Interview Questions and Answers for 'Java' | Search Interview Question

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Q1351. Which of the following syntax are correct ?a. LinkedList<Integer> l=new LinkedList<int>();b. List<Integer> l=new LinkedList<int>();c. LinkedList<Integer> l=new LinkedList<Integer>();d. List<Integer> l = new LinkedList<Integer>();

Core Java

Ans. All are correct.

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Q1352. Which of the following code is correct ?

a. Date date = DateFormat.newInstance(DateFormat.LONG, Locale.US).parse(str);

b. Date date = DateFormat.newInstance(DateFormat.LONG, Locale.US).format(str);

c. Date date = DateFormat.getDateInstance(DateFormat.LONG, Locale.US).parse(str);

Core Java

Ans. c

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Q1353. What's wrong with this code ?

public static void main(String[] args) {
String regex = "(\\w+)*";
String s = "Java is a programming language.";

Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(s);
while (matcher.next()) {
System.out.println("The e-mail id is: " + matcher.group());
}
}

Core Java

Ans. matcher.find() should have been used instead of matcher.next() within while.

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Q1354. Which methods of the Pattern class have equivalent methods in the String class?

Core Java

Ans. split() and macthes()

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Q1355. Can we compare Integers by using equals() in Java ?

Core Java

Ans. Yes for the Wrapper class Integer but not for the primitive int.

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Q1356. 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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Q1357. Which are the sorted collections ?

Core Java

Ans. TreeSet and TreeMap

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Very Frequently asked. Favorite question in walkins and telephonic interviews. Usually among first few questions. Asked in different variants. Must know for intermediate and expert professionals.Among Top 10 frequently asked questions.

Q1358. What is rule regarding overriding equals and hashCode method ?

Core Java

Ans. A Class must override the hashCode method if its overriding the equals method.

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Q1359. What is the difference between Collection and Collections ?

Core Java

Ans. Collection is an interface whereas Collections is a utility class.

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Q1360. Is Java a statically typed or dynamically typed language ?

Ans. Statically typed

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Q1361. What do you mean by "Java is a statically typed language" ?

Core Java

Ans. It means that the type of variables are checked at compile time in Java.The main advantage here is that all kinds of checking can be done by the compiler and hence will reduce bugs.

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Q1362. How can we reverse the order in the TreeMap ?

Ans. Using Collections.reverseOrder()

Map tree = new TreeMap(Collections.reverseOrder());

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Q1363. TreeMap orders the elements on which field ?

Ans. Keys

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Q1364. How TreeMap orders the elements if the Key is a String ?

Ans. As String implements Comparable, It refers to the String compareTo method to identify the order relationship among those elements.

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Q1365. Can we add heterogeneous elements into TreeMap ?

Ans. No, Sorted collections don't allow addition of heterogeneous elements as they are not comparable.

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Q1366. Will it create any problem if We add elements with key as user defined object into the TreeMap ?

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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Q1367. Can we have null keys in TreeMap ?

Core Java

Ans. No, results in exception.

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Q1368. Can value be null in TreeMap ?

Core Java

Ans. Yes.

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Q1369. Which interface TreeMap implements ?

Ans. TreeMap implements NavigableMap, SortedMap, Serializable and Clonable.

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Q1370. What is a ConcurrentHashMap ?

Core Java

Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

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Q1371. What is the difference between these two approaches of creating singleton Class ?

//Double Checked Locking Code
public static Singleton createInstance() {
   if(singleton == null){
      synchronized(Singleton.class) {
         if(singleton == null) {
            singleton = new Singleton();
         }
      }
   }
   return singleton;
}

//Single checked locking code
public static Singleton createInstance() {
   synchronized(Singleton.class) {
      if(singleton == null) {
         singleton = new Singleton();
      }
   }
   return singleton;
}

Design

Ans. In First Case , Lock for the synchronized block will be received only if singleton == null whereas in second case every thread will acquire the lock before executing the code.
The problem of synchronization with singleton will only happen when the object has not be instantiated. Once instantiated , the check singleton == null will always generate true and the same object will be returned and hence no problem. First condition will make sure that synchronized access ( acquiring locks ) will only take place if the object has not been created so far.

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Q1372. 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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Q1373. 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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Frequently asked in Phone Interviews.

Q1374. What are the various Auto Wiring types in Spring ?

Ans. By Name , By Type and Constructor.

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Q1375. Is It Good to use Reflection in an application ? Why ?

Core Java

Ans. no, It's like challenging the design of application.

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Q1376. Why is Reflection slower ?

Ans. Because it has to inspect the metadata in the bytecode instead of just using precompiled addresses and constants.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q1377. What is the difference between ArrayList and LinkedList ?

Core Java

Ans. Underlying data structure for ArrayList is Array whereas LinkedList is the linked list and hence have following differences -

1. ArrayList needs continuous memory locations and hence need to be moved to a bigger space if new elements are to be added to a filled array which is not required for LinkedList.

2. Removal and Insertion at specific place in ArrayList requires moving all elements and hence leads to O(n) insertions and removal whereas its constant O(1) for LinkedList.

3. Random access using index in ArrayList is faster than LinkedList which requires traversing the complete list through references.

4. Though Linear Search takes Similar Time for both, Binary Search using LinkedList requires creating new Model called Binary Search Tree which is slower but offers constant time insertion and deletion.

5. For a set of integers you want to sort using quicksort, it's probably faster to use an array; for a set of large structures you want to sort using selection sort, a linked list will be faster.

Sample Code for ArrayList

Sample Code for LinkedList

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Q1378. How many SessionFactory and Session objects are created ?

Java EE

Ans. Single SessionFactory object and multiple session objects for opening different session. Hibernate creates new Session object per thread.

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Q1379. What is the difference between int[] x; and int x[]; ?

Core Java

Ans. No Difference. Both are the acceptable ways to declare an array.

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Q1380. What are the annotations used in Junit with Junit4 ?

Ans. @Test

The Test annotation indicates that the public void method to which it is attached can be run as a test case.

@Before

The Before annotation indicates that this method must be executed before each test in the class, so as to execute some preconditions necessary for the test.

@BeforeClass

The BeforeClass annotation indicates that the static method to which is attached must be executed once and before all tests in the class.

@After

The After annotation indicates that this method gets executed after execution of each test.

@AfterClass

The AfterClass annotation can be used when a method needs to be executed after executing all the tests in a JUnit Test Case class so as to clean-up the set-up.

@Ignores

The Ignore annotation can be used when you want temporarily disable the execution of a specific test.

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