The Java Collections Framework (JCF) is one of the cornerstones of Java's utility classes, enabling developers to manage groups of objects efficiently. While the basics—such as Lists, Sets, and Maps—are essential, real-world enterprise and high-performance applications often demand advanced usage of the framework. This chapter delves deep into these advanced concepts, helping you build high-performing, maintainable, and scalable applications using the collections API.

By the end of this chapter, you will be able to:
• Understand internal workings of key collection classes
• Utilize advanced features of collections like synchronization, immutability, and navigable views
• Apply comparator and comparable interfaces effectively
• Master generic collections and wildcard usage
• Perform stream-based collection operations
• Optimize performance using concurrent and custom collections

Java Collections are broadly divided into:
• List (ArrayList, LinkedList, Vector)
• Set (HashSet, LinkedHashSet, TreeSet)
• Queue/Deque (PriorityQueue, ArrayDeque)
• Map (HashMap, LinkedHashMap, TreeMap, ConcurrentHashMap)
Each of these implements either Collection or Map interface.

• ArrayList: Backed by an array. Allows random access. Resize-costly.
• LinkedList: Doubly linked list. Efficient insertions/deletions.
• HashSet: Backed by HashMap. No duplicates.
• TreeSet: Uses a Red-Black Tree. Maintains sorted order.
• HashMap: Bucketed key-value pairs using hashing.
• TreeMap: Sorted Map using Red-Black Tree. Implements NavigableMap.

Collections.sort(list, new Comparator() {
public int compare(Student s1, Student s2) {
return s1.getMarks() - s2.getMarks();
}
});

Use Comparable when natural ordering is needed. Use Comparator for custom multi-field sorting.

List readOnlyList = Collections.unmodifiableList(myList);
List threadSafeList = Collections.synchronizedList(new ArrayList<>());

Useful for concurrency and immutability in multi-threaded environments.

NavigableMap map = new TreeMap<>();
map.put(10, "A"); map.put(20, "B"); map.put(30, "C");
System.out.println(map.ceilingEntry(15)); // Entry >= 15
System.out.println(map.floorEntry(25)); // Entry <= 25

Provides sorted views, range queries (subMap, headMap, tailMap).

• Iterator: Basic forward iteration.
• ListIterator: Bidirectional, with modification capabilities.
• Spliterator: Used for parallel processing with Streams.

myList.forEach(System.out::println);
myList.removeIf(name -> name.startsWith("A"));
myList.replaceAll(String::toUpperCase);

These methods enhance readability and reduce boilerplate.

• Prefer ArrayList unless you need frequent insertions/deletions.
• Use HashMap unless order or sorting is needed.
• Avoid premature synchronization. Use concurrent collections when truly needed.
• Use generics with wildcards for reusable APIs.
• For read-heavy applications, CopyOnWriteArrayList is better than synchronizedList.