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 Q11. What are the design considerations while making a choice between using interface and abstract class ?Core Java
Anonymous
Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

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 Q12. Can we use static method definitions in Interfaces ?Core Java
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Ans. Yes, Effective Java 8.

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 Q13. How is Abstraction implemented in Java ?Core Java
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Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

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 Q14. What is the use of runnable interface if we can always create a new thread using Thread class ?Core Java
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Ans. It's a choice to be made whether to use first approach ( Thread class ) or second approach ( runnable interface ) by the programmer. The second facility has been given for cases where your class is already extending some parent class and hence cannot extend another class ( for Thread ) as Java doesn't allow multiple inheritance.

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   Like         Discuss         Correct / Improve     java   threads   multithreading   runnable interface  concurrency


 Q15. Interface can only have ...

a. Member elements and Methods.
b. Static Variables and Static Methods.
c. Static Final Variables and Instance Method Declarations.
d. Member Elements , Instance Methods, Static variables and Static Methods.
Anonymous
Ans. Static Final Variables and Instance Method Declarations.

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 Q16. What will happen if there are no interfaces in Java ?Core Java2018-12-11 16:16:37

Ans. Abstract classes can take care of that to a certain extent. Though they are little heavier than Interfaces but An abstract class with all abstract methods and no instance variables will be able to help with everything that currently an interface does.

The only problem is that a class can only extend one class whereas it can implements multiple interfaces and that is the reason Interfaces were introduced in Java, i.e to get over the problem of multiple inheritance.

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 Q17. Write a program to show thread usage in Java by implementing runnable interface2017-08-03 12:16:07

Ans. public class MyClass {
   static class MyThreadClass implements Runnable{

      public void start() {
         Thread t = new Thread(this,"threadName");
         t.start();
      }
      
      @Override
      public void run() {
         System.out.println("Hello");
         try {
            Thread.sleep(1000);
            System.out.println("Hello Again");
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
         
      }
      
   }
   
   public static void main(String[] args){
      MyThreadClass myThreadClass = new MyThreadClass();
      myThreadClass.start();
      
      MyThreadClass myThreadClass2 = new MyThreadClass();
      myThreadClass2.start();
   }
}

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 Q18. Is runnable a Marker interface ?Core Java
Anonymous
Ans. No , it has run method declared.

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  Q19. what is the difference between collections class vs collections interface ?Core Java
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Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

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 Q20. Whats the purpose of marker interfaces ?
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Ans. They just tell the compiler that the objects of the classes implementing the interfaces with no defined methods need to be treated differently.

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 Q21. What will happen if class implement two interface having common method?
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Ans. That would not be a problem as both are specifying the contract that implement class has to follow.
If class C implement interface A & interface B then Class C thing I need to implement print() because of interface A then again Class think I need to implement print() again because of interface B, it sees that there is already a method called test() implemented so it's satisfied.

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 Q22. What is a cloneable interface and what all methods does it contain?Core Java
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Ans. Cloneable is a declaration that the class implementing it allows cloning or bitwise copy of it's object state. It is not having any method because it is a MARKER interface.

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  Q23. What is Comparable Interface?Core Java
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Ans. It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.

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 Q24. Explain Set Interface?Core Java
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Ans. It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

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 Q25. What is Externalizable interface?Core Java
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Ans. Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.

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 Q26. What is an enumeration?Core Java
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Ans. An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.

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 Q27. How TreeMap orders the elements if the Key is a String ?
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Ans. As String implements Comparable, It refers to the String compareTo method to identify the order relationship among those elements.

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 Q28. Can we add heterogeneous elements into TreeMap ?
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Ans. No, Sorted collections don't allow addition of heterogeneous elements as they are not comparable.

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 Q29. Will it create any problem if We add elements with key as user defined object into the TreeMap ?
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Ans. It won't create any problem if the objects are comparable i.e we have that class implementing Comparable interface.

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 Q30. Can we access Interface static method using Interface references ?
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Ans. No, only using Interface Name.

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 Q31. Can we have default method with same name and signature in the derived Interface as the static method in base Interface and vice versa ?
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Ans. Yes , we can do that as static methods are not accessible using references and hence cannot lead to conflict. We cannot do inverse as Default methods cannot be overridden with the static methods in derived interface.

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 Q32. What is Closeable ?Core Java
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Ans. A Closeable is an interface which is a source or destination of data that can be closed.

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Not frequently asked as it was introduced with Java 8.
 Q33. What is the @FunctionalInterface annotation ?Core Java
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Ans. This is an informative annotation that specify that the interface is a functional interface. A Function Interface has only one abstract method and many default methods. Compiler generates an error if the interface specified with the annotation doesn't abide by the specifications for functional interface.

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 Q34. Name few interfaces that extends Collection Interface ?
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Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

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 Q35. Name few classes that implement Collection interface ?
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Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

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 Q36. Is it correct to say that Interfaces are abstract data types ?
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Ans. No.

Data Type holds data whereas Interface doesn't hold anything. Interface is a contract about how to communicate with the underlying Class.

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 Q37. What is the package name for CertStoreParameters class?
Anonymous
Ans. java.lang

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 Q38. Which is the Parent Class of CertStoreParameters class?
Anonymous
Ans. Clonable

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 Q39. Variables of an interface are intrinsically ...

a. transient
b. final
c. public
d. static
Anonymous
Ans. b,c and d i.e final , public and static

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 Q40. Which is the Parent Class of CertStoreParameters class?
Anonymous
Ans. Clonable

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