java实现本学期期末学员共参加了3门课的考试，即Java，C，SQL，编写方法计算5名学员3门课的平均分。

在Java中，我们可以创建一个简单的类`Student`来存储每位学员的成绩，并提供一个方法来计算所有学员的平均成绩。下面是一个示例：

class Student {
    private String name;
    private double javaScore;
    private double cScore;
    private double sqlScore;

    public Student(String name, double java, double c, double sql) {
        this.name = name;
        this.javaScore = java;
        this.cScore = c;
        this.sqlScore = sql;
    }

    // 获取方法
    public double getJavaScore() { return javaScore; }
    public double getCScore() { return cScore; }
    public double getSqlScore() { return sqlScore; }

    // 计算总分
    public double getTotalScore() {
        return javaScore + cScore + sqlScore;
    }

    // 计算平均分
    public double calculateAverage() {
        if (javaScore + cScore + sqlScore == 0) {
            throw new IllegalArgumentException("Total scores cannot be zero.");
        }
        return getTotalScore() / 3.0;
    }
}

public class ExamAverageCalculator {
    public static void main(String[] args) {
        // 创建5位学员实例
        Student student1 = new Student("学员1", 85, 90, 95);
        Student student2 = new Student("学员2", 90, 87, 92);
        Student student3 = new Student("学员3", 95, 92, 96);
        Student student4 = new Student("学员4", 88, 89, 91);
        Student student5 = new Student("学员5", 92, 93, 94);

        // 存储学员列表
        List<Student> students = Arrays.asList(student1, student2, student3, student4, student5);

        // 计算所有学员的平均分
        double totalAverage = students.stream()
                .map(Student::calculateAverage)
                .average()
                .orElse(Double.NaN); // 避免除以零的情况

        System.out.printf("五名学员三门课的平均分为 %.2f 分.\n", totalAverage);
    }
}

在这个例子中，我们首先定义了`Student`类，然后在`ExamAverageCalculator`类里，通过流(Stream)操作计算所有学员的平均分。