将book与booktest分成两个类

将`Book`和`BookTest`分别放在两个独立的类中可以使代码更清晰、易于维护。以下是两个类的分离版本：

**Book.java**

public class Book {
    private String title;
    private int pageNum = 200;

    public Book(String title) {
        if (title == null || title.isEmpty()) {
            throw new IllegalArgumentException("Book title cannot be empty");
        }
        this.title = title;
    }

    // getter and setter methods
    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        if (title == null || title.isEmpty()) {
            throw new IllegalArgumentException("Book title cannot be empty");
        }
        this.title = title;
    }

    public int getPageNum() {
        return pageNum;
    }

    public void setPageNum(int pageNum) {
        if (pageNum < 200) {
            System.out.println("Error: Page number should not be less than 200. Setting default to 200.");
            this.pageNum = 200;
        } else {
            this.pageNum = pageNum;
        }
    }

    // detail method
    public void detail() {
        System.out.println("Title: " + title + ", Pages: " + pageNum);
    }
}

**BookTest.java**

import org.junit.Test;

public class BookTest {
    @Test
    public void testBook() {
        // 创建Book对象并调用detail方法
        Book book = new Book("Java Programming");
        book.detail();

        // 修改页数并验证
        try {
            book.setPageNum(150); // 尝试设置无效页数
        } catch (IllegalArgumentException e) {
            System.out.println(e.getMessage());
        }

        // 再次调用detail方法
        book.detail();
    }
}

现在，`Book`类专注于书籍实体的定义，而`BookTest`类则是一个JUnit测试类，用于测试`Book`类的行为。这样的设计使得每个类都有明确的责任，提高了代码的复用性和可测试性。