Collections Framework

tags: Java, Collection

 Collections framework provides an architecture to store and manipulate group of objects.

How does the Java Collections hierarchy look?

                Iterable
                   |
               Collection
            /      |      \
         List     Set     Queue
                           |
                         Deque

• Map is separate and not part of this hierarchy.

What is a framework in Java?

• It provides readymade architecture.
• It represents a set of classes and interfaces.
• It is optional.

What is the Collections Framework in Java?

Java Collections Framework is a set of classes and interfaces in the java.util package that provides data structures and algorithms to store, manipulate, and access groups of objects efficiently.

What are the main benefits of using the Collections Framework?

• Reduces programming effort by using ready-made data structures
• Increases performance with efficient implementations
• Provides interoperability between different collection types
• Offers algorithms (like sorting, searching) and utilities

What is the difference between Collection and Collections?

• Collection: an interface (root of the hierarchy)
• Collections: a utility class with static methods (e.g., sort(), reverse(), shuffle())

How is Map different from other Collection types?

Map is not a subtype of Collection.
While Collection stores single elements, Map stores key-value pairs.

What is the role of Iterator in Collections?

Iterator provides a unified way to traverse any collection.

• It has methods:
  • hasNext()
  • next()
  • remove()

What is Iterable interface in Java?

• Iterable is an interface in java.lang package.
• It represents a collection of elements that can be iterated (looped) one by one.
• It has one method:

Iterator<T> iterator();

• All major collection classes like ArrayList, HashSet, etc., implement Iterable.
• It allows the use of enhanced for loop (for-each) in Java:

List<String> list = List.of("A", "B", "C");
	for (String s : list) {
	    System.out.println(s); // works because List implements Iterable
}

What is Iterator interface in Java?

• Iterator is an interface in java.util package.
• It is used to traverse or iterate over a collection manually.
• It has three main methods:

boolean hasNext();      // checks if more elements exist
T next();               // returns the next element
void remove();          // removes the current element (less used)

Example:

List<String> names = List.of("Alice", "Bob", "Charlie");
 
Iterator<String> it = names.iterator();
 
while (it.hasNext()) {
    String name = it.next();
    System.out.println(name);
}

What is the difference Between Iterable and Iterator

Feature	Iterable	Iterator
Type	Interface	Interface
Package	java.lang	java.util
Purpose	Represents a collection that can return an iterator	Used to iterate over elements
Key Method	iterator()	hasNext(), next(), remove()
Used In	for-each loop	while loop with hasNext()
Who Implements	ArrayList, HashSet, etc.	Implemented internally by collections

What is the difference between Iterator and ListIterator?

• Iterator works for all collections
• ListIterator works only for List
• ListIterator supports bidirectional traversal and element modification

What is the purpose of Comparable and Comparator?

• Comparable: natural ordering using compareTo() (used in TreeSet, TreeMap)
• Comparator: custom ordering using compare() method

How do you sort a collection?

Use Collections.sort() for a List:

List<String> list = Arrays.asList("C", "A", "B");
Collections.sort(list);

For a custom order, use Comparator:

Collections.sort(list, (a, b) -> b.compareTo(a)); // descending

What is the difference between fail-fast and fail-safe iterators?

Feature	Fail-Fast	Fail-Safe
Behavior	Throws ConcurrentModificationException on change	Works on a copy, no exception
Examples	ArrayList, HashSet	CopyOnWriteArrayList, ConcurrentHashMap

How to make a collection thread-safe?

• Use synchronized versions:
  Collections.synchronizedList(new ArrayList<>())
• Use concurrent classes:
  ConcurrentHashMap, CopyOnWriteArrayList, etc.

What are some utility methods in the Collections class?

• sort(), reverse(), shuffle()
• min(), max()
• binarySearch()
• fill(), copy()
• synchronizedList(), unmodifiableList()

What are some real-world use cases for the Collections Framework?

• Storing form data or DB rows (List)
• Caching unique records (Set)
• Key-based lookup (Map)
• Message queues or task scheduling (Queue/Deque)