Collections framework: ArrayList, HashMap, HashSet, etc.

The Java Collections framework provides a set of classes and interfaces to store and manipulate groups of objects. The commonly used collections are:

ArrayList

An ArrayList is a resizable array implementation in Java. It dynamically grows and shrinks as elements are added or removed.

List names = new ArrayList();
names.add("Alice");
names.add("Bob");
names.add("Charlie");

HashMap

A HashMap is a key-value pair data structure. It allows fast retrieval of values based on keys and does not allow duplicate keys.

Map ages = new HashMap();
ages.put("Alice", 30);
ages.put("Bob", 35);
ages.put("Charlie", 40);

HashSet

A HashSet is an implementation of the Set interface. It stores unique elements and does not maintain any order.

Set uniqueNames = new HashSet();
uniqueNames.add("Alice");
uniqueNames.add("Bob");
uniqueNames.add("Charlie");

LinkedList

A LinkedList is a doubly linked list implementation in Java. It provides efficient insertion and deletion operations.

TreeMap

A TreeMap is a sorted map implementation in Java. It stores key-value pairs in sorted order based on the natural ordering of keys or a custom comparator.

TreeSet

A TreeSet is a sorted set implementation in Java. It stores unique elements in sorted order based on the natural ordering of elements or a custom comparator.

FAQ (interview questions and answers)

1. What is the purpose of ArrayList?
   To store a dynamic list of elements
   To store key-value pairs
   To maintain unique elements
   
2. How does HashMap store data?
   As key-value pairs
   In a sequential order
   As a single value
   
3. What is the main advantage of using HashSet?
   Maintaining order of elements
   Ensuring uniqueness of elements
   Storing key-value pairs
   
4. Can ArrayList contain duplicate elements?
   Yes
   No
   Sometimes
   
5. How do you access elements in a HashMap?
   Using an index
   Using keys
   Using values
   

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