Java Collections Framework

Key Interfaces

• Collection: Represents a group of objects. Includes subinterfaces:
  • List: Ordered collection allowing duplicates. Examples: ArrayList, LinkedList.
  • Set: Collection without duplicates. Examples: HashSet, TreeSet.
  • Queue: Holds elements before processing. Examples: PriorityQueue, LinkedList.
• Map: Maps keys to values, no duplicate keys. Examples: HashMap, TreeMap.

Important Classes

• ArrayList: Resizable-array implementation of List.
• LinkedList: Doubly-linked list implementation of List.
• HashSet: Implements Set using a hash table.
• TreeSet: Implements Set using a tree.
• HashMap: Hash table based implementation of Map.
• TreeMap: Red-black tree based implementation of Map.

Main Concepts

• Generics: Provides type-safe collections, e.g., ArrayList<String>.
• Iterators: Allows iterating over elements of a collection.
• Comparable and Comparator: Interfaces for natural ordering and custom sorting.
• Concurrency: Thread-safe collections like ConcurrentHashMap.
• Utility Classes: Provides methods for sorting, searching, etc.

Common Operations

• Adding: add(E element), addAll(Collection<? extends E> c).
• Removing: remove(Object o), clear().
• Accessing: get(int index), contains(Object o).
• Sorting: Collections.sort(List<T> list).

Best Practices

• Choose collection type based on requirements (e.g., List vs. Set).
• Use generics for type safety.
• Consider concurrency needs for thread safety.

Example Usage

List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add("C++");

for (String lang : list) {
    System.out.println(lang);
}

Map<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

System.out.println(map.get("two")); // prints 2
    

The Java Collections Framework provides powerful tools for managing collections of objects efficiently, essential for a wide range of Java applications.