My controller class:

@PostMapping(value = "/uniqueUrl")
        @ResponseBody
        public ResponseEntity<MyResponse> urlGenerator(@RequestBody MyRequest myRequest){
            log.info("Request for url : ", myRequest); 
            MyResponse myResponse= this.generateUrlService.urlGenerator(myRequest);
            log.info("generateUniqueUrl response: ", myResponse.getLongUniqueUrlList());
            return ResponseEntity.accepted().body(myResponse);
        }

MyRequest class:

@Data
public class MyRequestimplements Serializable {

    @NotNull(message = "Url cannot be null or empty")
    private String url;
    @NotNull(message = "count cannot be null or empty")
    private int cound;
}

My service implemantation :

@Override
    public myResponse urlGenerator(MyRequest myRequest) {

        log.info("urlGenerator started..");

        myUrlRequestValidator.validate(myRequest);

        String longUrl = myRequest.getUrl();
        int count = myRequest.getCount();

        List<String> uniqueUrlList = Arrays.asList(new String[count]);

        for (String string : uniqueUrlList) {
            string = longUrl   "/?";

            for (int i = 0; i < rand.nextInt(11) 4; i  ) {
                string  = letters.get(rand.nextInt(35));
            }
            uniqueUrlList.add(string);
            log.info(string);
        }

        MyResponse response = new MyResponse();
        response.setLongUniqueUrlList(uniqueUrlList);
        return response;
    }

MyResponse class:

@Data public class MyResponse extends BaseResponse {

private List<String> longUniqueUrlList;
private List<String> shortUrlList;

}

In the method where my Controller and Service class is as follows, the result of uniqueUrlList returns null. I want to add each string formed by the add method to the list, but it does not add it. Can you help me where am I going wrong?

CodePudding user response：

It is null because your List<String> uniqueUrlList is initialized with Arrays.asList which are fixed in size and unmodifiable, as specified in the Javadoc. The Arrays.asList(new String[count]) is also empty as there are no elements inside the new String[count].

Instead you should initialize it with a new ArrayList<String>():

List<String> uniqueUrlList = new ArrayList<String>();

Where you can then modify the list as you please, using a loop to add to your uniqueUrlList as many as myRequest.getCount() times.

CodePudding user response：

You should initialize a list by

List<String> uniqueUrlList = new ArrayList<>();