以下是一个示例Java代码，用于按列分组并找到平均值：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class GroupByAndAverage {

    public static void main(String[] args) {
        // 创建示例数据
        List students = new ArrayList<>();
        students.add(new Student("John", "Math", 90));
        students.add(new Student("John", "Science", 95));
        students.add(new Student("Jane", "Math", 85));
        students.add(new Student("Jane", "Science", 92));
        students.add(new Student("Bob", "Math", 80));
        students.add(new Student("Bob", "Science", 88));

        // 按列分组并计算平均值
        Map averageScoresBySubject = new HashMap<>();
        Map countsBySubject = new HashMap<>();
        for (Student student : students) {
            String subject = student.getSubject();
            double score = student.getScore();

            if (averageScoresBySubject.containsKey(subject)) {
                averageScoresBySubject.put(subject, averageScoresBySubject.get(subject) + score);
                countsBySubject.put(subject, countsBySubject.get(subject) + 1);
            } else {
                averageScoresBySubject.put(subject, score);
                countsBySubject.put(subject, 1);
            }
        }

        for (String subject : averageScoresBySubject.keySet()) {
            double averageScore = averageScoresBySubject.get(subject) / countsBySubject.get(subject);
            System.out.println("Subject: " + subject + ", Average Score: " + averageScore);
        }
    }

    static class Student {
        private String name;
        private String subject;
        private double score;

        public Student(String name, String subject, double score) {
            this.name = name;
            this.subject = subject;
            this.score = score;
        }

        public String getName() {
            return name;
        }

        public String getSubject() {
            return subject;
        }

        public double getScore() {
            return score;
        }
    }
}

在此示例中，我们创建了一个名为Student的内部类来表示学生对象。每个学生对象都有姓名、科目和分数。

然后，我们创建了一个List来存储学生对象，并将示例数据添加到该列表中。

接下来，我们使用两个Map来分别存储每个科目的分数总和和学生人数。我们迭代学生列表，并根据科目进行分组。如果averageScoresBySubject和countsBySubject中已经存在该科目，则将分数和计数相应地增加；否则，将新的科目、分数和计数添加到这两个Map中。

最后，我们遍历averageScoresBySubject，计算每个科目的平均分数，并打印输出结果。

这是一个简单的示例，你可以根据自己的需求进行修改和扩展。