一句话，
拿到完整源码工程

public class QuestionExamService {
    private static final String RESPONSIBILITY =
        "题库、组卷、考试提交和成绩计算";
    private static final List<String> DOMAIN_RULES = Arrays.asList(
        "报名成功后才允许进入班级学习。",
        "课程未发布时不得展示给学员。",
        "考试提交时间不得晚于截止时间。",
        "证书发放必须校验完课率和考试成绩。");
    private static final List<String> STATUS_FLOW = Arrays.asList(
        "not_started", "learning", "completed",
        "submitted", "grading", "passed", "failed");

    public void transitionStatus(String before, String after) {
        if (!STATUS_FLOW.contains(before))
            throw new IllegalArgumentException("前置状态无效：" + before);
        if (!STATUS_FLOW.contains(after))
            throw new IllegalArgumentException("目标状态无效：" + after);
        if (STATUS_FLOW.indexOf(after) < STATUS_FLOW.indexOf(before))
            throw new IllegalArgumentException("不允许回退到更早的业务状态");
    }
    // ……完整实现见交付工程
}