jdk8自定義Collector實現對象的求和和求平均值

1、首先定義兩個接口SumOperator、AverageOperator；所有要實現求和功能的類都必須實現SumOperator接口，要實現求平均值功能的類都必須實現AverageOperator接口，以下是兩個接口定義：

public interface SumOperator<T> {

    /**
     * sum
     * @desc: 
     * @author: 
     * @createTime: 2020/4/29 12:37
     * @param t
     * @return: T
     */
    T sum(T t);

}

public interface AverageOperator<T> extends SumOperator<T>{

    /**
     * divide
     * @desc: 
     * @author: 
     * @createTime: 2020/4/29 12:37
     * @param number
     * @return: T
     */
    T divide(long number);

}

2、接下來自定義一個類來生成這兩個Collector：代碼如下

public class CustomCollectors{

    public static<T extends SumOperator<T>> Collector<T, List<T>, T> ofSum() {
        return new CollectorImpl<T>(
                list -> list.stream().reduce((t1, t2) -> t1.sum(t2)).get()
        );
    }

    public static<T extends AverageOperator<T>> Collector<T, List<T>, T> ofAverage() {
        return new CollectorImpl<T>(
                list -> list.stream().reduce((t1, t2) -> t1.sum(t2)).get().divide(list.size())
        );
    }

    static class CollectorImpl<T> implements Collector<T, List<T>, T>{

        private Function<List<T>, T> finisher;

        private CollectorImpl(Function<List<T>, T> finisher){
            this.finisher = finisher;
        }

        @Override
        public Supplier<List<T>> supplier() {
            return ArrayList::new;
        }

        @Override
        public BiConsumer<List<T>, T> accumulator() {
            return List::add;
        }

        @Override
        public BinaryOperator<List<T>> combiner() {
            return (left, right) -> { left.addAll(right); return left; };
        }

        @Override
        public Function<List<T>, T> finisher() {
            return finisher;
        }

        @Override
        public Set<Characteristics> characteristics() {
            return Collections.emptySet();
        }

    }

}

3、接下來我們做個測試：

public class Test {

    @Getter
    @Setter
    @Accessors(chain = true)
    static class Student implements AverageOperator<Student> {

        private String name;

        private int term;

        private double mathScore;

        private double englishScore;

        /**
         * student
         * @desc: 爲了不影響List中存儲的Student對象，不需要可以去掉
         * @author: 
         * @createTime: 2020/4/29 16:25
         * @param
         * @return: Student
         */
        private Student student(){
            Student  student = this;
            if(!StringUtils.isEmpty(student.name)){
                student = new Student();
                student.mathScore = mathScore;
                student.englishScore = englishScore;
            }
            return student;
        }

        @Override
        public Student divide(long number) {
            Student student = student();
            student.mathScore /= number;
            student.englishScore /= number;
            return student;
        }

        @Override
        public Student sum(Student student) {
            Student s = student();
            s.mathScore += student.mathScore;
            s.englishScore += student.englishScore;
            return s;
        }

    }

    public static void main(String[] args) {
        List<Student> students = new ArrayList<Student>(){
            {
                add(new Student().setName("張三").setTerm(1).setMathScore(90).setEnglishScore(80));
                add(new Student().setName("張三").setTerm(2).setMathScore(85).setEnglishScore(77));
                add(new Student().setName("張三").setTerm(3).setMathScore(95).setEnglishScore(78));
                add(new Student().setName("張三").setTerm(4).setMathScore(88).setEnglishScore(75));
                add(new Student().setName("張三").setTerm(5).setMathScore(95).setEnglishScore(78));
                add(new Student().setName("張三").setTerm(6).setMathScore(88).setEnglishScore(75));

                add(new Student().setName("李四").setTerm(1).setMathScore(90).setEnglishScore(60));
                add(new Student().setName("李四").setTerm(2).setMathScore(95).setEnglishScore(67));
                add(new Student().setName("李四").setTerm(3).setMathScore(98).setEnglishScore(60));
                add(new Student().setName("李四").setTerm(4).setMathScore(98).setEnglishScore(70));
                add(new Student().setName("李四").setTerm(5).setMathScore(100).setEnglishScore(68));
                add(new Student().setName("李四").setTerm(6).setMathScore(96).setEnglishScore(75));
            }
        };

        //下面分組求出學生的平均分
        Collector<Student, List<Student>, Student> collector = CustomCollectors.ofAverage();
        Map<String, Student> map = students.stream().collect(Collectors.groupingBy(Student::getName, collector));
        map.forEach((name, student) -> {
            System.out.println(String.format("學生【%s】的平均分爲；(數學：%f, 英語：%f)", name, student.getMathScore(), student.getEnglishScore()));
        });

    }

}

打印結果如下：

學生【李四】的平均分爲；(數學：96.166667, 英語：66.666667)
學生【張三】的平均分爲；(數學：90.166667, 英語：77.166667)