Unit Testing with JUnit

Unit testing is a way to test small parts of your code.

We test one “unit” at a time, like one method or class.

JUnit is a tool to help you write and run these tests in Java.

Why Testing is Important

Imagine you bake a cake.

Before giving it to someone, you taste a small piece.

You check if it is good or has problems.

Unit testing is the same idea for software.

We check parts of our program to make sure they work.

What is JUnit?

JUnit is a testing framework for Java.

It lets you write code to test your other code.

It tells you if your code works or breaks.

To use JUnit:

• Add junit to your project (use Maven or Gradle or add the JAR).
• Create a test class.
• Write test methods with @Test annotation.

Simple Example

Let’s say we have a class to add two numbers:

public class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
}
  

Now we write a test for it using JUnit:

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

public class CalculatorTest {
    @Test
    public void testAdd() {
        Calculator calc = new Calculator();
        int result = calc.add(2, 3);
        assertEquals(5, result);
    }
}
  

Here:

• @Test tells JUnit this is a test method.
• assertEquals checks if the result is what we expect.

More Test Examples

Imagine a method that returns the bigger of two numbers:

public class MathUtils {
    public int max(int a, int b) {
        return a > b ? a : b;
    }
}
  

Test it like this:

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

public class MathUtilsTest {
    @Test
    public void testMax() {
        MathUtils mu = new MathUtils();
        assertEquals(8, mu.max(5, 8));
        assertEquals(10, mu.max(10, 3));
    }
}
  

Daily Life Example

Let’s say you are writing an app for a bakery.

You have a method that gives a discount if someone buys more than 5 cupcakes.

public class Bakery {
    public double getPrice(int count) {
        double price = count * 2.0;
        if (count > 5) {
            price *= 0.9; // 10% discount
        }
        return price;
    }
}
  

Now test it:

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

public class BakeryTest {
    @Test
    public void testGetPrice() {
        Bakery b = new Bakery();
        assertEquals(10.0, b.getPrice(5));
        assertEquals(10.8, b.getPrice(6), 0.001);
    }
}
  

We use 0.001 for floating point accuracy.

When Tests Fail

JUnit will show a red bar or error message.

It tells you what failed and where.

This helps you fix your code faster.

Benefits of Unit Testing

• You catch bugs early.
• Your code is more reliable.
• Refactoring is safer.
• You can work faster and with more confidence.

Conclusion

Testing is part of good software engineering.

JUnit makes testing easy in Java.

Write small tests. Run them often. Fix bugs quickly.

Happy testing!