What is the Collections Framework?
It is a unified architecture for representing and manipulating collections. It allows you to store, retrieve, and manipulate groups of data (objects) efficiently.
1. The ArrayList (Dynamic Array)
The ArrayList class is a resizable array. While a standard array is fixed in size, an ArrayList grows automatically as you add elements.
import java.util.ArrayList; // Step 1: Import
public class ListDemo {
public static void main(String[] args) {
ArrayList students = new ArrayList<>();
// Step 2: Add elements
students.add("John");
students.add("Jane");
students.add("Craby");
// Step 3: Access and Remove
System.out.println("Student at index 0: " + students.get(0));
students.remove("Jane");
System.out.println("List Size: " + students.size());
}
}
// Output:
Student at index 0: John
List Size: 2
2. The HashMap (Key-Value Pairs)
A HashMap stores items in "Key/Value" pairs. You can access them by an index of another type (e.g., a String). Think of it like a real dictionary: the word is the Key, and the definition is the Value.
import java.util.HashMap;
public class MapDemo {
public static void main(String[] args) {
HashMap userAges = new HashMap<>();
// Add keys and values (Name, Age)
userAges.put("Alice", 25);
userAges.put("Bob", 30);
// Accessing a value using its key
System.out.println("Alice's Age: " + userAges.get("Alice"));
}
}
// Output:
Alice's Age: 25
3. Key Theory: List vs. Set
List: An ordered collection that allows duplicate elements (e.g., ArrayList, LinkedList).
Set: A collection that does not allow duplicates (e.g., HashSet). If you try to add the same item twice, it simply ignores the second one.
Map: An object that maps keys to values. Keys must be unique!
Human Note: Why use these? In modern development, we almost never use standard Arrays. Whether you are building an Android app or a web server, ArrayList and HashMap are your best friends for handling data that changes over time.
