Can an 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.
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Should an interface have a constructor?

Constructor in an interface

Since we cannot call the methods in the interface, there is no need of creating an object for an interface and there is no need of having a constructor in it.
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Can an interface contain a constructor Java?

Interfaces can contain instance methods, properties, events, indexers, or any combination of those four member types. Interfaces may contain static constructors, fields, constants, or operators.
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Can you construct an interface object?

No, you cannot instantiate an interface. Generally, it contains abstract methods (except default and static methods introduced in Java8), which are incomplete.
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What happens when a constructor is defined in an interface?

What happens when a constructor is defined for an interface? Explanation: Constructor is not provided by interface as objects cannot be instantiated.
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CAN interface have concrete methods?

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”.
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Can an interface be 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”.
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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.
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CAN interface have variables?

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).
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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).
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Can a class extend 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.
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Can abstract methods have constructor?

Like any other classes in Java, abstract classes can have constructors even when they are only called from their concrete subclasses.
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Can you inherit multiple interfaces?

No you cannot inherit multiple interfaces, because interfaces cannot be inherited. Interfaces are IMPLEMENTED, not inherited.
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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.
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CAN interface have private 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.
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CAN interface have main method?

Yes, from Java8, interface allows static method. So we can write main method and execute it.
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Can an interface have constants?

It's possible to place widely used constants in an interface. If a class implements such an interface, then the class can refer to those constants without a qualifying class name.
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Can you declare constructor inside an interface if not why?

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.
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CAN interface have private variables?

Therefore, the members of an interface cannot be private. If you try to declare the members of an interface private, a compile-time error is generated saying “modifier private not allowed here”.
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Can we declare interface as abstract?

Interface contains only abstract methods that can't be instantiated and it is declared by keyword interface. A class that is declared with the abstract keyword is known as an abstract class in Java.
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CAN interfaces 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{
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Can we override default method in interface?

If you have default method in an interface, it is not mandatory to override (provide body) it in the classes that are already implementing this interface. In short, you can access the default methods of an interface using the objects of the implementing classes.
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Can we override methods in interface?

No. Interface methods can be implemented. To overload/override a method, that method must be defined first. Interface do not contain method definition.
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CAN interface have multiple default methods?

Multiple Defaults

With default functions in interfaces, there is a possibility that a class is implementing two interfaces with same default methods. The following code explains how this ambiguity can be resolved. First solution is to create an own method that overrides the default implementation.
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Can a class be defined inside an interface?

Yes, you can define a class inside an interface. In general, if the methods of the interface use this class and if we are not using it anywhere else we will declare a class within an interface.
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