Collections Framework

☕ Java Lesson 14 Advanced

Unlike raw arrays, Java Collections provide dynamic resizing frameworks. The framework includes lists, sets, and key-value maps to store objects.

1 Collection Architecture: List, Set, and Map

The primary elements of the collection framework are:

List (ArrayList): An ordered container that allows duplicates. Excellent for random index access.
Set (HashSet): An unordered container that rejects duplicates. Quick validation checks for existence.
Map (HashMap): Stores key-value pairings (e.g. usernames paired with user IDs). Keys must be unique.

⚠️ Wrapper Class Notice: Collections can only store Objects, not raw primitives. If you want to store integers inside an ArrayList, Java uses Auto-boxing to convert `int` primitives into their object wrappers automatically: `ArrayList<Integer> list = new ArrayList<>();`.

2 Collections Operations

Let's run a program declaring Lists, Sets, and Maps, performing insertions and reads:

Java — Collections Framework ▶ Run Code

import java.util.ArrayList;
import java.util.HashSet;
import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // ArrayList: Ordered list
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Apple"); // Duplicates allowed
        System.out.println("List: " + list);

        // HashSet: Unique values
        HashSet<String> set = new HashSet<>();
        set.add("Apple");
        set.add("Banana");
        set.add("Apple"); // Duplicate ignored
        System.out.println("Set: " + set);

        // HashMap: Key-Value pairs
        HashMap<String, Integer> map = new HashMap<>();
        map.put("Alice", 95);
        map.put("Bob", 88);
        System.out.println("Map: " + map);
        System.out.println("Bob's Score: " + map.get("Bob"));
    }
}

3 Code Challenge

Challenge: Write a program that instantiates an `ArrayList` of integers, adds 5 arbitrary values, removes the value at index 2, and prints the list. Then declare a `HashMap` to map product names (String) to prices (Double). Add items and print price values.