Java
Test layer
Write tests with JUnit and Mockito.
LedgerService and its test
In this lesson you will write a LedgerService class and its unit test. The service is a thin layer that adds records and counts them.
class LedgerService {
ArrayList<Transaction> ledger = new ArrayList<>();
void add(Transaction t) { ledger.add(t); }
int getCount() { return ledger.size(); }
}
The test class LedgerServiceTest adds one record and verifies getCount() with assertEquals(1, ...). This adds a reliable test layer to the app.
Verifying with assertEquals
assertEquals(expected, actual) compares the expected and actual values. If they are equal the test passes; otherwise it throws and fails. This is the basic verification tool for unit tests.
@Test
void testGetCount() {
LedgerService s = new LedgerService();
s.add(new Transaction("market", 120));
assertEquals(1, s.getCount());
}
Here, after adding one record, getCount() should return 1. The first argument is expected, the second is actual. Keeping tests in a separate class (not in main) keeps production code clean.
Exercise Write LedgerServiceTest
Define a LedgerServiceTest class. Write a @Test method testGetCount that adds one record and calls assertEquals(1, s.getCount()). In Main, create a LedgerService, add two records and print service.getCount() with System.out.println.
Show example solution
import java.util.ArrayList;
import java.util.concurrent.*;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class Transaction {
String category;
int amount;
Transaction(String category, int amount) {
this.category = category;
this.amount = amount;
}
}
class LedgerService {
ArrayList<Transaction> ledger = new ArrayList<>();
void add(Transaction t) { ledger.add(t); }
int getCount() { return ledger.size(); }
}
class LedgerServiceTest {
@Test
void testGetCount() {
LedgerService s = new LedgerService();
s.add(new Transaction("market", 120));
assertEquals(1, s.getCount());
}
}
public class Main {
public static void main(String[] args) throws Exception {
LedgerService service = new LedgerService();
service.add(new Transaction("market", 120));
service.add(new Transaction("fuel", 80));
System.out.println(service.getCount());
}
}
Test yourself
-
Complete the @Test annotation
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class LedgerServiceTest {
____
void testGetCount() {
LedgerService s = new LedgerService();
s.add(new Transaction("market", 120));
assertEquals(1, s.getCount());
}
}
Show answer
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class LedgerServiceTest {
@Test
void testGetCount() {
LedgerService s = new LedgerService();
s.add(new Transaction("market", 120));
assertEquals(1, s.getCount());
}
}
-
When does assertEquals(1, s.getCount()) fail?
- When @Test is missing
- When the class compiles
- When there is no main
- When getCount() does not return 1
Show answer
When getCount() does not return 1 — If getCount() returns something other than 1, the assertion fails.
-
Match the test concepts
- LedgerService
- Adds and counts
- LedgerServiceTest
- Test class
- @Test
- Test method marker
- assertEquals
- Value verification
Next lesson: Putting it together
