为什么55.5必须加f才能赋值给float变量。。。

package Book;

public class Test {

public static void main(String[] args) {

// TODO 自动生成的方法存根

Book one=new Book("红楼梦","曹雪芹","人民文学出版社",10);

System.out.println(one.information());

System.out.println("=======================");

Book two=new Book("小李飞刀","古龙","中国长安出版社",55.5f);

System.out.println(two.information());

}

}

package Book;

public class Book {

  //私有属性：书名、作者、出版社、价格

    private String bookName,author,publisher;

    private float price;

  //通过构造方法实现属性赋值

    public Book(String bookName,String author,String publisher,float price) {

    this.setBookName(bookName);

    this.setAuthor(author);

    this.setPublisher(publisher);

    this.setPrice(price);

    }

    /*通过公有的get/set方法实现属性的访问，其中：+

    1、限定图书价格必须大于10，如果无效需进行提示，并强制赋值为10

    2、限定作者、书名均为只读属性

    */

public String getBookName() {

return bookName;

}

public void setBookName(String bookName) {

this.bookName = bookName;

}

public String getAuthor() {

return author;

}

public void setAuthor(String author) {

this.author = author;

}

public String getPublisher() {

return publisher;

}

public void setPublisher(String publisher) {

this.publisher = publisher;

}

public double getPrice() {

return price;

}

public void setPrice(float price) {

//if(price>10)

this.price = price;

/*else

{

System.out.println("图书价格最低10元");

this.price=10;

}

*/

}

  //信息介绍方法，描述图书所有信息

public String information() {

    return "书名："+this.getBookName()+"\n作者："+this.getAuthor()+"\n出版社："+this.getPublisher()

+"\n价格："+this.getPrice();

}

}

为什么55.5必须加f才能赋值给float变量，而上面的10为啥不用修改就可以直接赋值float变量呢？