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 Q11. 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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 Q12. 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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 Q13. Difference between Map and HashMap ?Core Java
Ans. Map is an interface where HashMap is the concrete class.

  Sample Code for map

  Sample Code for hashmap

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 Q14. What is a Property class ?Core Java
Ans. The properties class is a subclass of Hashtable that can be read from or written to a stream.

  Sample Code for Load Properties using Property Class

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   Like         Discuss         Correct / Improve     java   collections   hashtable   map   synchronization   synchronized

 Q15. Why String is popular HashMap key in Java?Core Java
Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

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 Q16. Can we add duplicate keys in a HashMap ? What will happen if we attempt to add duplicate values ?
Ans. No, We cannot have duplicate keys in HashMap. If we attempt to do so , the previous value for the key is overwritten.

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 Q17. Which are the sorted collections ?Core Java
Ans. TreeSet and TreeMap

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 Q18. 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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 Q19. Which of the following collection maintain its elements in Natural Sorted order ?

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

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 Q20. If we try to add duplicate key to the HashMap, What will happen ?

a. It will throw an exception.
b. It won't add the new Element without any exception.
c. The new element will replace the existing element.
d. Compiler will identify the problem and will throw an error.
Ans. The new element will replace the existing element.

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 Q21. Which interface does java.util.Hashtable implement?Core Java
Ans. Java.util.Map

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 Q22. Which interface provides the capability to store objects using a key-value pair?Core Java
Ans. java.util.map

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  Q23. What is Comparable Interface?Core Java
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. Which Java collection class can be used to maintain the entries in the order in which they were last accessed?
Ans. LinkedHashMap

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 Q25. How can we reverse the order in the TreeMap ?
Ans. Using Collections.reverseOrder()

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

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 Q26. TreeMap orders the elements on which field ?
Ans. Keys

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 Q27. 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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 Q28. 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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 Q29. 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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  Q30. Can we have null keys in TreeMap ?Core Java
Ans. No, results in exception.

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 Q31. Can value be null in TreeMap ?Core Java
Ans. Yes.

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 Q32. Which interface TreeMap implements ?
Ans. TreeMap implements NavigableMap, SortedMap, Serializable and Clonable.

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 Q33. How is HashSet maintained in memory by Java ?Core Java
Ans. HashSet is maintained as HashMap by Java with values of the HashSet as Keys of the HashMap and value of the HashMap as the constant PRESENT.

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 Q34. Name few Collections Map implementations ?
Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

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 Q35. How do we specify the criteria if it involves mapping between two entities or join between tables ?Hibernate
Ans. The following code returns the list of Employee objects having employee name starting with A and Dept Name ( Department , Employee Mapped ).

session.createCriteria(Employee.class,"emp")
.createAlias("emp.department", "dept",Criteria.INNER_JOIN)
.add( Restrictions.like("name", "A%") )
.add(Restrictions.eq("dept.name","Finance")
.list();

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 Q36. Name few classes that implement Map interface ?
Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_82.html

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 Q37. Difference between HashMap and WeakHashMap ?
Ans. WeakHashMap uses weak reference for keys, which means if the key object doesn't have any reference then both key/value mapping will become eligible for garbage collection.

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 Q38. Which tree is used to implement TreeMap in Java?
Ans. Red Black Tree

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 Q39. If we add Enum constants to a sorted collection ( Treemap , TreeSet ), What will be the order in which they will be maintained ?

a. Sorted Collection wont maintain them in any order.
b. Insertion Order
c. Order in which constants are declared.
d. Natural Sorting Order.
Ans. Order in which constants are declared.

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 Q40. Which of the following is false ?

a. HashMap came before HashTable.
b. HashMap allows null values whereas Hashtable doesn’t allow null values.
c. HashTable and HashMap allow Key-Value pairs.
d. Hashtable is synchronized whereas HashMap is not.
Ans. HashMap came before HashTable.

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