直接上程式碼

1.學生成績類

public class StudentScore {

    private String stuName;
    private String subject;
    private int score;

    public String getStuName() {
        return stuName;
    }

    public void setStuName(String stuName) {
        this.stuName = stuName;
    }

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }
}

　　2.計算總分

/**
 * 要求求得每個學生的總成績
 */
public class MapDemo {


    public static void main(String[] args) throws JsonProcessingException {

        //獲取準備資料
        List<StudentScore> list = buildATestList();

        //傳統方法
        long start = System.currentTimeMillis();
        ObjectMapper objectMapper 
 
= new ObjectMapper();
        Map<String, Integer> studentScoreMap = new HashMap<>();
        list.forEach(studentScore -> {
            if (studentScoreMap.containsKey(studentScore.getStuName())) {
                studentScoreMap.put(studentScore.getStuName(),
                        studentScoreMap.get(studentScore.getStuName()) 
 
+ studentScore.getScore());
            } else {
                studentScoreMap.put(studentScore.getStuName(), studentScore.getScore());
            }
        });

        System.out.println(objectMapper.writeValueAsString(studentScoreMap));
        long end = System.currentTimeMillis();
        System.out.println(end - start);

        //新方法
        long start2 = System.currentTimeMillis();
        Map<String, Integer> studentScoreMap2 = new HashMap<>();

        list.forEach(studentScore -> studentScoreMap2.merge(
                studentScore.getStuName(),
                studentScore.getScore(),
                Integer::sum));

        System.out.println(objectMapper.writeValueAsString(studentScoreMap2));
        long end2 = System.currentTimeMillis();
        System.out.println(end2 - start2);
    }


    //準備資料
    private static List<StudentScore> buildATestList() {
        List<StudentScore> studentScoreList = new ArrayList<>();
        StudentScore studentScore1 = new StudentScore() {{
            setStuName("張三");
            setSubject("語文");
            setScore(70);
        }};
        StudentScore studentScore2 = new StudentScore() {{
            setStuName("張三");
            setSubject("數學");
            setScore(80);
        }};
        StudentScore studentScore3 = new StudentScore() {{
            setStuName("張三");
            setSubject("英語");
            setScore(65);
        }};
        StudentScore studentScore4 = new StudentScore() {{
            setStuName("李四");
            setSubject("語文");
            setScore(68);
        }};
        StudentScore studentScore5 = new StudentScore() {{
            setStuName("李四");
            setSubject("數學");
            setScore(70);
        }};
        StudentScore studentScore6 = new StudentScore() {{
            setStuName("李四");
            setSubject("英語");
            setScore(90);
        }};
        StudentScore studentScore7 = new StudentScore() {{
            setStuName("王五");
            setSubject("語文");
            setScore(80);
        }};
        StudentScore studentScore8 = new StudentScore() {{
            setStuName("王五");
            setSubject("數學");
            setScore(85);
        }};
        StudentScore studentScore9 = new StudentScore() {{
            setStuName("王五");
            setSubject("英語");
            setScore(70);
        }};

        studentScoreList.add(studentScore1);
        studentScoreList.add(studentScore2);
        studentScoreList.add(studentScore3);
        studentScoreList.add(studentScore4);
        studentScoreList.add(studentScore5);
        studentScoreList.add(studentScore6);
        studentScoreList.add(studentScore7);
        studentScoreList.add(studentScore8);
        studentScoreList.add(studentScore9);

        return studentScoreList;
    }
}

3.最後結果

{"李四":228,"張三":215,"王五":235}
410
{"李四":228,"張三":215,"王五":235}
8