Run JUnit Test Cases From the Command Line

package com.gfg;

public class StringUtility {

    // Reverses the given string
    public String reverse(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Input cannot be null"); // Check for null input
        }
        return new StringBuilder(input).reverse().toString(); // Reverse the string
    }

    // Converts the string to uppercase
    public String toUpperCase(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Input cannot be null"); // Check for null input
        }
        return input.toUpperCase(); // Convert string to uppercase
    }

    // Counts the number of vowels in a string
    public int countVowels(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Input cannot be null"); // Check for null input
        }
        int count = 0; // Initialize vowel count
        for (char c : input.toLowerCase().toCharArray()) { // Convert to lowercase and iterate
            if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
                count++; // Increment count if a vowel is found
            }
        }
        return count; // Return the total count of vowels
    }
}

package com.gfg;

// Main class to demonstrate StringUtility functionality
public class Main {
    public static void main(String[] args) {
        StringUtility stringUtility = new StringUtility(); // Instantiate StringUtility

        // Test reverse method
        String reversed = stringUtility.reverse("example");
        System.out.println("Reversed: " + reversed); // Output reversed string

        // Test toUpperCase method
        String upper = stringUtility.toUpperCase("example");
        System.out.println("Uppercase: " + upper); // Output uppercase string

        // Test countVowels method
        int vowelCount = stringUtility.countVowels("example");
        System.out.println("Vowel count: " + vowelCount); // Output count of vowels
    }
}

import com.gfg.StringUtility;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

// Test class for StringUtility
class StringUtilityTest {

    private final StringUtility stringUtility = new StringUtility(); // Instantiate StringUtility

    @Test
    void testReverse() {
        assertEquals("tac", stringUtility.reverse("cat"), "Reverse test failed"); // Test reverse method
        assertEquals("", stringUtility.reverse(""), "Reverse empty string failed"); // Test empty string
        assertThrows(IllegalArgumentException.class, () -> stringUtility.reverse(null), "Null input failed"); // Test null input
    }

    @Test
    void testToUpperCase() {
        assertEquals("HELLO", stringUtility.toUpperCase("hello"), "Uppercase conversion failed"); // Test lowercase input
        assertEquals("WORLD", stringUtility.toUpperCase("WoRlD"), "Mixed case conversion failed"); // Test mixed case
        assertThrows(IllegalArgumentException.class, () -> stringUtility.toUpperCase(null), "Null input failed"); // Test null input
    }

    @Test
    void testCountVowels() {
        assertEquals(3, stringUtility.countVowels("example"), "Vowel count failed"); // Test count of vowels
        assertEquals(0, stringUtility.countVowels("xyz"), "Vowel count for consonants failed"); // Test no vowels
        assertThrows(IllegalArgumentException.class, () -> stringUtility.countVowels(null), "Null input failed"); // Test null input
    }
}