Java中Map.merge()方法使用示例详解

default V merge(K key, V value,
            BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        V newValue = (oldValue == null) ? value :
                   remappingFunction.apply(oldValue, value);
        if(newValue == null) {
            remove(key);
        } else {
            put(key, newValue);
        }
        return newValue;
    }

/**
     * 简单实例
     */
    @Test
    public void someTest01() {
        Map<String, Integer> map = new HashMap<>();
        map.put("key1", 10);
        map.put("key2", 20);
        // 当key存在时，进行old+value；当key不存在时，将key的值设置为1
        map.merge("key", 1, Integer::sum);
        // 当key1存在时，进行old+value；当key1不存在时，将key1的值设置为20
        map.merge("key1", 20, Integer::sum);
        map.forEach((k, v) -> System.out.println("k:" + k + "---" + "v:" + v));
    }
    // ------------------------------------------------------------------
    // 打印结果
    k:key1---v:30
    k:key2---v:20
    k:key---v:1

@Data
public class Student {
    /**
     * 学生姓名
     */
    private String studentName;
    /**
     * 学科
     */
    private String subject;
    /**
     * 分数
     */
    private Integer score;
    public Student(String studentName, String subject, Integer score) {
        this.studentName = studentName;
        this.subject = subject;
        this.score = score;
    }
}
// ------------------------------------------------------------------
    /**
     * 统计学生成绩
     */
    @Test
    public void someTest02() {
        List<Student> students = Arrays.asList(
                new Student("yoni", "English", 80),
                new Student("yoni", "Chiness", 98),
                new Student("yoni", "Math", 95),
                new Student("taohai.wang", "English", 50),
                new Student("taohai.wang", "Chiness", 72),
                new Student("taohai.wang", "Math", 41),
                new Student("Seely", "English", 88),
                new Student("Seely", "Chiness", 89),
                new Student("Seely", "Math", 92)
        );
        Map<String, Integer> scoreMap = new HashMap<>();
        students.forEach(student -> scoreMap.merge(student.getStudentName(), student.getScore(), Integer::sum));
        scoreMap.forEach((k, v) -> System.out.println("key:" + k + "---" + "value:" + v));
    }
// ------------------------------------------------------------------
// 运行结果
姓名:Seely---成绩:269
姓名:taohai.wang---成绩:163
姓名:yoni---成绩:273