Free course · Lesson 3 of 5

Arrays & Basic Collections

In this lesson you will

Store several values in an array and loop over them
Use ArrayList and HashMap for data that grows

An array holds a fixed number of values of the same type. For lists that need to grow or shrink, Java's Collections Framework provides ArrayList — flexible, but still type-safe thanks to generics like ArrayList<String>.

int[] scores = {78, 91, 64}; — fixed-size, fast, type-checked.
ArrayList<String> names = new ArrayList<>(); — resizable list.
HashMap<String, Integer> stores key-value pairs, Java's equivalent of a dictionary.
The enhanced for loop (for (String name : names)) reads cleanly.

💡 Practical Skill: Choosing the right collection — array, list, or map — is one of the most common real-world Java decisions, from processing form submissions to building reports.

Worked exampleFiltering Passing Scores

Java

import java.util.ArrayList;import java.util.HashMap; public class Main {    public static void main(String[] args) {        HashMap<String, Integer> scores = new HashMap<>();        scores.put("Amina", 78);        scores.put("Deng", 91);        scores.put("Grace", 64);         ArrayList<String> passed = new ArrayList<>();        for (String name : scores.keySet()) {            if (scores.get(name) >= 70) {                passed.add(name);            }        }         System.out.println("Passed: " + passed);    }}

Practice exercise

Store seven daily temperatures in an array, then print the highest, the lowest and the average.

Key takeaways

Array indexes start at 0 and end at length - 1
ArrayList grows as you add items; HashMap finds values by key
Most data tasks are a loop that totals, counts or finds a maximum

In the full program Module 3: Arrays, Strings and ArrayList See how this topic is taught in depth, with practical activities and an assessment.

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