Testing: JUnit 5 + Mockito — Ship Code You Trust

8. Practice Problems

Try each one, then reveal the answer.

1. Write three plain-Java assertions for an add(int,int) method: 2+3=5, 0+0=0, and -1+1=0. Run them.
   সাধারণ Java দিয়ে একটি add(int,int) method-এর জন্য তিনটি assertion লিখুন — 2+3=5, 0+0=0, -1+1=0।
   ✨ Show Answer

   Main.java

   class Main {
       static int add(int a, int b) { return a + b; }
       static void check(int exp, int got, String t) {
           if (exp != got) throw new AssertionError(t + " failed");
           System.out.println("[OK] " + t);
       }
       public static void main(String[] args) {
           check(5, add(2, 3), "2+3");
           check(0, add(0, 0), "0+0");
           check(0, add(-1, 1), "-1+1");
       }
   }

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2. Why is @BeforeEach preferred over a constructor-initialised field for the "fixture"?
   @BeforeEach-কে constructor দিয়ে field initialise করার চেয়ে কেন বেশি পছন্দ করা হয়?
   ✨ Show Answer

   Answer: JUnit 5 creates a new instance of the test class for every test method, so a field assignment in the constructor also works. BUT @BeforeEach is clearer to the reader, survives any future change to JUnit's lifecycle, and pairs naturally with @AfterEach cleanup — it makes the setup/teardown intent explicit.

3. Build a tiny hand-rolled mock: an Emailer interface with send(to, body), a fake that records every call into a List, and verify the count.
   একটি ছোট Mockito-মতো mock বানান — Emailer interface, একটি ভুয়া implementation যা সব call record করবে, তারপর count verify করুন।
   ✨ Show Answer

   Main.java

   import java.util.*;
   
   interface Emailer { void send(String to, String body); }
   
   class FakeEmailer implements Emailer {
       List<String> calls = new ArrayList<>();
       public void send(String to, String body) { calls.add(to + ":" + body); }
   }
   
   class Notifier {
       private final Emailer e;
       Notifier(Emailer e) { this.e = e; }
       void welcome(String user) { e.send(user, "Welcome!"); }
   }
   
   class Main {
       public static void main(String[] args) {
           FakeEmailer fake = new FakeEmailer();
           Notifier n = new Notifier(fake);
           n.welcome("fatima@abcltech.com");
           if (fake.calls.size() != 1) throw new AssertionError("expected 1 email");
           System.out.println("[OK] recorded " + fake.calls);
       }
   }

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4. Name three signs of a "flaky" test and one strategy to eliminate each.
   একটি test flaky কিনা বোঝার তিনটি লক্ষণ বলুন ও প্রতিটির সমাধান দিন।
   ✨ Show Answer

   Answer: (1) Depends on wall-clock time — use a Clock that can be injected. (2) Depends on a shared database — use per-test transactions that roll back, or Testcontainers for isolation. (3) Depends on iteration order of a HashMap — use LinkedHashMap or assert on a sorted view.

   (১) wall-clock-এর উপর নির্ভর — inject-করা Clock ব্যবহার করুন। (২) shared database-এর উপর নির্ভর — per-test transaction বা Testcontainers। (৩) HashMap-এর iteration order — LinkedHashMap বা sorted view-এর উপর assert।

5. Demonstrate parameterized testing manually: loop over three test-case arrays and run the same assertion for each.
   হাতে-হাতে parameterized test বানান: তিনটি test-case array-র উপর loop চালিয়ে একই assertion করুন।
   ✨ Show Answer

   Main.java

   class Main {
       static boolean isEven(int n) { return n % 2 == 0; }
   
       public static void main(String[] args) {
           int[] inputs   = { 0, 1, 2, 3, 100 };
           boolean[] expect = { true, false, true, false, true };
           for (int i = 0; i < inputs.length; i++) {
               boolean got = isEven(inputs[i]);
               if (got != expect[i]) throw new AssertionError("case " + i);
               System.out.println("[OK] isEven(" + inputs[i] + ") = " + got);
           }
       }
   }

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