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What is a NullPointerException, and how do I fix it?（12个答案）
12小时前关门了。
我在SpringBoot中有一个API，当我更新我的资源时，SpringBoot抛出了NullPointerException，我不知道为什么。
SpringBoot告诉我：出现错误（类型=内部服务器错误，状态= 500）。无法调用"String. equalsIgnoreCase（String）"，因为"com. packages. appRest. survey. Question. getId（）"的返回值为空。无法调用"String. equalsIgnoreCase（String）"，因为"com. packages. appRest. survey. Question. getId（）"的返回值为空
服务类中的方法更新：

public void updateSurveyQuestion(String surveyId, String questionId, Question question) {
        List<Question> questions = retrieveAllSurveyQuestions(surveyId);
        questions.removeIf(q -> q.getId().equalsIgnoreCase(questionId));
        questions.add(question);
    }

控制器方法：

@RestController
public class SurveyResource {
    

    private SurveyService surveyService;

    public SurveyResource(SurveyService surveyService) {
        super();
        this.surveyService = surveyService;
    }

    @RequestMapping("/surveys/{surveyId}")
    public Survey retrieveSurveyById(@PathVariable String surveyId){
        Survey survey = surveyService.retrieveSurveyById(surveyId);

        if(survey == null){
            throw new ResponseStatusException(HttpStatus.NOT_FOUND);
        }

        return survey;
    }

    ////////
    @RequestMapping("/surveys/{surveyId}/questions")
    public List<Question> retrieveAllSurveyQuestions(@PathVariable String surveyId){
        List<Question> questions = surveyService.retrieveAllSurveyQuestions(surveyId);

        if(questions == null){
            throw new ResponseStatusException(HttpStatus.NOT_FOUND);
        }

        return questions;
    }

    ///////
    @RequestMapping("/surveys/{surveyId}/questions/{questionId}")
    public Question retrieveSpecificSurveyQuestions(@PathVariable String surveyId, @PathVariable String questionId){
        Question question = surveyService.retrieveSpecificSurveyQuestions(surveyId, questionId);

        if(question == null){
            throw new ResponseStatusException(HttpStatus.NOT_FOUND);
        }

        return question;
    }
    
    ////////
    @RequestMapping(value="/surveys/{surveyId}/questions", method=RequestMethod.POST)
    public ResponseEntity<Object> addNewSurveyQuestion(@PathVariable String surveyId, @RequestBody Question question){

        String questionId = surveyService.addNewSurveyQuestion(surveyId, question);

        URI location = ServletUriComponentsBuilder.fromCurrentRequest().path("/{questionId}").buildAndExpand(questionId).toUri();

        return ResponseEntity.created(location).build();
    }
    ////////

    ////////
    @RequestMapping(value="/surveys/{surveyId}/questions/{questionId}", method=RequestMethod.DELETE )
    public ResponseEntity<Object> deleteSurveyQuestion(@PathVariable String surveyId, @PathVariable String questionId){

        surveyService.deleteSurveyQuestion(surveyId, questionId);

        return ResponseEntity.noContent().build();
    }
    ////////

    @RequestMapping(value="/surveys/{surveyId}/questions/{questionId}", method = RequestMethod.PUT)
    public ResponseEntity<Object> updateSurveyQuestion(@PathVariable String surveyId, @PathVariable String questionId, @RequestBody Question question){

        surveyService.updateSurveyQuestion(surveyId, questionId, question);
        
        return ResponseEntity.noContent().build();
    }


}

实体类：

public class Question {

    public Question(){

    }

    public Question(String id, String description, List<String> options, String correctAnswer) {
        super();
        this.id = id;
        this.description = description;
        this.options = options;
        this.correctAnswer = correctAnswer;
    }

    ///////////////////

    private String id;

    private String description;

    private List<String> options;

    private String correctAnswer;

    //////////////////

    public String getId() {
        return this.id;
    }
    public void setId(String id) {
        this.id = id;
    }

    public String getDescription() {
        return this.description;
    }
    public void setDescription(String description) {
        this.description = description;
    }

    public List<String> getOptions() {
        return this.options;
    }
    public void setOptions(List<String> options) {
        this.options = options;
    }

    public String getCorrectAnswer() {
        return this.correctAnswer;
    }
    public void setCorrectAnswer(String correctAnswer) {
        this.correctAnswer = correctAnswer;
    }

    ///////////
    @Override
    public String toString() {
        return "{" +
            " id='" + getId() + "'" +
            ", description='" + getDescription() + "'" +
            ", options='" + getOptions() + "'" +
            ", correctAnswer='" + getCorrectAnswer() + "'" +
            "}";
    } 
}

我看不到错误