Collections In Java and How to Implement Them?

Collections are essential to Java programming, helping you manage and organize data efficiently. This beginner-friendly guide will walk you through the basics of Java Collection Frameworks, features, components, interfaces, and classes. Let’s get started!

What is a Java Collection?

Collections in Java provide an architecture for storing and manipulating a group of objects. They are a set of interfaces and classes designed to hold multiple elements (like lists of data, sets of unique items, queues, etc.) and perform operations such as adding, removing, searching, sorting, and iterating over those elements.

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What is a Collection Framework in Java?

The Collection Framework in Java (JCF) is a unified architecture that provides a set of interfaces, classes, and algorithms to store, retrieve, and manipulate groups of objects in a standardized way. It’s a toolbox full of ready-made data structures (like Lists, Sets, and Maps) and utilities (like sorting and searching) that you can use to handle data easily and efficiently.

The framework is part of the java.util package and greatly simplifies programming by providing powerful data-handling capabilities.

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Features of Collections in Java Framework

• Consistent API: Common methods across different collection classes (like add(), remove(), and size())
• Reusability: You don’t need to reinvent data structures like linked lists, hash tables, or trees
• Efficiency: Collections are highly optimized for performance
• Interoperability: Easy to switch between different collection types thanks to interfaces
• Extensibility: You can create your own collection classes if needed

Components of Collections in Java Framework

The JCF mainly consists of:

import java.util.ArrayList;
import java.util.Collections;
public class Example {
public static void main(String[] args) {
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(5);
numbers.add(3);
numbers.add(8);
Collections.sort(numbers); // Sort the list
System.out.println(numbers); // Output: [3, 5, 8]
}
}

• ArrayList is a class from the Collection Framework in Java
• Collections.sort() is an algorithm provided by the framework

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Hierarchy of Collection Framework in Java

The following image depicts the Java Collections Hierarchy.

Java Collections Interfaces

Collection interfaces in Java are blueprints that define standard behaviors for groups of objects. They are part of the Collection Framework in Java and allow developers to work with different types of collections polymorphically (i.e., coding to an interface, not an implementation).

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Collection Interfaces in Java

Java Collection Interfaces Hierarchy

Collection (Interface)

|

----------------------------

|             |            |

List        Set      Queue

(ArrayList) (HashSet) (PriorityQueue)

|

Deque

(ArrayDeque)

Map (Separate hierarchy)

(HashMap, TreeMap)

Code Example using ArrayList:

import java.util.List;
import java.util.ArrayList;
public class ListExample {
public static void main(String[] args) {
List<String> fruits = new ArrayList<>();        
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Mango");
System.out.println("Fruits List: " + fruits);
}
}

Java Collection Classes

Java Collection Classes are the concrete implementations of the collection interfaces in the Java Collection Framework. They provide ready-to-use data structures like ArrayList, LinkedList, HashSet, TreeSet, PriorityQueue, HashMap, and many others. Each class is designed to handle specific types of data operations efficiently.

Collection Classes in Java

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Types of Collection Classes

1. ArrayList

• Resizable array implementation of List
• Fast random access but slow insertion/deletion (middle)

List<String> list = new ArrayList<>();

2. LinkedList

• Doubly linked list implementation of List and Deque
• Fast insertion/deletion

List<String> list = new LinkedList<>();

3. HashSet

• Implements Set
• No duplicate elements, unordered storage

Set<Integer> set = new HashSet<>();

4. LinkedHashSet

• Like HashSet, but maintains insertion order

Set<String> set = new LinkedHashSet<>();

5. TreeSet

• Sorted set using a Red-Black tree
• Elements stored in ascending order

Set<Integer> set = new TreeSet<>();

6. HashMap

• Stores key-value pairs
• No ordering of keys

Map<String, Integer> map = new HashMap<>();

7. LinkedHashMap

• Like HashMap, but it maintains insertion order

Map<String, Integer> map = new LinkedHashMap<>();

8. TreeMap

• Stores key-value pairs in sorted (ascending) order by keys

Map<String, Integer> map = new TreeMap<>();

Conclusion

Learning Collection Frameworks in Java is essential for efficient real-time programming and easy handling of complex data operations. Once you have a solid grasp of collections, the next important step is to build a strong foundation in software development principles.

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