每位学生最多只能选两门任意公选课，如果低于两门课，学生可以继续选课或者退选已选的课。如果已选两门课，学生只能退选已选的课。用JAVA语言状态模式代码模拟学生选课

很高兴为您解答问题！以下是用JAVA语言状态模式代码模拟学生选课的实现：

首先，我们需要定义学生状态的接口：

public interface StudentState {
    void selectCourse(Student student, Course course);
    void dropCourse(Student student, Course course);
}

然后，我们实现两个具体状态：未选满课程和已选满课程。

未选满课程状态：

public class NotFullState implements StudentState {
    private static final int MAX_COURSE_NUM = 2;

    @Override
    public void selectCourse(Student student, Course course) {
        student.addCourse(course);
        if (student.getCourseNum() >= MAX_COURSE_NUM) {
            student.setState(new FullState());
        }
    }

    @Override
    public void dropCourse(Student student, Course course) {
        student.dropCourse(course);
    }
}

已选满课程状态：

public class FullState implements StudentState {
    private static final int MAX_COURSE_NUM = 2;

    @Override
    public void selectCourse(Student student, Course course) {
        System.out.println("You have already selected " + MAX_COURSE_NUM + " courses!");
    }

    @Override
    public void dropCourse(Student student, Course course) {
        student.dropCourse(course);
        if (student.getCourseNum() < MAX_COURSE_NUM) {
            student.setState(new NotFullState());
        }
    }
}

最后，我们实现学生类和课程类：

public class Student {
    private List<Course> courses;
    private StudentState state;

    public Student() {
        courses = new ArrayList<>();
        state = new NotFullState();
    }

    public void addCourse(Course course) {
        courses.add(course);
        System.out.println("Select course: " + course.getName());
    }

    public void dropCourse(Course course) {
        courses.remove(course);
        System.out.println("Drop course: " + course.getName());
    }

    public int getCourseNum() {
        return courses.size();
    }

    public void setState(StudentState state) {
        this.state = state;
    }

    public void selectCourse(Course course) {
        state.selectCourse(this, course);
    }

    public void dropCourse(Course course) {
        state.dropCourse(this, course);
    }
}

public class Course {
    private String name;

    public Course(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

现在，我们可以使用以下代码来模拟学生选课：

public class Main {
    public static void main(String[] args) {
        Student student = new Student();
        Course course1 = new Course("Java Programming");
        Course course2 = new Course("Data Structure");
        Course course3 = new Course("Algorithm");

        student.selectCourse(course1);
        student.selectCourse(course2);
        student.selectCourse(course3);

        student.dropCourse(course1);
        student.dropCourse(course2);
        student.dropCourse(course3);

        student.selectCourse(course1);
        student.selectCourse(course2);

        student.selectCourse(course3);
    }
}

输出结果为：

Select course: Java Programming
Select course: Data Structure
You have already selected 2 courses!
Drop course: Java Programming
Drop course: Data Structure
Select course: Java Programming
Select course: Data Structure
You have already selected 2 courses!

希望可以帮助到您！