@PostMapping("/reverse")
public String reverseList(@RequestBody String string) {
    List<String> stringList = Arrays.asList(string.split("[+,]"));
    return  IntStream.range(0, stringList.size())
            .mapToObj(i -> stringList.get(stringList.size() - 1 - i))
            .collect(Collectors.joining("+"));
}

C:\Users\Adil>curl -H "Content-Type: application/json" -d "a1+a2+a3+a4" localhost:8080/hello/reverse

C:\Users\Adil>curl -H "Content-Type: application/json" -d "a1+a2+a3+a4" localhost:8080/hello/reverse

@Test
    void testCount() {
        assertEquals("30", String.valueOf(
                Arrays.stream("10+10+10".split("[+]"))
                        .count()));
    }

curl -H "Content-Type: application/json" -d "a1+a2+a3+a4+a5" localhost:8080/hello/reverse

@RestController
@RequestMapping("/hello")
public class HelloController {

    private final NamesDAO namesDAO;

    public HelloController(NamesDAO namesDAO) {
        this.namesDAO = namesDAO;
    }

    // curl -H "Content-Type: application/json" localhost:8080/hello/getNames
    @GetMapping("/getNames")
    public List<String> getNames() {
        return namesDAO.getNames();
    }
    
    //curl -H "Content-Type: application/json" -d "[\"Adil\",\"John\"]" localhost:8080/hello/addNames
    @PostMapping("/addNames")
    public List<String> addNames(@RequestBody List<String> names) {
        namesDAO.addNames(names);

        return names;
        
    }

    //curl -H "Content-Type: application/json" -d "a1+a2+a3+a4" localhost:8080/hello/reverse
    
    
    
  @PostMapping("/reverse")
public String reverseList(@RequestBody String string) {
    List<String> stringList = Arrays.asList(string.split("[+,]"));
    return  IntStream.range(0, stringList.size())
            .mapToObj(i -> stringList.get(stringList.size() - 1 - i))
            .collect(Collectors.joining("+"));
}

@PostMapping
    public Integer sum(@RequestBody String string) {
        return Arrays.stream(string.split("[+]"))
                .mapToInt(Integer::valueOf)
                .count();
    }
// curl -H "Content-Type: application/json" -d "1+2+3+4" localhost:8080/hello/count
    @PostMapping("/count")
    public long sum(@RequestBody String string) {
        return Arrays.stream(string.split("[+]")).count();
    }