Solving Code

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class ReverseString {
public static void main(String[] args) { }

public static String reverseString(String s) {
Stack<Character> stack = new Stack<>();
String reverse = "";
for (char c : s.toCharArray()) {
stack.push(c);
}
while (!stack.isEmpty()) {
reverse += stack.pop();
}
return reverse;
}

public static String reverStringArrayList(String s) {
List<Character> list = new ArrayList<>();
String reverse = "";
for (char c : s.toCharArray()) {
list.add(c);
}
for (int i = list.size() - 1; i >=0; i--) {
reverse += list.get(i);
}
return reverse;
}
}

Test code with JUnit

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

public class ReverseStringTest {

@Test
void testNormalWord() {
String hello = "hello";
String expected = "olleh";
String result = ReverseString.reverseString(hello);
assertEquals(expected, result);
}

@Test
void testSpaceWithWords() {
String value = "This is a pen.";
String expected = ".nep a si sihT";
String result = ReverseString.reverStringArrayList(value);
assertEquals(expected, result);
}
}