1.Why Map interface doesn’t extend Collection interface?

• Set is unordered collection and does not allows duplicate elements.
• List is ordered collection allows duplicate elements.
• Where as Map is key-value pair.
• It is viewed as set of keys and collection of values.
• Map is a collection of key value pairs so by design they separated from collection interface.

2.What is difference between HashMap and Hashtable?

• Synchronization or Thread Safe 
• Null keys and null values 
• Iterating the values 
•  Default Capacity

3.Differences between comparable and comparator?

• Comparable Interface is actually from java.lang package.
• It will have a method compareTo(Object obj)to sort objects
• Comparator Interface is actually from java.util package.
• It will have a method compare(Object obj1, Object obj2)to sort objects

4.How can we sort a list of Objects?

•  To sort the array of objects we will use  Arrays.sort() method.
• If we need to sort collection of object we will use Collections.sort().

5.What is difference between fail-fast and fail-safe?

• Fail fast is nothing but immediately report any failure. whenever a problem occurs fail fast system fails. 
• in java Fail fast iterator while iterating through collection of objects sometimes concurrent modification exception will come there are two reasons for this.
• If one thread is iterating a collection and another thread trying to modify the collection.
• And after remove() method call if we try to modify collection object

6. What is difference between Iterator ,ListIterator and Enumeration?

• Enumeration interface implemented in java 1.2 version.So Enumeration is legacy interface.
• Enumeration uses elements() method.
• Iterator is implemented on all Java collection classes.
• Iterator uses iterator() method.
• Iterator can traverse in forward direction only.
• ListIterator is implemented only for List type classes
• ListIterator uses listIterator() method.

What is difference between Iterator ,ListIterator and Enumeration?

7.What is difference between Set and List in Java?

• A set is a collection that allows unique elements.
• Set having no index.
• Set allows only one null value.
• Set having classes like :

• HashSet
• LinkedHashMap
• TreeSet
• List having index.
• List allows n number of null values.
• List will display Insertion order with index.

• List having classes like :
• Vector
• ArrayList
• LinkedList

8.Differences between arraylist and vector?

• Vector was introduced in  first version of java . that's the reason only vector is legacy class.
• ArrayList was introduced in java version1.2, as part of java collections framework.
• Vector is  synchronized. 
• ArrayList is not synchronized.

9.What are the classes implementing List interface?

• ArrayList     
• LinkedList     
• Vector

10. Which all classes implement Set interface ?

• HashSet
• LinkedHashSet
• TreeSet

11.How to make a collection thread safe?

• Vector, Hashtable, Properties and Stack are synchronized classes, so they are thread-safe and can be used in multi-threaded environment.
• By using Collections.synchronizedList(list)) we can make list classes thread safe.
• By using 
  java.util.Collections.synchronizedSet()  we can make set classes thread safe.

12.Can a null element added to a TreeSet or HashSet?

• One null element can be added to hashset.
• TreeSet also allow null value once.

13. Explain Collection’s hierarchy?

14.Which design pattern Iterator follows?

• Iterator design pattern

15.Which data structure HashSet implements

• Hashset implements hashmap internally.

16.Why doesn't Collection extend Cloneable and Serializable?

• List and Set and queue extends Collection interface.
• SortedMap extends Map interface.

17.What is the importance of hashCode() and equals() methods? How they are used in Java?

• equals() and hashcode() methods defined in "object" class. 
• If equals() method return true on comparing two objects then hashcode() of those two objects must be same.

18.What is difference between array & arraylist?

• Array is collection of similar type of objects and fixed in size.
• Arraylist is collection of homogeneous and heterogeneous elements.

19.What is the Properties class?

• Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key and the value is String.

20.How to convert a string array to arraylist?

• Using arrayList.toArray();