老师，麻烦讲解一下，为什么构造方法是这样写的？

public class BookTest {

	public static void main(String[] args) {
		Book a=new Book("红楼梦","曹雪芹");
		Book b=new Book("小李飞刀", "古龙");
		a.setBookHouse("人民文学出版社");
		b.setBookHouse("中国长安出版社");
		a.setPrices(8);
		b.setPrices(55.5);
		System.out.println(a.info());
		System.out.println("================================");
		System.out.println(b.info());
	}

}

public class Book {
	private String bookName,author,bookHouse;
	private double prices;
	public String getBookName() {
		return bookName;
	}

	public String getAuthor() {
		return author;
	}
	public Book(String bookName,String author) {
		this.bookName=bookName;
		this.author=author;
	}
	public String getBookHouse() {
		return bookHouse;
	}
	public void setBookHouse(String bookHouse) {
		this.bookHouse = bookHouse;
	}
	public double getPrices() {
		return prices;
	}
	public void setPrices(double prices) {
		if(prices<=10) {
			System.out.println("图书价格最低为10元");
			this.prices=10.0;
		}else
			this.prices = prices;
	}
	public String info() {
		String info="书名："+this.getBookName()+"\n作者："+this.getAuthor()+"\n出版社："+this.getBookHouse()+"\n价格"+this.getPrices()+"元";
		return info;
	}
}