我在BookTest课程上收到三条错误消息，表示在getCode()，getCategory和calculateTax上找不到符号。我该如何解决？我试图在对话框中打印这些，但这三个是唯一不起作用的。
import javax.swing. JOptionPane;
public class BookTest
{
public static void main(String args[])
{
double charge;
double grandTotal= 0;
String dataArray[][] = {{"NonFiction", "Abraham Lincoln Vampire Hunter","Grahame-Smith","978-0446563079","13.99","Haper","NY","US","Political"},
{"NonFiction", "Frankenstein","Shelley","978-0486282114","7.99","Pearson", "TX","England", "Historical"},
{"Fiction", "Dracula","Stoker","978-0486411095","5.99","Double Day", "CA","4918362"},
{"NonFiction", "Curse of the Wolfman"," Hageman","B00381AKHG","10.59","Harper", "NY","Transylvania","Historical"},
{"Fiction", "The Mummy","Rice","978-0345369949","7.99","Nelson","GA","3879158"}};
Book bookArray[] = new Book[dataArray.length];
int quantityArray[] = {12, 3, 7, 23, 5};
for (int i = 0; i < dataArray.length; i++)
{
if (dataArray[i][0] == "NonFiction")
{
bookArray[i] = new NonFictionBook(dataArray[i][1], dataArray[i][2], dataArray[i][3], Double.parseDouble(dataArray[i][4]),
new Publisher(dataArray[i][5], dataArray[i][6]), dataArray[i][7], dataArray[i][8]);
}
else
{
bookArray[i] = new FictionBook(dataArray[i][1], dataArray[i][2], dataArray[i][3], Double.parseDouble(dataArray[i][4]),
new Publisher(dataArray[i][5], dataArray[i][6]), Integer.parseInt(dataArray[i][7]));
}
}
String msg = "";
for (int i = 0; i < bookArray.length; i++)
{
charge = bookArray[i].calculateTotal(quantityArray[i]);
grandTotal = charge + grandTotal;
msg += String.format("%s %s %d $%.2f $%.2f\n", bookArray[i].getTitle(), bookArray[i].getCategory(), bookArray[i].getCode(), bookArray[i].calculateTax, charge); //this is where i get the 3 error messages. im trying to print all in one dialog box the title, category of the book, charge and tax for each book.
}
msg += String.format("Grand Total $%.2f ", grandTotal);
JOptionPane.showMessageDialog(null, msg);
}
}
**************************************************************
public abstract class Book implements Tax
{
private String title;
private String author;
private String isbn;
private Double price;
private Publisher publisher;
public Book()
{
setTitle("");
setAuthor("");
setIsbn("");
setPrice(0.0);
setPublisher(new Publisher());
}
public Book(String t, String a, String i, double p, Publisher n)
{
setTitle(t);
setAuthor(a);
setIsbn(i);
setPrice(p);
setPublisher(n);
}
public void setTitle(String t)
{
title = t;
}
public String getTitle()
{
return title;
}
public void setAuthor(String a)
{
author = a;
}
public String getAuthor()
{
return author;
}
public void setIsbn(String i)
{
isbn = i;
}
public String getIsbn()
{
return isbn;
}
public void setPrice(double p)
{
price = p;
}
public double getPrice()
{
return price;
}
public void setPublisher(Publisher n)
{
publisher = n;
}
public Publisher getPublisher()
{
return publisher;
}
public abstract double calculateTotal(int quantity);
public double calculateTax(double a)
{
return a * .08;
}
public String toString()
{
return( " Title " + title + " Author " + author + " Isbn " + isbn
+ " Price " + price + " Publisher " + publisher.toString());
}
}
********************************************************
public class NonFictionBook extends Book
{
private String country;
private String category;
public NonFictionBook()
{
super();
setCountry("");
setCategory("");
}
public NonFictionBook(String t, String a, String i, double p, Publisher n, String c, String ca)
{
super(t,a,i,p,n);
setCountry(c);
setCategory(ca);
}
public void setCountry(String c)
{
country = c;
}
public void setCategory(String ca)
{
category = ca;
}
public String getCountry()
{
return country;
}
public String getCategory()
{
return category;
}
public String toStirng()
{
return( super.toString() + "Country " + country + " Category " + category);
}
public double calculateTotal(int quantity)
{
double charge =0;
charge = (quantity * getPrice());
if( country != "US" )
charge += 50;
return charge;
}
}
*********************************************
public class FictionBook extends Book
{
private int code;
public FictionBook()
{
super();
setCode(0);
}
public FictionBook(String t, String a, String i, double p, Publisher n, int c)
{
super(t,a,i,p,n);
setCode(c);
}
public void setCode(int c)
{
code = c;
}
public int getCode()
{
return code;
}
public String toString()
{
return (super.toString() + " Code " + code);
}
public double calculateTotal(int quantity)
{
double charge =0;
charge = (quantity * getPrice());
if (quantity > 5)
charge += 5 * (quantity - 5);
return charge;
}
}
答案 0 :(得分：1)
树方法在Books的子类中实现。所以你必须将值转换为子类。
if (bookArray[i] instanceof FictionBook){
FictionBook fb = (FictionBook)bookArray[i];
msg += String.format("%s %s %d $%.2f $%.2f\n", fb.getTitle(), "", fb.getCode(), 0, charge);
}
if (bookArray[i] instanceof NonFictionBook){
NonFictionBook fb = (NonFictionBook)bookArray[i];
msg += String.format("%s %s %d $%.2f $%.2f\n", nfb.getTitle(), nfb.getCategory(), nfb.getCode(), nfb.calculateTax, charge);
}
等等
此外，您必须使用equals()来比较字符串。不是==
答案 1 :(得分：0)
在您遇到错误的行中，您尝试打印的字段不是Book类的实例，而是其子类的实例。 你有一系列的书籍，你迭代它们并尝试打印信息。但并非所有的书籍都有getCode（）方法（只有小说书籍），只有非小说类书籍才有getCategory（）方法。所以你不能为所有书籍做这件事。
您可以根据书籍类型更改您的打印，或者您可以在Book类中创建一个方法，每个子类都可以覆盖它，打印您为此类提供的信息（如toString方法）。然后在主类中使用它。
并且如评论中所指出的那样，测试字符串是否与equals运算符相等而不是==或！=
答案 2 :(得分：0)
您的数组使用类型簿：
Book bookArray[] = new Book[dataArray.length];
稍后，您将子类添加到此数组中，例如NonFictionBook。由于code，category等只是子类的一部分，因此您无法使用Book数组引用访问它们。
// won't work because class Book does not have these methods, properties
bookArray[i].getCategory(), bookArray[i].getCode(), bookArray[i].calculateTax
您需要强制转换对象（基于数组中的类型）。