Introduction to Collections in Java

Introduction to Collections in Java

Java Collections Framework provides a set of interfaces and classes to store and manipulate groups of objects efficiently. It is a unified architecture to represent and manage collections such as lists, sets, queues, and maps.

Main Collection Interfaces

• List: Ordered collection that allows duplicate elements. Examples include ArrayList and LinkedList.
• Set: Collection that does not allow duplicate elements. Examples include HashSet and TreeSet.
• Queue: Ordered collection designed for holding elements prior to processing. Examples include PriorityQueue and LinkedList.
• Map: Key-value pairs collection that does not allow duplicate keys. Examples include HashMap and TreeMap.

Hierarchy of Collections

The Java Collections Framework is organized as follows:

Collection
|
|-- List       (e.g., ArrayList, LinkedList)
|
|-- Set        (e.g., HashSet, TreeSet)
|
|-- Queue      (e.g., LinkedList, PriorityQueue)
|
|-- Map        (e.g., HashMap, TreeMap)
            

Examples of Key Collections

List Example

List list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
list.add("Apple"); // Allows duplicates
for (String item : list) {
    System.out.println(item);
}
            

Set Example

Set set = new HashSet<>();
set.add("Apple");
set.add("Banana");
set.add("Apple"); // Duplicate ignored
for (String item : set) {
    System.out.println(item);
}
            

Map Example

Map map = new HashMap<>();
map.put("Apple", 1);
map.put("Banana", 2);
map.put("Apple", 3); // Overwrites key
for (Map.Entry entry : map.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}
            

Conclusion

The Java Collections Framework simplifies the storage, retrieval, and manipulation of data in Java programs. By choosing the right collection, developers can optimize their code for performance and clarity.