Java Collection Framework

05 Oct 2022

Java collection framework provides all the interfaces and their implementation to manipulate data and reflect the common data structures used in computer science. In other words, collection framework is a representation of the data structures used in computer sciennce. The key points that needs to be considered while learning Java Collection Framework are following:

• Java collections can store only "Objects"
• Collections are made based on a list of interfaces and implementation classes
• All standard Java collection classes and interfaces are defined in a package called java.util

Collection Interfaces

1. List - List focuses on index and allows duplicate values to be inserted in the collection. List also ensures the order of the collection.

2. Set - Set focuses on hashcode and does not allow duplicate value. It does not retain any order (unordered collection).

3. Map - Map represents a dictionary which contains key-value pair. Any key-value pair is maintained in Map category.

Implementation Classes of List Interface

• ArrayList - Index based dynamically growable array, most suitable for READ operations, not suitable for write operations
• LinkedList - Node based collection, suitable for WRITE operations, not suitable for READ operations
• Vector - Vector is similar to ArrayList but Vector is synchronized.

public class Main {

    public static void main(String[] args){
        ArrayList<Integer> evenNumbers = new ArrayList<>();

        evenNumbers.add(2);
        evenNumbers.add(4);

        System.out.println("Size of the list: " + evenNumbers.size());

        for (int value : evenNumbers) {
            System.out.println(value);
        }
    }
}

Implementation Classes of Set Interface

1. HashSet - Unordered collection, focuses on hashing, avoids duplicates

2. LinkedHashSet - Maintain order based on insertion, avoid duplicates

3. TreeSet - Implements SortedSet (a subtype of Set interface)

Implementation Classes of Map Interface

1. HashMap - Similar to HashSet maintaining Key-Value pair, unordered

2. LinkedHashMap - Similar to LinkedHashSet, maintains key-value pair, retain insertion order

3. TreeMap - Similar to TreeSet, maintains ascending order(Items are sorted based on Key)

4. Hashtable - Legacy class which is unordered, maintains unique keys but synchronized (thread-safe)