Unit Testing in Java – Beginner Friendly Guide

Beginner Friendly

Unit Testing in Java – A Simple, Beginner Friendly Guide

Unit testing is one of the most important skills every Java developer should learn early. It saves time, improves code quality, and is expected in interviews at most modern companies.

What is Unit Testing?

Unit testing means writing small automated tests that verify whether a specific method or function is working correctly. Each test focuses on one “unit” of behavior.

Why Should You Learn Unit Testing?

• You catch bugs early
• You write cleaner and modular code
• You refactor with confidence
• Most product companies expect basic testing knowledge

Tools You Need (JUnit 5)

JUnit 5 is the most commonly used Java testing framework.

For Maven Projects

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.9.1</version>
    <scope>test</scope>
</dependency>

Your First Example – Calculator Class

Let’s test a simple Java class.

Calculator.java

public class Calculator {
  public int add(int a, int b) { return a + b; }
  public int subtract(int a, int b) { return a - b; }
  public int multiply(int a, int b) { return a * b; }
  public int divide(int a, int b) {
    if (b == 0) throw new IllegalArgumentException("Cannot divide by zero");
    return a / b;
  }
}

CalculatorTest.java (JUnit 5)

import org.junit.jupiter.api.*;

public class CalculatorTest {

  private Calculator calculator;

  @BeforeEach
  void init() {
    calculator = new Calculator();
  }

  @Test
  void testAdd() {
    Assertions.assertEquals(8, calculator.add(5, 3));
  }

  @Test
  void testSubtract() {
    Assertions.assertEquals(6, calculator.subtract(10, 4));
  }

  @Test
  void testMultiply() {
    Assertions.assertEquals(10, calculator.multiply(2, 5));
  }

  @Test
  void testDivide() {
    Assertions.assertEquals(5, calculator.divide(10, 2));
  }

  @Test
  void testDivideByZero() {
    Assertions.assertThrows(IllegalArgumentException.class,
      () -> calculator.divide(10, 0));
  }
}

Pro Tip: Write one test for one behavior. This ensures clear, readable tests and easier debugging.

How to Run Tests

• Right-click test → Run (IntelliJ/Eclipse)
• Maven: mvn test
• Gradle: gradle test

Best Practices

• Follow Arrange → Act → Assert
• Do not test too many things in one method
• Avoid external systems (DB, APIs)
• Use mocks for dependencies
• Cover edge cases

What to Learn Next

• Mockito (mocking dependencies)
• TDD (Test Driven Development)
• JaCoCo for coverage
• Integration tests

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