Java Collections

A collection is a object that groups multiple elements into a single unit.

List of core collection Java interfaces.

  • Collection – This interface is the least common denominator that all collections implement and is used to pass collections around and manipulate them where maximum generality is desired.
  • Set – A collection that cannot contain duplicate elements
  • List – A ordered collection that can contain duplicate elements. Elements can be accessed by their integer index.
  • Queue – A collection which orders elements in first-in, first-out manner.
  • Map – An object which maps keys to values. There cannot be duplicate keys.
  • SortedSet – A set that maintains its elements in ascending order.
  • SortedMap – A map that maintains its mappings in ascending order.

Ways to traverse a collection.

  • for-each
for (Object o : collection)
  • Iterators – It enables to traverse a collection and to remove elements selectively. Iterator.remove is the only safe way to modify a collection during iteration
static void filter(Collection<?> c) {
    for (Iterator<?> it = c.iterator(); it.hasNext(); )
        if (!cond(

toArray method – translates collection to array.

Object[] a = c.toArray();
String[] a = c.toArray(new String[0]);

Set Implementations.

  • HashSet – Stores elements in hash table. Good performance, but makes no guarantee about order.
  • TreeSet – Stores elements in red-black tree, orders elements by values, slower than HashSet. SortedSet implementation.
  • LinkedHashSet – Implements a hash table with a linked list running through it, orders elements based on the order in which they were inserted.

List Implementations.

  • ArrayList – provides constant time positional access and is fast
  • LinkedList – offers better performance under certain circumstances. If you add elements to beginning of list or iterate over the list to delete elements.

Map Implementations.

  • HashMap – Provides maximum speed, but not ordered.
  • TreeMap – Ordered by the keys. SortedMap implementation.
  • LinkedHashMap – Provides intermediate performance of the above two implementation. Ordered by insertion.



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