📊 Java Data Structures – Master Collections, Lists, and Maps (2025 Guide)

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🧲 Introduction – Why Learn Java Data Structures?

Java provides a robust Collections Framework to handle, store, and manipulate dynamic groups of objects efficiently. Understanding Java data structures helps in writing scalable, efficient, and organized programs.

🎯 In this guide, you’ll learn:

• What Java data structures are and why they matter
• How to use ArrayList, LinkedList, and HashMap
• How to sort lists, use iterators, and understand wrapper classes

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📌 Topics Covered

🔢 Topic	📘 Description
Java Data Structures	Overview of collection types and utilities
Java ArrayList	Dynamic arrays with fast access
Java LinkedList	Node-based list with quick insertions/deletions
Java List Sorting	Sorting elements using Collections.sort()
Java HashMap	Key-value pair data structure
Java Iterator	Sequential access and safe element removal
Java Wrapper Classes	Converting primitives into objects (e.g., int → Integer)

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📂 Java Data Structures – The Foundation

Java’s data structures allow you to store and organize data in different ways:

• List → Ordered collection
• Set → Unordered unique elements
• Map → Key-value pairs

We’ll focus on List and Map, two of the most commonly used.

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📋 Java ArrayList – Resizable Array

The ArrayList class provides a dynamic array with fast random access.

🧪 Example:

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> colors = new ArrayList<>();
    colors.add("Red");
    colors.add("Blue");
    colors.add("Green");

    System.out.println(colors.get(1)); // Output: Blue
  }
}

🔍 Line-by-Line Explanation:

• ArrayList<String> → Declares a dynamic list of strings.
• .add(...) → Adds items to the list.
• .get(1) → Retrieves element at index 1.

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🔗 Java LinkedList – Efficient Insertions

LinkedList allows faster insertions/removals compared to ArrayList.

🧪 Example:

import java.util.LinkedList;

public class Main {
  public static void main(String[] args) {
    LinkedList<String> items = new LinkedList<>();
    items.add("Milk");
    items.addFirst("Bread");
    items.addLast("Eggs");

    System.out.println(items); // Output: [Bread, Milk, Eggs]
  }
}

🔍 Line-by-Line Explanation:

• addFirst() → Inserts at the beginning.
• addLast() → Inserts at the end.

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🗂️ Java List Sorting – Organize Data

Use Collections.sort() to sort a list of elements.

🧪 Example:

import java.util.*;

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(5, 2, 8, 1));
    Collections.sort(numbers);
    System.out.println(numbers); // Output: [1, 2, 5, 8]
  }
}

🔍 Line-by-Line Explanation:

• Arrays.asList(...) → Creates list from array.
• Collections.sort(...) → Sorts the list in ascending order.

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🧠 Java HashMap – Key-Value Store

Use HashMap when you want to store data with unique keys.

🧪 Example:

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    HashMap<String, Integer> ageMap = new HashMap<>();
    ageMap.put("Alice", 28);
    ageMap.put("Bob", 35);

    System.out.println(ageMap.get("Bob")); // Output: 35
  }
}

🔍 Line-by-Line Explanation:

• .put(key, value) → Inserts key-value pair.
• .get(key) → Retrieves value using the key.

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🔁 Java Iterator – Traverse Collections Safely

Iterators allow sequential access and element removal during iteration.

🧪 Example:

import java.util.*;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> fruits = new ArrayList<>(List.of("Apple", "Banana", "Cherry"));
    Iterator<String> it = fruits.iterator();

    while (it.hasNext()) {
      System.out.println(it.next());
    }
  }
}

🔍 Line-by-Line Explanation:

• .iterator() → Creates an iterator object.
• .hasNext() → Checks for more elements.
• .next() → Retrieves next element.

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🧱 Java Wrapper Classes – Primitives as Objects

Wrapper classes encapsulate primitive types into objects.

Primitive	Wrapper Class
int	Integer
char	Character
boolean	Boolean

🧪 Example:

public class Main {
  public static void main(String[] args) {
    int num = 10;
    Integer wrappedNum = num;  // Autoboxing
    System.out.println(wrappedNum);
  }
}

🔍 Line-by-Line Explanation:

• Integer wrappedNum = num; → Converts primitive to object automatically.

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📌 Summary – Recap & Next Steps

Understanding and applying data structures is essential for real-world Java programming.

🔍 Key Takeaways:

• ArrayList and LinkedList manage dynamic lists efficiently
• HashMap offers quick key-based access to data
• Iterator lets you loop and remove safely
• Wrapper Classes let primitives act like objects
• Collections.sort() helps organize your list data

⚙️ What’s Next?

• Explore HashSet, TreeMap, and PriorityQueue
• Learn custom sorting with Comparator
• Build CRUD apps using ArrayList and HashMap

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❓ Frequently Asked Questions (FAQs)

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❓ When should I use ArrayList vs LinkedList?
✅ Use ArrayList for fast access and LinkedList for frequent insertions/deletions.

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❓ Can I store primitive data types in collections?
❌ No. Use wrapper classes like Integer, Double, etc., since collections store objects.

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❓ Is HashMap ordered?
❌ No. Use LinkedHashMap or TreeMap if you need insertion or sorted order.

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❓ How do I remove elements while iterating a list?
✅ Use an iterator and its remove() method to safely remove elements.

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❓ What’s the difference between autoboxing and unboxing?
✅ Autoboxing converts primitives to objects.
✅ Unboxing converts objects back to primitives.

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