Java Collections

What is the Collections API?

The Collections API is a set of classes and interfaces that support operations on collections of objects.

1. Collection extends Iterable
2. An iterator is an object that enables t traverse through a collection and to remove elements from the collection selectively, if required.
3. Set

• Set is a Collection that cannot contain duplicate elements.
• Set interface contains only methods from Collection.
• Two Set instances are equal if they have same elements.
• There are three Set implementations

      HashSet –

• Stores the elements in a hash table
• Uses hashtable to store the elements.It extends AbstractSet class and implements Set interface.
• Contains unique elements only.

      TreeSet – orders the elements based on their values

       List<String> li = new ArrayList<String>();
       TreeSet<String> myset = new TreeSet<String>(li);

• Removing duplicates from array.

               String[] strArr = {"one","two","three","four","four","five"};
               List<String> tmpList = Arrays.asList(strArr);

               TreeSet<String> unique = new TreeSet<String>(tmpList);

      LinkedHashSet – orders the elements based on the order in which it is inserted

• Bulk operations:

              If s1 and s2 are two sets:
              s1. containsAll(s2) – returns true if s2 is a subset of s1
              s1.addAll(s1) – transforms s1 into the union of s1 and s2
              s1.retainAll(s2) – transforms s1 into the intersection of s1 and s2
              s1.removeAll(s2) – transforms s1 into the set difference of s1 and s24

1. Arrays,Collections are utility class available in java.util package.