Writing Your First Tests

Small, focused, readable tests

A JUnit Jupiter test is a method annotated with @Test in a test class; neither needs to be public. Each test should check one behaviour and follow the arrange-act-assert (AAA) structure, also called given-when-then: set up inputs and objects, call the method under test, then verify the result. Name tests so a failure explains itself, such as withdrawFailsWhenBalanceIsInsufficient, or use @DisplayName for readable sentences. Use assertions from org.junit.jupiter.api.Assertions (assertEquals(expected, actual), assertTrue, assertThrows) or the fluent AssertJ library (assertThat(actual).isEqualTo(expected)). Remember argument order in JUnit: expected first, then actual, otherwise failure messages are misleading. Keep tests independent: never rely on another test having run first, and create fresh objects in each test. Run them from the IDE, with mvn test or with gradle test, ideally on every save and in CI for every change.

Arrange, act, assert

One behaviour per test, readable names and expected-before-actual.

import static org.junit.jupiter.api.Assertions.*;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

class WalletTest {

    @Test
    @DisplayName("paying less than the balance reduces it by the amount")
    void payReducesBalance() {
        // arrange
        Wallet wallet = new Wallet(1_000);
        // act
        wallet.pay(300);
        // assert
        assertEquals(700, wallet.balance());
    }

    @Test
    void payingMoreThanBalanceThrowsAndLeavesBalanceUnchanged() {
        Wallet wallet = new Wallet(200);

        InsufficientFundsException ex =
            assertThrows(InsufficientFundsException.class, () -> wallet.pay(500));

        assertEquals("requested 500 but only 200 available", ex.getMessage());
        assertEquals(200, wallet.balance());
    }
}

// mvn test          or          ./gradlew test

Make the failure message do the work

A test named test1 failing with "expected 700 but was 1000" sends you hunting. A test named payReducesBalance tells you which rule broke before you open the file.

त्वरित जाँच: In JUnit's assertEquals, what is the conventional argument order?

• actual, expected
• message, actual
• expected, actual
• Any order; it makes no difference to the output