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 Q161. what is fail fast and Fail Safe in collections?Core Java
Ans. Iterators in java are used to iterate over the Collection objects.

Fail-Fast iterators immediately throw ConcurrentModificationException if there is any addition, removal or updation of any element.

Fail-Safe iterators don't throw any exception if a collection is structurally modified while iterating over it. This is because, they operate on the clone of the collection and not on the original collection.

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   Like         Discuss         Correct / Improve     ail fast   fail saf     Asked in 9 Companies      intermediate

 Q162. Will the static block be executed in the following code ? Why ?

class Test
{
static
{
System.out.println("Why I am not executing ");
}
public static final int param=20;
}

public class Demo
{
public static void main(String[] args)
{
System.out.println(Test.param);
}
}
Ans. No the static block won't get executed as the referenced variable in the Test class is final. Compiler replaces the content of the final variable within Demo.main method and hence actually no reference to Test class is made.

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   Like         Discuss         Correct / Improve     java   oops   static   static block   final variable
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 Q163. What will be the output of following code ?

public static void main(String[] args){
String parent = null;
File file = new File(parent, "myfile.txt");
try {
file.createNewFile();
} catch (IOException e) {
e.printStackTrace();
}
}		Core Java
Ans. It will create the file myfile.txt in the current directory.

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   Like         Discuss         Correct / Improve     java   io   file   fileio   coding   code   file handling

 Q164. What is comparator interface used for ?Core Java
Ans. The purpose of comparator interface is to compare objects of the same class to identify the sorting order. Sorted Collection Classes ( TreeSet, TreeMap ) have been designed such to look for this method to identify the sorting order, that is why class need to implement Comparator interface to qualify its objects to be part of Sorted Collections.

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   Like         Discuss         Correct / Improve     java   collections   treemap   treeset   comparator     Asked in 2 Companies      Intermediate
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 Q165. Why are Getter and Setter better than directly changing and retrieving fields ?

or

What is the significance of variable hiding in Java ?

or

Why it's preferred to keep variables private and allow their access through getter and setter methods ?		Core Java
Ans. 1. Methods can participate in runtime polymorphism whereas member variables cannot.

For example -

Vehicle vehicle = new Car();

car.getObjVariable();// will return variable defined in Car

whereas

car.variable

will return the variable from Car class irrespective of the object it holds.

2. Validations can be performed before setting the variables.

3. If the input format changes or some other value or calculated value needs to be returned , that can be absorbed by making change ( wrapping ) in the setter and getter. By this the call to method from outside or interface to the object will remain the same.

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   Like         Discuss         Correct / Improve     java   beans   getter   setter

  Q166. Can we overload main method in Java ?Core Java
Ans. Yes, but the overloaded main methods without single String[] argument doesn't get any special status by the JVM. They are just another methods that needs to be called explicitly.

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   Like         Discuss         Correct / Improve     java   main method   overloading   yes-no     Asked in 6 Companies      intermediate        frequent

 Q167. If I try to add Enum constants to a TreeSet, What sorting order will it use ?
Ans. Tree Set will sort the Values in the order in which Enum constants are declared.

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   Like         Discuss         Correct / Improve     java   collections   set   treeset   enum   sorting   technical lead

 Q168. Can we override compareTo method for Enumerations ?Core Java
Ans. No. compareTo method is declared final for the Enumerations and hence cannot be overriden. This has been intentionally done so that one cannot temper with the sorting order on the Enumeration which is the order in which Enum constants are declared.

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   Like         Discuss         Correct / Improve     java   compareto   final methods   enum   enumeration      expert

Very Frequently asked.
 Q169. Explain throw, throws , try and catch in Java ?Core Java
Ans. throw is used to re throw an exception.throws is used to declare that the method throws the respective exceptions.try block is used to identify if the respective block has thrown any exception.catch is used to catch the exception that has been thrown by the respective try block.

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   Like         Discuss         Correct / Improve     java   exception handling   throw   throws   try   catch     Asked in 5 Companies      basic        frequent

 Q170. What is ConcurrentModificationException ?Core Java
Ans. This is the exception that is thrown when we try to modify the non concurrent collection class while iterating through it.

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   Like         Discuss         Correct / Improve     java   collections   concurrentmodificationexception   exception  concurrency     Asked in 14 Companies      intermediate

 Q171. What is the use of runnable interface if we can always create a new thread using Thread class ?Core Java
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

 Q172. What is a BlockingQueue?Data Structures
Ans. BlockingQueue is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

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   Like         Discuss         Correct / Improve     blockingqueue   collections   java   concurrent collections     Asked in 10 Companies

 Q173. Which of the following collections stores its elements in insertion Order ?

a. HashMap
b. TreeMap
c. LinkedHashMap
d. LinkedMap		Core Java
Ans. LinkedHashMap

  Sample Code for LinkedHashMap

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   Like         Discuss         Correct / Improve     linkedhashmap   collections   java   map

 Q174. In what order the elements of a HashSet are retrieved ?

a. Random Order
b. Insertion Order
c. Natural Sorting Order
d. Inverse Natural Sorting Order
Core Java
Ans. Random Order

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   Like         Discuss         Correct / Improve     collections   set   hashset   java
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 Q175. Overridden methods must have the same ...

a. name
b. name and argument list
c. name, argument list, and return type
d. name, argument list, return type and belong to the same class
Ans. name and argument list

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 Q176. Which of the following file is called deployment descriptor ?

a. application.xml
b. project.xml
c. web.xml
d. build.xml		Java EE
Ans. web.xml

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   Like         Discuss         Correct / Improve     configuration files   deployment   server   java

 Q177. Which of the following is false about Constructors ?

a. Constructor can be overloaded
b. A no argument constructor is provided by the compiler if we declare only constructors with arguments.
c. Constructors shouldn't have any return types , not even void.
d. If super is not explicitly called, still super() is intrinsically added by the compiler.
Core Java
Ans. A no argument constructor is provided by the compiler if we declare only constructors with arguments.

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 Q178. Which of the following doesn't extend Collection interface ?

a. Set
b. List
c. Map
d. Queue
Core Java
Ans. Map

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   Like         Discuss         Correct / Improve     java   collections   collection interface

 Q179. Which of the following Java feature promotes Code Re usability ?

a. Abstraction
b. Encapsulation
c. Inheritance
d. Polymorphism
Core Java
Ans. Inheritance

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 Q180. Which of the following is not a build tool or plugin ?

a. Maven
b. Ant
c. Gradle
d. svn		Tool
Ans. svn

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 Q181. enums are intrinsically ..

a. private
b. public
c. static
d. final
Core Java
Ans. static

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   Like         Discuss         Correct / Improve     java   enum

 Q182. In which cases , moving methods to utility class could be useful ?Core Java
Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases.

Don't belong to one group through is-a relationship ( You can share through parent class method )

Don't implement a specific interface ( java 8 default methods )

Doesn't involve complex computing as you will be loosing the benefit of object state with just static method.

Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism.

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   Like         Discuss         Correct / Improve     utility classes   util classes   static methods   application design        rare

 Q183. What are different types of variables a class can have ?Core Java
Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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  Q184. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.		Core Java
Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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   Like         Discuss         Correct / Improve     multiple inheritance  object oriented programming (oops)  oops concepts  diamond problem     Asked in 20 Companies      basic        frequent

 Q185. Design an app to find 10 most visited urlsCore Java
 This question was recently asked at 'Wayfair'.This question is still unanswered. Can you please provide an answer.


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 Q186. Trim a string without using String library methodCore Java
 This question was recently asked at 'Ariba'.This question is still unanswered. Can you please provide an answer.


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Usually asked to entry level software developers.
  Q187. Write a program to swap two variables without using thirdCore Java
Ans. public static void main(String[] args) {
   int num1 = 1;   
   int num2 = 2;
   num1 = num1^num2;
   num2 = num1^num2;
   num1 = num1^num2;
   System.out.print("num1 = " + num1 +", num2 = "+num2);
}

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   Like         Discuss         Correct / Improve     code  coding     Asked in 37 Companies      basic        frequent

 Q188. Write a Program to print pattern like following

*
**
***
****
*****
*****
****
***
**
*		Core Java
Ans. public class BuggyBread {
   public static void main(String args[]) {
      for(int x=1;x<=5;x++){
         for(int y=1;y<=x;y++){
            System.out.print("*");
         }
         
         System.out.println("");
      }
      
      for(int x=5;x>=1;x--){
         for(int y=1;y<=x;y++){
            System.out.print("*");
         }
         
         System.out.println("");
      }   
   }
}

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 Q189. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java
Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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   Like         Discuss         Correct / Improve     abstract class   interfaces     Asked in 2 Companies      Basic

 Q190. Can we overload method as following ?

void method(int... x){};
void method(int[] x){}; 		Core Java
Ans. No. Because java internally treats var args as arrays and hence both method declarations will generate the same byte code and hence would result in ambiguity while determining call binding.

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   Like         Discuss         Correct / Improve     var args  methods  functions   method declaration  scjp  ocjp

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