Introduction

• Collection Interface is the root interface of the Collections Framework in Java.
• It is present in the java.util package.
• It was introduced in JDK 1.2 version.

• Hierarchy of Collection Interface:
  • 

• It represents a group of objects, known as elements.
• It defines the most common methods that every collection class (like List, Queue, Set) should implement.

• Syntax :

  public interface Collection<E> extends Iterable<E>
  {
      // methods (abstract and default)
  }

Methods of Collection Interface:

• Below are the methods defined specifically in the Collection interface:

Program:

• Collection simple generic program that works with any type of collection (List, Set, Queue).

  import java.util.*;
  
  public class CollectionDemo
  {
      public static void printCollection(Collection<String> collection)
      {
          for (String element : collection)
          {
              System.out.println(element);
          }
      }
  
      public static void main(String[] args)
      {
          Collection<String> list = new ArrayList<>();
          list.add("Amit");
          list.add("Deepak");
          printCollection(list);
  
          System.out.println("-----------");
  
          Collection<String> set = new HashSet<>();
          set.add("Kamal");
          set.add("Rahul");
          printCollection(set);
      }
  }

  Output:

  Amit
  Deepak
  -----------
  Rahul
  Kamal

Use of Collection Interface:

• Maximum Flexibility:
  • Use a Collection reference when we want our code should work with any type of collection (List, Set, or Queue) and we don't need special features like indexed access.
• Easy to Maintain:
  • If a method only needs basic operations like add(), remove(), or iterate, using Collection makes it easy to switch between different collection types later.
• Used in APIs:
  • Many libraries and frameworks use Collection in their APIs so that developers can pass any kind of collection without changing their code.

Properties of Collection Interface:

• It is the root interface in the collection hierarchy.
• It is a generic interface — Collection<E> (E represents the type of elements).
• All major collection classes (like ArrayList, HashSet, LinkedList) implement it, directly or indirectly.
• It provides basic operations like add, remove, iterate, size etc. which are common for all collections.
• It can store both homogeneous and heterogeneous elements (but to ensure type-safety we should use generics).