Why Tests & JUnit's Shape

You already test - every time you run a program and squint at the output, you're testing. The problem: you have to rerun everything after every change, and you eventually get lazy. Automated tests are programs whose only job is to run your other code and complain loudly when it's wrong.

What the compiler cannot catch

static double average(int[] scores) {
    int sum = 0;
    for (int s : scores) sum += s;
    return sum / scores.length;        // compiles. runs. wrong.
}

Integer division silently truncates: average({1, 2}) is 1.0, not 1.5. The compiler saw valid types and valid syntax - the logic is your problem. Tests are how logic bugs get caught, ideally before your users find them.

JUnit: the shape

JUnit 5 is the standard Java test framework. In real Maven projects the tests live in src/test/java, one test class per production class:

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

class ScoreAverageTest {

    @Test
    void averagesTwoScores() {
        assertEquals(1.5, ScoreAverage.average(new int[]{1, 2}));
    }

    @Test
    void emptyScoresThrow() {
        assertThrows(IllegalArgumentException.class,
            () -> ScoreAverage.average(new int[0]));
    }
}

The vocabulary:

• @Test - marks a method the runner executes. Each test is independent and runs in a fresh instance.
• assertEquals(expected, actual) - the workhorse. Note the order: expected first. The failure message prints both, so order makes failures readable.
• assertTrue / assertFalse, assertNull / assertNotNull, assertThrows - the rest of the everyday set.
• @BeforeEach - a method run before each test: build fresh fixtures there instead of copy-pasting setup.

Arrange, Act, Assert

Read every test as three beats:

@Test
void withdrawReducesBalance() {
    // Arrange
    Account a = new Account(100);
    // Act
    a.withdraw(30);
    // Assert
    assertEquals(70, a.balance());
}

One behavior per test. If you can't name the test's single behavior in a sentence, it's two tests.