Java Collections

Working with groups of objects in Java

📦 What are Java Collections?

Java Collections are frameworks that provide data structures like lists, sets, and maps to store and manipulate groups of objects efficiently with built-in methods. 


// Simple ArrayList example
import java.util.ArrayList;

ArrayList<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
System.out.println(fruits); // [Apple, Banana]

Types of Collections

📋

ArrayList

Dynamic arrays that can grow 

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(10);
numbers.add(20);
🔗

LinkedList

Doubly-linked list implementation 

LinkedList<String> names = new LinkedList<>();
names.addFirst("John");
names.addLast("Jane");
🎯

HashSet

Unique elements only 

HashSet<String> colors = new HashSet<>();
colors.add("Red");
colors.add("Blue");
🗺️

HashMap

Key-value pairs storage 

HashMap<String, Integer> ages = new HashMap<>();
ages.put("Alice", 25);
ages.put("Bob", 30);

🔹 ArrayList Methods

Common methods for working with ArrayLists: 

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        
        // Adding elements
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        
        // Getting elements
        System.out.println("First fruit: " + fruits.get(0));
        
        // Size of list
        System.out.println("Total fruits: " + fruits.size());
        
        // Removing elements
        fruits.remove("Banana");
        
        // Check if contains
        if (fruits.contains("Apple")) {
            System.out.println("We have apples!");
        }
        
        // Print all fruits
        for (String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}

Output:

First fruit: Apple
Total fruits: 3
We have apples!
Apple
Orange

🔹 HashMap Example

Working with key-value pairs: 

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> studentGrades = new HashMap<>();
        
        // Adding key-value pairs
        studentGrades.put("Alice", 95);
        studentGrades.put("Bob", 87);
        studentGrades.put("Charlie", 92);
        
        // Getting values
        System.out.println("Alice's grade: " + studentGrades.get("Alice"));
        
        // Check if key exists
        if (studentGrades.containsKey("Bob")) {
            System.out.println("Bob's grade found!");
        }
        
        // Print all entries
        for (String name : studentGrades.keySet()) {
            System.out.println(name + ": " + studentGrades.get(name));
        }
    }
}

Output:

Alice's grade: 95
Bob's grade found!
Alice: 95
Bob: 87
Charlie: 92

🔹 HashSet Example

Working with unique elements: 

import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        HashSet<String> uniqueColors = new HashSet<>();
        
        // Adding elements
        uniqueColors.add("Red");
        uniqueColors.add("Blue");
        uniqueColors.add("Red"); // Duplicate - won't be added
        
        System.out.println("Colors: " + uniqueColors);
        System.out.println("Size: " + uniqueColors.size());
        
        // Check if contains
        if (uniqueColors.contains("Blue")) {
            System.out.println("Blue is in the set!");
        }
    }
}

Output:

Colors: [Red, Blue]
Size: 2
Blue is in the set!

🧠 Test Your Knowledge

Which collection allows duplicate elements?