JAVA Stream的正确使用姿势

矫情吗；* 2023-10-11 22:41 25阅读 0赞

**目录**

1.业务场景

2.循环写法

3.stream写法

4.总结

--------------------

## 1.业务场景 ##

一种选择题试卷（TestPaper）由N个试卷项（TestPaperItem）组成，一种试卷由N个学生作答，会产生N份答卷（answer），一份答卷由N个答卷项（answerItem）组成，一个答卷项由TestPaperItem的一个问题项、一个选项组成。

实体关系如下：

![51a04ba57857407fb732446faf9a1494.png][]

相应的表关系和实体关系一直，就不赘述。

**需求：**

**查出所有考试的答卷列表。**

## 2.循环写法 ##

原生写法，先获取testPaper列表，再通过testPaperId获取answer列表，再通过answerId获取answerItem列表。

整个代码就是多层嵌套循环，去拼最后的结果：

@RestController
    public class TestController {
        @Autowired
        TestPaperService testPaperService;
        @Autowired
        TestPaperItemService testPaperItemService;
        @Autowired
        AnswerService answerService;
        @Autowired
        AnswerItemService answerItemService;
    
        @GetMapping("getTestPaperList")
        public List<TestPaper>  getTestPaperList(){
            //获取试卷列表
            List<TestPaper> testPaperList = testPaperService.getList();
            for (TestPaper testPaper : testPaperList) {
                Answer answerQueryParam = new Answer();
                answerQueryParam.setPaperId(testPaper.getId());
                List<Answer> answerList = answerService.getList(answerQueryParam);
                testPaper.setAnswerList(answerList);
                for (Answer answer : answerList) {
                    AnswerItem answerItemQueryParam = new AnswerItem();
                    List<AnswerItem> answerItemList = answerItemService.getList(answerItemQueryParam);
                    answer.setAnswerItems(answerItemList);
                }
            }
            return testPaperList;
        }
    }

可以看到循环写法，代码过于冗长，看起来很吃力，改起来也很吃力。

## 3.stream写法 ##

接下来我们看改成stream后整个流程会多么简洁、易读：

@RestController
    public class TestController {
        @Autowired
        TestPaperService testPaperService;
        @Autowired
        TestPaperItemService testPaperItemService;
        @Autowired
        AnswerService answerService;
        @Autowired
        AnswerItemService answerItemService;
    
        @GetMapping("getTestPaperList")
        public List<TestPaper> getTestPaperList() {
            return testPaperService.getList().stream()
                    .peek(this::setAnswerList)
                    .collect(Collectors.toList());
        }
    
        private void setAnswerList(TestPaper testPaper) {
            Answer answerQueryParam = new Answer();
            answerQueryParam.setPaperId(testPaper.getId());
            List<Answer> answerList = answerService.getList(answerQueryParam);
            testPaper.setAnswerList(answerList);
    
            answerList.forEach(this::setAnswerItems);
        }
    
        private void setAnswerItems(Answer answer) {
            AnswerItem answerItemQueryParam = new AnswerItem();
            List<AnswerItem> answerItemList = answerItemService.getList(answerItemQueryParam);
            answer.setAnswerItems(answerItemList);
        }
    }

## 4.总结 ##

通过上面的示例，我们可以看到，java stream在业务流程中，很适合当一条组装逻辑的“主线”，将冗长的代码逻辑单独剔出来封装成方法，在主线上拼接逻辑，使得代码具有优秀的可读性。这是stream在实际生产中非常重要的价值。

关于stream的更多、更详细的知识可以移步作者另一篇文章：  
[详解JAVA Stream\_\_BugMan的博客-CSDN博客][JAVA Stream_BugMan_-CSDN]

[51a04ba57857407fb732446faf9a1494.png]: https://img-blog.csdnimg.cn/51a04ba57857407fb732446faf9a1494.png
[JAVA Stream_BugMan_-CSDN]: https://blog.csdn.net/Joker_ZJN/article/details/129648877?spm=1001.2014.3001.5501