Collections: ArrayList & HashMap

Store growing lists and key–value data with Java’s powerful collection classes.

🎯 What you will learn

Use ArrayList for growing lists
Use HashMap for key–value data
Loop with for-each

👋 Let’s get started!

Arrays have a fixed size — but real apps grow! 📈 Java’s collections handle lists that grow and lookups by key, just like Python lists and dictionaries.

🪜 Step by step

1. 📋 ArrayList

Import it, then add(), get() and size().

import java.util.ArrayList;

ArrayList<String> songs = new ArrayList<>();
songs.add("Believer");
songs.add("Happy");
System.out.println(songs.get(0) + " / " + songs.size());

2. 🗝️ HashMap

put() a key and value, get() it back.

import java.util.HashMap;

HashMap<String, Integer> stock = new HashMap<>();
stock.put("apples", 12);
stock.put("pears", 4);
System.out.println(stock.get("apples"));

3. 🔁 for-each

Loop through every item easily.

for (String s : songs) {
    System.out.println("♪ " + s);
}

🧪 Try it!

Read the code, guess what it prints, then press 👀 Show output.

Word counter

String text = "the cat and the dog and the bird";
HashMap<String, Integer> counts = new HashMap<>();
for (String w : text.split(" ")) {
    counts.put(w, counts.getOrDefault(w, 0) + 1);
}
System.out.println("the = " + counts.get("the"));
System.out.println("and = " + counts.get("and"));

Shopping list

ArrayList<String> list = new ArrayList<>();
list.add("milk");
list.add("eggs");
System.out.println(list.size());

📖 Key words

Tap a card to flip it over!

ArrayList: A list that can grow 📋
HashMap: Key–value storage 🗝️
Generics <T>: Says what type a collection holds 🏷️
for-each: Loops over every item 🔁

🏆 Challenge

Predict the output.

Challenge

ArrayList<Integer> nums = new ArrayList<>();
nums.add(5); nums.add(10); nums.add(15);
nums.remove(0);
System.out.println(nums);

Knowledge Check Quiz

How do you add to an ArrayList?
What does map.get("k") return?
What does ArrayList<String> mean?