Collections API 1

Collection:-

• Collection is a single object which manages a group of objects. And in collection we call object as an elements.
• Before the release of the JAVA SE 5.0 collections maintained references to object of type Object so, from that any type of an object can be stored in the collections.
• After Java SE 5.0 onwards, we can use the generic collections.
• By using generic collection we can maintain the object without type casting.

Collection API which contains interfaces that group objects as one of the following:-

• Collection:-

A group of an objects known as an elements; implementations determine whether there is specific ordering and whether duplicates are permitted.

• Set:-

Set is an unordered collection and in Set interface no duplicates are permitted.

• List:-

List is an ordered collection and in List interface duplicates are permitted.

Collection implementations:-

In the collection framework Set, List, Map and Deque are the general purpose implementations of the core interfaces.

A Set Example:-

1. import java.util.*;
2. public class SetDemo
3. {
4. public static void main(String args[])
5. {
6. Set set = new HashSet();
7. set.add(“one”);
8. set.add(“second”);
9. set.add(“3rd”);
10. set.add(new Integer(4));
11. set.add(new Float(5.0F));
12. set.add(“second”); // duplicate is not added
13. set.add(new Integer(4)); // duplicate is not added
14. System.out.println(set);
15. }
16. }

Output:-

[one, second, 5.0, 3rd 4]

• In above example, set variable is declared of type Set which is initialized to a new HashSet Object.
• Then it uses it’s add method to add few elements and prints the set standard output.
• Lines 12 and 13 attempt to add duplicate values to set. Because the duplicates are not allowed in Set interface add method returns false.
• And also we can see that the order of the elements in the output is not the same as the order which they were added.
• Line 14 prints the set object to the standard output. This works cause the HashSet class overrides the inherited toString method and creates a comma separated sequence.

A List Example:-

1. import java.util.*;
2. public class ListDemo
3. {
4. public static void main(String args[])
5. {
6. List list = new ArrayList();
7. list.add(“one”);
8. list.add(“second”);
9. list.add(“3rd”);
10. list.add(new Integer(4));
11. list.add(new Float(5.0F));
12. list.add(“second”); // duplicate is added
13. list.add(new Integer(4)); // duplicate is added
14. System.out.println(list);
15. }
16. }

Output:-

[one, second, 3rd, 4, 5.0, second, 4]

• In the above example, List interface is almost similar as a Set interface.
• In above example, list variable is declared of type List and which is assigned to the new ArrayList Object.
• After that by using add method few elements are added and prints the list to the standard output.
• Lines 12 and 13 attempt to add duplicate values to the list. Because the duplicates are allowed in List interface add method returns true.
• And also we can see that the order of the elements in the output is same as the order which they were added.
• Line 14 prints the list object to the standard output.

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