Sometime back I wrote about Collections Framework. Here let me take a slightly deeper look into some of the concrete implementations with examples.

List Implementations

ArrayList: Simply put, this provides dynamic array implementation in Java.

LinkedList: Implementation of Linked List data structure. Provides methods like add and remove.

Vector: It is a synchronized ArrayList.

Stack: Implementation of Stack data structure. Provides methods like push and pop.

Queue Implementations

PriorityQueue: Implement First In, First Out. If Comparator is provided, it will be used for Priority management.

public class PriorityQueueDemo {
    
  public static void main(String args[])
  {
      PriorityQueue<Integer> pQueue = new PriorityQueue<Integer>();

      pQueue.add(10);
      pQueue.add(20);
      pQueue.add(15);

      System.out.println(pQueue.peek());
      System.out.println(pQueue.poll());
      System.out.println(pQueue.peek());
  }
}

Output

10
10
15

Dequeue Implementation

ArrayDequeue: Double-ended queue implementation. Provides methods like add, addFirst, addLast.

Set Implementations

HashSet: Implementation of hash table data structure. Do not guarantee order.

LinkedHashSet: Uses a doubly linked list and hence maintains the order.

Sorted Set Implementation

TreeSet: Ordering is maintained as natural order or explicit comparator based ordering.

public class TreeSetDemo {  
  public static void main(String args[])
  {
      TreeSet<String> ts = new TreeSet<String>();

      ts.add("this");
      ts.add("is");
      ts.add("just");
      ts.add("a");
      ts.add("test");

      Iterator<String> itr = ts.iterator();
      while (itr.hasNext()) {
          System.out.println(itr.next());
      }
  }
}

Output

a
is
just
test
this