Project setup

Tests live in the same project as the code they check. Maven needs two things to run them: the JUnit library in the dependencies and the Surefire plugin, which finds test classes and runs them in the test phase. This lesson does not teach Maven itself — only the part you need for tests.

Creating the project

Any Maven project will do. A new one is created by the standard archetype:

mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes \
  -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.5 \
  -DgroupId=com.example -DartifactId=blog -Dpackage=com.example.blog \
  -DinteractiveMode=false
cd blog

The archetype adds JUnit 5.11 and the sample classes App and AppTest — delete both and replace pom.xml with the version below.

pom.xml

<!-- pom.xml -->
<project xmlns="http://maven.apache.org/POM/4.0.0">
  <modelVersion>4.0.0</modelVersion>
 
  <groupId>com.example</groupId>
  <artifactId>blog</artifactId>
  <version>1.0-SNAPSHOT</version>
 
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.release>25</maven.compiler.release>
  </properties>
 
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.junit</groupId>
        <artifactId>junit-bom</artifactId>
        <version>6.1.3</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
 
  <dependencies>
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>
 
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>3.6.0</version>
      </plugin>
    </plugins>
  </build>
</project>

Part	Why it is needed
`junit-bom`	A BOM (bill of materials) fixes the versions of all JUnit modules in one place, so the dependencies below have no `version`
`junit-jupiter`	The API for writing tests, parameterized tests and the engine that runs them
`scope test`	The library is available only to tests and does not get into the finished program
`maven-surefire-plugin`	Runs the tests; the version is pinned so the build does not depend on Maven defaults

Where the code goes

blog/
├── pom.xml
└── src/
    ├── main/java/com/example/shop/PriceCalculator.java      the code
    └── test/java/com/example/shop/PriceCalculatorTest.java  its test

A test lives in src/test/java in the same package as the class it tests. That way it sees package-private members, and the structure of both folders matches.

The class under test

Create src/main/java/com/example/shop/PriceCalculator.java:

// src/main/java/com/example/shop/PriceCalculator.java
package com.example.shop;
 
import java.math.BigDecimal;
import java.math.RoundingMode;
 
public class PriceCalculator {
 
    public BigDecimal total(BigDecimal price, int quantity) {
        if (quantity <= 0) {
            throw new IllegalArgumentException("Quantity must be positive: " + quantity);
        }
        return price.multiply(BigDecimal.valueOf(quantity))
                .setScale(2, RoundingMode.HALF_UP);
    }
 
    public BigDecimal applyDiscount(BigDecimal amount, int percent) {
        if (percent < 0 || percent > 100) {
            throw new IllegalArgumentException("Discount must be between 0 and 100: " + percent);
        }
        BigDecimal factor = BigDecimal.valueOf(100 - percent).movePointLeft(2);
        return amount.multiply(factor).setScale(2, RoundingMode.HALF_UP);
    }
}

total() multiplies the price by the quantity, applyDiscount() takes a percentage off. Both round to cents. Run mvn test — Maven downloads JUnit and reports BUILD SUCCESS, there are no tests yet.

Common mistake

If you put a test into src/main/java, the project stops compiling: JUnit is available only to code in src/test/java because of scope test.

[ERROR] /.../src/main/java/com/example/shop/WrongPlaceTest.java:[3,29] package org.junit.jupiter.api does not exist

Move the test class to src/test/java, keeping the package.

Thanks for reading!