18.How do you prevent a method from being overridden?
A).To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                           public final void exampleMethod() {
                          //  Method statements
                          }


19.What is an Interface?
A).An interface is a description of a set of methods that conforming implementing classes must have.
Note:

    You can’t mark an interface as final.
    Interface variables must be static.
    An Interface cannot extend anything but another interfaces.


20.Can we instantiate an interface?
A).You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

21.Can we create an object for an interface?
A).Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

22.Do interfaces have member variables?
A).Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

23.What modifiers are allowed for methods in an Interface?
A).Only public and abstract modifiers are allowed for methods in interfaces.

24.What is a marker interface?
A).Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

25.What is an abstract class?
A).Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:
    If even a single method is abstract, the whole class must be declared abstract.
    Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
    You can’t mark a class as both abstract and final.


26.Can we instantiate an abstract class?
A).An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

27.What are the differences between Interface and Abstract class?
A).Abstract Class VS    Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.     An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.     A Class may implement several interfaces.
An abstract class can have non-abstract methods.     All methods of an Interface are abstract.
An abstract class can have instance variables.     An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.     An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.     If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .     An Interface cannot contain constructors .
Abstract classes are fast.     Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

28.When should I use abstract classes and when should I use interfaces?
A).Use Interfaces when…
    You see that something in your design will change frequently.
    If various implementations only share method signatures then it is better to use Interfaces.
    you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when…

    If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
    When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
    Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.


29.When you declare a method as abstract, can other nonabstract methods access it?
A).Yes, other nonabstract methods can access a method that you declare as abstract.

30.Can there be an abstract class with no abstract methods in it?
A).Yes, there can be an abstract class without abstract methods.

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