用Java编写设计一个教材类，一个课程类，及一个为某门课程指定参考教材的类。一门课程可以有多本参考教材，再设计一个测试类。

好的，以下是一个简单的教材类、课程类、为某门课程指定参考教材的类以及测试类的Java代码：

教材类 `Textbook.java`：

public class Textbook {
    private String title;
    private String author;
    private int yearPublished;

    public Textbook(String title, String author, int yearPublished) {
        this.title = title;
        this.author = author;
        this.yearPublished = yearPublished;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }

    public int getYearPublished() {
        return yearPublished;
    }
}

课程类 `Course.java`：

import java.util.ArrayList;

public class Course {
    private String name;
    private int credit;
    private ArrayList<Textbook> textbooks;

    public Course(String name, int credit) {
        this.name = name;
        this.credit = credit;
        this.textbooks = new ArrayList<>();
    }

    public String getName() {
        return name;
    }

    public int getCredit() {
        return credit;
    }

    public void addTextbook(Textbook textbook) {
        textbooks.add(textbook);
    }

    public ArrayList<Textbook> getTextbooks() {
        return textbooks;
    }
}

为某门课程指定参考教材的类 `CourseTextbook.java`：

public class CourseTextbook {
    private Course course;
    private Textbook textbook;

    public CourseTextbook(Course course, Textbook textbook) {
        this.course = course;
        this.textbook = textbook;
    }

    public Course getCourse() {
        return course;
    }

    public Textbook getTextbook() {
        return textbook;
    }
}

测试类 `Test.java`：

public class Test {
    public static void main(String[] args) {
        // 创建教材
        Textbook textbook1 = new Textbook("Java编程思想", "Bruce Eckel", 2000);
        Textbook textbook2 = new Textbook("Effective Java", "Joshua Bloch", 2008);

        // 创建课程
        Course course1 = new Course("Java程序设计", 4);
        Course course2 = new Course("高级Java程序设计", 3);

        // 为课程1指定参考教材
        CourseTextbook course1Textbook1 = new CourseTextbook(course1, textbook1);
        CourseTextbook course1Textbook2 = new CourseTextbook(course1, textbook2);
        course1.addTextbook(textbook1);
        course1.addTextbook(textbook2);

        // 输出课程1的信息及参考教材
        System.out.println("课程名称：" + course1.getName());
        System.out.println("学分：" + course1.getCredit());
        System.out.println("参考教材：");
        for (Textbook textbook : course1.getTextbooks()) {
            System.out.println(textbook.getTitle() + "（" + textbook.getAuthor() + "，" + textbook.getYearPublished() + "）");
        }

        // 为课程2指定参考教材
        CourseTextbook course2Textbook1 = new CourseTextbook(course2, textbook2);
        course2.addTextbook(textbook2);

        // 输出课程2的信息及参考教材
        System.out.println("课程名称：" + course2.getName());
        System.out.println("学分：" + course2.getCredit());
        System.out.println("参考教材：");
        for (Textbook textbook : course2.getTextbooks()) {
            System.out.println(textbook.getTitle() + "（" + textbook.getAuthor() + "，" + textbook.getYearPublished() + "）");
        }
    }
}

在测试类中，我们创建了两个教材、两门课程以及为这两门课程指定了参考教材，并输出了课程信息以及参考教材信息。