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.

Are all methods in interface abstract?

All methods in an interface are abstract. This statement is True. It is mandatory for an interface to have abstract methods only to apply multiple inheritance.

CAN interface have normal methods?

It cannot have a method body. Java Interface also represents the IS-A relationship. Like a class, an interface can have methods and variables, but the methods declared in an interface are by default abstract (only method signature, no body). Interfaces specify what a class must do and not how.

CAN interface have non-abstract methods C#?

We cannot create an object of an interface, we can only create a reference or Interface Variable. An interface will have only abstract methods (method declaration).

Do interface methods have to be declared abstract?

Note: Methods in an interface (see the Interfaces section) that are not declared as default or static are implicitly abstract, so the abstract modifier is not used with interface methods. (It can be used, but it is unnecessary.)

Abstract Classes and Methods - Learn Abstraction in Java

Can an interface have private methods?

As of Java 9, methods in an interface can be private. A private method can be static or an instance method, but it cannot be a default method since that can be overridden.

CAN interface have static methods?

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 concrete methods C#?

The simplest form of this feature is the ability to declare a concrete method in an interface, which is a method with a body. A class that implements this interface need not implement its concrete method. Within an instance member of an interface, this has the type of the enclosing interface.

Can 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.

Is it possible an interface to 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 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”.

Can functional interface have 2 abstract methods?

A functional interface is an interface that contains only one abstract method. They can have only one functionality to exhibit.

Why interface has default method?

The reason we have default methods in interfaces is to allow the developers to add new methods to the interfaces without affecting the classes that implements these interfaces.

Why interface have only abstract methods?

Because by default all methods are abstract inside the interface. So this example states that we can not have final methods inside the interfaces. Hence, this example also shows that we can have abstract methods only inside the interface.

CAN interface have multiple methods?

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class. An interface can contain any number of methods.

Can we mark 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 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 interface be data type?

Interface can be considered as reference data type. A data type in programming language is a set of data with values having predefined characteristics . In Object Oriented programming, a programmer can create new data types to suit the application requirements.

CAN interface have private methods in C#?

Interface cannot include private, protected, or internal members. All the members are public by default. Interface cannot contain fields, and auto-implemented properties. A class or a struct can implement one or more interfaces implicitly or explicitly.

Can we create non static variables in an interface?

No you cannot have non-static variables in an interface. By default, All the members (methods and fields) of an interface are public. All the methods in an interface are public and abstract (except static and default).

CAN interface have virtual methods in C#?

Yes, you can have virtual interface members in C# 8. Any interface member whose declaration includes a body is a virtual member unless the sealed or private modifier is used. So by default, all the default interface methods are virtual unless the sealed or private modifier is used.

Can interface contain nested types?

Yes, if we define a class inside the interface, the Java compiler creates a static nested class. Let's see how can we define a class within the interface: interface M{

Why interface Cannot have static methods?

All methods in an interface are explicitly abstract and hence you cannot define them as static because static methods cannot be abstract.

Can an interface contain constants?

Interface Constants

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.g. in calculations, or as parameters to some of the methods in the interface.

CAN interface have non public methods?

An interface can have private methods since Java 9 version. These methods are visible only inside the class/interface, so it's recommended to use private methods for confidential code. That's the reason behind the addition of private methods in interfaces.