Java Collections
Understanding Java's powerful data structure framework
📦 What are Java Collections?
Java Collections Framework provides pre-built data structures and algorithms to store, organize, and manipulate groups of objects efficiently in your programs.
// Simple Collections example
import java.util.*;
List<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
System.out.println(fruits); // [Apple, Banana]
Output:
[Apple, Banana]
Key Collection Types
List
Ordered collection that allows duplicates
List<Integer> numbers = new ArrayList<>();Set
Collection with unique elements only
Set<String> unique = new HashSet<>();Map
Key-value pairs for data mapping
Map<String, Integer> ages = new HashMap<>();Queue
First-in-first-out data structure
Queue<String> tasks = new LinkedList<>();🔹 Collections Hierarchy
Understanding the relationship between different collection types:
// Collection Interface (Root)
// ├── List Interface
// │ ├── ArrayList (most common)
// │ ├── LinkedList
// │ └── Vector
// ├── Set Interface
// │ ├── HashSet (most common)
// │ ├── LinkedHashSet
// │ └── TreeSet
// └── Queue Interface
// ├── LinkedList
// └── PriorityQueue
// Map is separate (not part of Collection)
// Map Interface
// ├── HashMap (most common)
// ├── LinkedHashMap
// └── TreeMap🔹 Basic Collections Operations
Common operations you'll use with all collections:
import java.util.*;
public class CollectionsBasics {
public static void main(String[] args) {
// Creating a collection
List<String> colors = new ArrayList<>();
// Adding elements
colors.add("Red");
colors.add("Blue");
colors.add("Green");
// Checking size
System.out.println("Size: " + colors.size());
// Checking if contains element
System.out.println("Has Red: " + colors.contains("Red"));
// Iterating through collection
for (String color : colors) {
System.out.println(color);
}
// Removing element
colors.remove("Blue");
System.out.println("After removal: " + colors);
}
}Output:
Size: 3
Has Red: true
Red
Blue
Green
After removal: [Red, Green]
🔹 When to Use Which Collection
Quick Selection Guide:
- ArrayList: When you need fast access by index and don't mind slower insertions/deletions
- LinkedList: When you frequently insert/delete elements in the middle
- HashSet: When you need unique elements and fast lookups
- HashMap: When you need key-value pairs with fast access
- TreeSet/TreeMap: When you need sorted collections