Problem Solving with Java

Combine variables, control flow, collections, and classes to solve a complete, small problem. Java rewards planning: sketch your classes and methods before typing, since the compiler will hold you to every declared type and signature.

• Identify the "things" in the problem — each usually becomes a class.
• Identify the "actions" on those things — each usually becomes a method.
• Compile early and often; fix the first error the compiler reports before moving to the next.
• Test with realistic sample data, then with edge cases (empty arrays, zero values).

💡 Practical Skill: This step-by-step discipline is exactly what's expected in a professional Java codebase — clear classes, clear methods, and code that compiles cleanly.

import java.util.HashMap;import java.util.ArrayList; public class Main {    static ArrayList<String> checkStock(HashMap<String, Integer> inventory, int threshold) {        ArrayList<String> lowStock = new ArrayList<>();        for (String item : inventory.keySet()) {            if (inventory.get(item) < threshold) {                lowStock.add(item);            }        }        return lowStock;    }     public static void main(String[] args) {        HashMap<String, Integer> inventory = new HashMap<>();        inventory.put("exercise books", 120);        inventory.put("pens", 45);        inventory.put("chalk boxes", 8);         ArrayList<String> reorder = checkStock(inventory, 10);         if (reorder.isEmpty()) {            System.out.println("All stock levels are healthy.");        } else {            System.out.println("Reorder needed for: " + reorder);        }    }}