3 · Reliability and project · 45 MIN

Project · Inventory with checked updates

An update should either preserve invariants or fail before mutation.

Inventory operations enforce nonnegative stock and checked integer addition. Looking up a missing product returns zero for this deliberately chosen contract. Validate a requested removal before writing the new value; otherwise a rejected purchase could still corrupt stock. Assertions provide a tiny dependency-free starting point for tests, but they are disabled unless java runs with -ea. An application with persistent inventory also needs a database transaction or conditional update because this local object does not coordinate multiple processes.

Test state after failure, not only the returned boolean.

Read the example

import java.util.*;
public class Main {
  static final class Inventory {
    private final Map<String,Integer> stock = new HashMap<>();
    int get(String id) { return stock.getOrDefault(id,0); }
    void add(String id,int count) {
      if (count < 0) throw new IllegalArgumentException("negative quantity");
      stock.put(id,Math.addExact(get(id),count));
    }
    boolean buy(String id,int count) {
      if (count <= 0) throw new IllegalArgumentException("positive quantity required");
      int available = get(id);
      if (available < count) return false;
      stock.put(id,available-count); return true;
    }
  }
  public static void main(String[] args) {
    var inventory = new Inventory(); inventory.add("book",3);
    assert inventory.buy("book",2);
    assert !inventory.buy("book",2);
    assert inventory.get("book") == 1;
    System.out.println("inventory checks passed");
  }
}

Check the expected output

Run java -ea Main: inventory checks passed

Your challenge

Move the assertions into a repeatable test suite; add validation for blank product IDs and document concurrency limitations.

Solution cost: Expected O(1) map operations per stock update. time · Account for collection storage separately from the returned result. space

Common trap

Without -ea, assertions are skipped; never put required application work in assert expressions outside this test-only main.

Study the project implementation

import java.util.*;
public class Main {
  static final class Inventory {
    private final Map<String,Integer> stock = new HashMap<>();
    int get(String id) { return stock.getOrDefault(id,0); }
    void add(String id,int count) {
      if (count < 0) throw new IllegalArgumentException("negative quantity");
      stock.put(id,Math.addExact(get(id),count));
    }
    boolean buy(String id,int count) {
      if (count <= 0) throw new IllegalArgumentException("positive quantity required");
      int available = get(id);
      if (available < count) return false;
      stock.put(id,available-count); return true;
    }
  }
  public static void main(String[] args) {
    var inventory = new Inventory(); inventory.add("book",3);
    assert inventory.buy("book",2);
    assert !inventory.buy("book",2);
    assert inventory.get("book") == 1;
    System.out.println("inventory checks passed");
  }
}

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