Core Java Interview Questions & Answers.
Part 7 - InterfacesWhat is an Interface?
Interfaces say what a class must do but does not say how a class must do it. Interfaces are 100% abstract.
Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?
No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.
Can a method inside a Interface be declared as final?
No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.
Can an Interface implement another Interface?
Intefaces doesn't provide implementation hence a interface cannot implement another interface.
Can an Interface extend another Interface?
Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.
Can a Class extend more than one Class?
Not possible. A Class can extend only one class but can implement any number of Interfaces.
Why is an Interface be able to extend more than one Interface but a Class can't extend more than one Class?
Basically Java doesn't allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn't have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.
Can an Interface be final?
Not possible. Doing so so will result in compilation error.
Can a class be defined inside an Interface?
Yes it's possible.
Can an Interface be defined inside a class?
Yes it's possible.
What is a Marker Interface?
An Interface which doesn't have any declaration inside but still enforces a mechanism.
Can we define private and protected modifiers for variables in interfaces?
No. Always all variables declared inside a interface are of public access.
What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
When can an object reference be cast to an interface reference?
An object reference can be cast to an interface reference when the object implements the referenced interface.
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