Using Java 8 Map.merge() to Aggregate Student Scores

Java 8 brings many functional‑programming features, such as lambda expressions, and introduces useful methods for the Map interface, among which Map.merge() simplifies common aggregation tasks.

The article starts with a concrete example: a list of StudentScore objects, each containing a student name, a subject, and a score, and the goal is to compute the total score for each student.

private List
buildATestList() {
    List
studentScoreList = new ArrayList<>();
    StudentScore studentScore1 = new StudentScore() {{
        setStuName("张三");
        setSubject("语文");
        setScore(70);
    }};
    // ... (similar objects for other subjects and students) ...
    studentScoreList.add(studentScore1);
    // add the rest of the objects
    return studentScoreList;
}

First, the conventional approach uses forEach to check whether a key already exists in the map and updates the value manually:

ObjectMapper objectMapper = new ObjectMapper();
List
studentScoreList = buildATestList();
Map
studentScoreMap = new HashMap<>();
studentScoreList.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));
// Result: {"李四":228,"张三":215,"王五":235}

Using Map.merge() the same logic becomes a single line:

Map
studentScoreMap2 = new HashMap<>();
studentScoreList.forEach(studentScore ->
    studentScoreMap2.merge(
        studentScore.getStuName(),
        studentScore.getScore(),
        Integer::sum));
System.out.println(objectMapper.writeValueAsString(studentScoreMap2));
// Result: {"李四":228,"张三":215,"王五":235}

The merge method has the signature default V merge(K key, V value, BiFunction remappingFunction) . If the key is absent, it behaves like put(key, value) ; otherwise it applies the provided remapping function (e.g., Integer::sum ) to combine the existing and new values.

This behavior makes merge especially useful for grouping and summing operations, offering a cleaner alternative to manual checks when processing collections.

The article also mentions other useful Map methods introduced in Java 8, such as putIfAbsent , compute() , computeIfAbsent() , and computeIfPresent() , and provides the source of compute() for reference.

In conclusion, Map.merge() provides a concise and readable way to aggregate map values, and understanding its source helps developers apply it correctly in real‑world scenarios.