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Q661. What do you mean by "Java is a statically typed language" ?

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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Q662. How can we reverse the order in the TreeMap ?

Ans. Using Collections.reverseOrder()

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

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Q663. TreeMap orders the elements on which field ?

Ans. Keys

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Q664. 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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Q665. 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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Q666. 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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Q667. Can we have null keys in TreeMap ?

Core Java

Ans. No, results in exception.

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Q668. Can value be null in TreeMap ?

Core Java

Ans. Yes.

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Q669. Which interface TreeMap implements ?

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

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Q670. 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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Q671. 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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Frequently asked in Phone Interviews.

Q672. What are the various Auto Wiring types in Spring ?

Ans. By Name , By Type and Constructor.

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Q673. What is the purpose of dialect configured in Hibernate configuration file ?

Hibernate

Ans. It tells the framework which SQL varient to generate.

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Very frequently asked Hibernate interview question. Frequently asked in TCS ( based on 2 feedback )

Q674. What are different types of associations in Hibernate ?

Hibernate

Ans. There are 4 types of associations in Hibernate

One to One
One to Many
Many to One
Many to Many

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Q675. What are the configuration files in Hibernate ?

Hibernate

Ans. hibernate.cfg.xml ( Main Configuration File )

and *.hbm.xml files ( Mapping Files )

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Q676. What are the contents of Hibernate configuration file ( hibernate.cfg.xml ) ?

Hibernate

Ans. HBM Files ( Mapping )
DB Connection ( DB Connection String , User Name , Password , Pool Size )
SQL Dialect ( SQL variant to be generated )
Show SQL ( Show / No show SQL on Console )
Auto Commit ( True / False )

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Very frequently asked if being interviewed for hibernate. Frequently asked in Tata Consultancy (TCS) and Overstock.com

Q677. Difference between load and get ?

Hibernate

Ans. If id doesnt exist in the DB load throws an exception whereas get returns null in that case.get makes the call to DB immediately whereas load makes the call to proxy.

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Q678. 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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Q679. 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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Q680. Difference between Assert and Verify ?

Testing

Ans. Assert works only if assertions ( -ea ) are enabled which is not required for Verify.Assert throws an exception and hence doesn't continue with the test if assert evaluates to false whereas it's not so with Verify.

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

Q681. 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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Q682. Which annotations are used in Hibernate ?

Hibernate

Ans. @Entity
@Table
@Id
@Column
@Temporal
@Basic
@Enumerated
@Access
@Embeddable
@Lob
@AttributeOverride
@Embedded
@GeneratedValue
@ElementCollection
@JoinTable
@JoinColumn
@CollectionId
@GenericGenerator
@OneToOne
@OneToMany
@ManyToOne
@ManyToMany
@NotFound

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Q683. What entries we make in the hibernate config file if we are not using hbm files but Annotations ?

Hibernate

Ans. We configure Entity classes having annotated mappings.

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Q684. 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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Q685. What is the use of hbm2ddl Configuration in Hibernate ?

Hibernate

Ans. This configuration specifies if hibernate should creates the Schema / Table on its own if the respective table is not found.

"update" doesn't create the table if it's not found whereas configuration set as "create" creates the schema automatically.

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Q686. What is the difference between these 2 annotations ?

@Entity

@Entity ( name="EMPLOYEES" )

Hibernate

Ans. The first annotation will try to map the Class with the Table as of same name as Class whereas the second annotation will specify the Entity name as "EMPLOYEES" and hence will try to map with Table Name "EMPLOYEES".

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Q687. What is the difference between these 2 annotations ?

@Entity ( name ="EMPLOYEES")
@Entity @Table ( name=""EMPLOYEES"" )

@Entity ( name="EMP")
@Table ( name="EMPLPYEES" )

Hibernate

Ans. First Annotation will set the Entity name as EMPLOYEES and hence will try to map with the same Table name.

The second annotation will make the Entity mapped to table EMPLOYEES irrespective of the Entity Name ( which is class name in this case ).

Third Annotations will set the different names for Enitity and Table and will explicitly map them.

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Q688. What are the different ID generating strategies using @GeneratedValue annotation ?

Hibernate

Ans. Auto , Identity , Sequence and Table.

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Q689. How to do Eager loading in Hibernate ?

Hibernate

Ans. Using

lazy = false in hibernate config file

or

@Basic(fetch=FetchType.EAGER) at the mapping

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Frequently asked question in companies using Hibernate.

Q690. What is Lazy Initialization in Hibernate ?

Hibernate

Ans. It's a feature to lazily initialize dependencies , relationship and associations from the Database. Any related references marked as @OneToMany or @ManyToMany are loaded lazily i.e when they are accessed and not when the parent is loaded.

Sample Code for Lazy Initialization

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