学校规定，每个学期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。请用java代码状态模式模拟学生选课的场景

好的，以下是状态模式的Java代码实现：

首先，我们需要定义学生选课的状态接口（State）：

public interface State {
void selectCourse();
void dropCourse();
}

然后，我们定义学生选课的三种状态：未选课状态（UnselectedState）、已选一门课程状态（OneCourseState）、已选两门课程状态（TwoCourseState）：

public class UnselectedState implements State {
private Student student;

public UnselectedState(Student student) {
this.student = student;
}

@Override
public void selectCourse() {
System.out.println("选了一门课程");
student.setState(student.getOneCourseState());
}

@Override
public void dropCourse() {
System.out.println("不能退选，未选课");
}
}

public class OneCourseState implements State {
private Student student;

public OneCourseState(Student student) {
this.student = student;
}

@Override
public void selectCourse() {
System.out.println("选了第二门课程");
student.setState(student.getTwoCourseState());
}

@Override
public void dropCourse() {
System.out.println("退选了一门课程");
student.setState(student.getUnselectedState());
}
}

public class TwoCourseState implements State {
private Student student;

public TwoCourseState(Student student) {
this.student = student;
}

@Override
public void selectCourse() {
System.out.println("不能再选课程，已选两门");
}

@Override
public void dropCourse() {
System.out.println("退选了一门课程");
student.setState(student.getOneCourseState());
}
}

最后，我们定义学生类（Student）：

public class Student {
private State unselectedState;
private State oneCourseState;
private State twoCourseState;

private State currentState;
private int courseCount;

public Student() {
unselectedState = new UnselectedState(this);
oneCourseState = new OneCourseState(this);
twoCourseState = new TwoCourseState(this);

currentState = unselectedState;
courseCount = 0;
}

public void setState(State state) {
currentState = state;
}

public State getUnselectedState() {
return unselectedState;
}

public State getOneCourseState() {
return oneCourseState;
}

public State getTwoCourseState() {
return twoCourseState;
}

public void selectCourse() {
currentState.selectCourse();
courseCount++;
}

public void dropCourse() {
currentState.dropCourse();
courseCount--;
}
}

现在，我们可以创建一个学生对象，然后模拟学生选课的场景：

Student student = new Student();

student.selectCourse(); // 选了一门课程
student.selectCourse(); // 选了第二门课程
student.selectCourse(); // 不能再选课程，已选两门

student.dropCourse(); // 退选了一门课程
student.dropCourse(); // 不能退选，未选课