one interface).

Can interface be overriden?

You can make the methods default in the interface itself, Default methods are introduced in interfaces since Java8 and if you have default methods in an interface it is not mandatory to override them in the implementing class.

Java Interface Tutorial - Learn Interfaces in Java

Can interface be overloaded?

Overloading methods of an interface

Yes, you can have overloaded methods (methods with the same name different parameters) in an interface. You can implement this interface and achieve method overloading through its methods.

Can we override functional interface?

We can override the default method as it is available to the class that implements the interface. In the same way, we can invoke it using the implementation class object from the interface directly without overriding it.

Can we declare interface as final?

If you make an interface final, you cannot implement its methods which defies the very purpose of the interfaces. Therefore, you cannot make an interface final in Java. Still if you try to do so, a compile time exception is generated saying “illegal combination of modifiers − interface and final”.

Can we instantiate runnable interface?

You cannot instantiate an interface. Nor an abstract class.

CAN interface have a constructor?

No, you cannot have a constructor within an interface in Java. You can have only public, static, final variables and, public, abstract, methods as of Java7. From Java8 onwards interfaces allow default methods and static methods. From Java9 onwards interfaces allow private and private static methods.

Can we instantiate an abstract class?

Abstract classes cannot be instantiated, but they can be subclassed. When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract .

Can interface extend another interface?

Defining an Interface

An interface can extend other interfaces, just as a class subclass or extend another class. However, whereas a class can extend only one other class, an interface can extend any number of interfaces. The interface declaration includes a comma-separated list of all the interfaces that it extends.

Can an interface declare an instance variable?

The final modifier ensures the value assigned to the interface variable is a true constant that cannot be re-assigned. In other words, interfaces can declare only constants, not instance variables.

Can we instantiate interface and abstract class?

No you can not instantiate an interface or abstract class. But you can instantiate an anonymous class that implements/extends the interface or abstract class without defining a class object.

What class Cannot be instantiated?

An abstract class cannot be instantiated.

Can we instantiate interface in C#?

Interface can not be directly instantiated. We can create an instance of a class that implements the interface, then assign that instance to a variable of the interface type.

What is Callable interface?

Interface Callable<V>

Implementors define a single method with no arguments called call. The Callable interface is similar to Runnable , in that both are designed for classes whose instances are potentially executed by another thread. A Runnable, however, does not return a result and cannot throw a checked exception.

Is functional interface runnable?

Interface Runnable

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference. The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread.

Can runnable throw exception?

Runnable can't throw checked exception but RuntimeException can be thrown from the run(). Uncaught exceptions are handled by the exception handler of the thread, if JVM can't handle or catch exceptions, it prints the stack trace and terminates the flow.

Can we make interface as static?

No,Because of static methods belolngs to a class not instance of class,and interfaces are not classes.In interface you define a behaviour taht has to be implemented in calss.So it is not possible to make a static method in interface.In java8 introduces static methods in interface.

Can we declare constructor as private?

Yes, we can declare a constructor as private. If we declare a constructor as private we are not able to create an object of a class. We can use this private constructor in the Singleton Design Pattern.

Can we put a static method in interfaces?

Static methods in an interface since java8

Since Java8 you can have static methods in an interface (with body). You need to call them using the name of the interface, just like static methods of a class.

CAN interface have non abstract methods?

Type of methods: Interface can have only abstract methods. An abstract class can have abstract and non-abstract methods.

Can we have non abstract method in functional interface?

A functional interface must have exactly one abstract method. A functional interface has any number of default methods because they are not abstract and implementation already provided by the same.

Can we have concrete methods in interface?

All the methods in an interface must be abstract, you cannot have a concrete method (the one which has body) if you try to do so, it gives you a compile time error saying “interface abstract methods cannot have body”.