Java中子类的实现

Question

I am completing a homework assignment for a Java programming course and I am having trouble understanding the concept of a subclcass.

Here is the question:

Create a class named Book that contains data fields for the title and number of pages. Include get and set methods for these fields. Next, create a subclass named Textbook, which contains an additional field that holds a grade level for the Textbook and additional methods to get and set the grade level field. Write an application that demonstrates using objects of each class. Save the files as Book.java, Textbook.java, and DemoBook.java.

Here is my code for Book.java:

public class Book
{
    String bookTitle;
    int numPages;

    private void setBTitle(String title)
    {
        bookTitle = title;
    }

    private void setBPages(int pages)
    {
        numPages = pages;
    }

    private String getBTitle()
    {
        return bookTitle;
    }

    private int getBPages()
    {
        return numPages;
    }

    public void displayBookInfo()
    {
        System.out.println("The book's title is: " + bookTitle + ".");
        System.out.println("The number of pages is: " + numPages + ".");
    }
}

Here is my code for Texbook.java:

public class Textbook extends Book
{
    int gradeLevel;

    public int getGLevel()
    {
        return gradeLevel;
    }

    public void setGLevel(int level)
    {
        gradeLevel = level;
    }
    }

If I do in fact have those two parts correct, how would I implement this in a DemoBook.java file?

Any help or direction would be appreciated.

Here is my code for the DemoBook.java file:

import java.util.Scanner;

public class DemoBook
{
    public static void main(String[] args)
    {
        String BTitle;
        int BPages;
        int BLevel;

        Book b = new Book();
        Textbook t = new Textbook();
        Book bt = new Textbook();

        Scanner input = new Scanner(System.in);
        System.out.println("Please enter the title of your book: ");
        BTitle = input.nextLine();
        System.out.println("Please enter the number of pages: ");
        BPages = input.nextInt();
        System.out.println("Please enter the grade level: ");
        BLevel = input.nextInt();

        b.setBTitle(BTitle);
        b.setBPages(BPages);
        t.setGLevel(BLevel);

        b.displayBookInfo();

    }
}

I changed the variables to private, and this is the compiler error I get:

DemoBook.java:33: error: setBTitle(String) has private access in Book
        b.setBTitle(BTitle);
         ^
DemoBook.java:34: error: setBPages(int) has private access in Book
        b.setBPages(BPages);
         ^
2 errors

I am still not grasping this. The chapter is titled Introduction to Inheritance.

Answer 1

Code V1:

I believe it's just asking you to make a DemoBook.java with a main() function that creates one of each type, and does some sets and gets. The code itself seems fine to me!

Code V2:

You have to have your getters and setters all stay public! What Tomas and other folks meant was make your variables themselves private, the String and two ints. You usually want all of your member variables to be private, and your getters and setters to be public. Also, if you make your getters and setters private, you can't even access them in derived classes!

I would also do a bit more getting and setting between your books. Try getting from one and setting into another, print out all three, go wild! =)

Answer 2

As Danalog said, the classes are fine, now use them! Try to do an example for everything you can think of trying.

I would suggest making some Book objects, Book b = new Book(); and calling some methods on them, make some TextBooks, TextBook t = new TextBook(); .
And finally Book bt = new TextBook(); .

Play around with them all and print out some results.

EDIT: Great suggestion by @Tomas above, make the variables private as you are using getters and setters.

Answer 3

Book.java

public class Book
{
    private String bookTitle;
    private int bookPages;

    public void setBookTitle(String bookTitle)
    {
        this.bookTitle = bookTitle;
    }
    public void setBookPages(int bookPages)
    {
        this.bookPages = bookPages;
    }
    public String getBookTitle()
    {
        return this.bookTitle;
    }
    public int getBookPages()
    {
        return this.bookPages;
    }
    public void displayBookInfo()
    {
        System.out.println("The book's title is: " + this.bookTitle + ".");
        System.out.println("The number of pages is: " + this.bookPages + ".");
    }
}

Textbook.java

public class Textbook extends Book
{
    private int gradeLevel;

    public int getGradeLevel()
    {
        return this.gradeLevel;
    }

    public void setGradeLevel(int gradeLevel)
    {
        this.gradeLevel = gradeLevel;
    }
    }

DemoBook.java

Answer 4

DemoBook.java

import java.util.Scanner;

public class DemoBook
{
    public static void main(String[] args)
    {
        String BTitle;
        int BPages;
        int BLevel;

        Book b = new Book();
        Textbook t = new Textbook();
        Book bt = new Textbook();

        Scanner input = new Scanner(System.in);
        System.out.println("Please enter the title of your book: ");
        BTitle = input.nextLine();
        System.out.println("Please enter the number of pages: ");
        BPages = input.nextInt();
        System.out.println("Please enter the grade level: ");
        BLevel = input.nextInt();

        b.setBookTitle(BTitle);
        b.setBookPages(BPages);
        t.setGradeLevel(BLevel);

        b.displayBookInfo();

    }
}

Answer 5

This assignment is trying to make you familiar with the concept of Polymorphism. There's a good overview here

Answer 6

Include get and set methods for these fields.

This generally means that your fields (ie. member variables) should be private , encapsulating them and requiring the use of the get and set methods as the only way to access and modify those variables.

If I do in fact have those two parts correct, how would I implement this in a DemoBook.java file?

This is the simplest class of all, it's just a test class to see if you know how to create objects of the base and derived type, and to use them properly. This is the entry point of the application, so you'll have a public class DemoBook and your main method inside there.

You should make some test cases to ensure that the functionality of the two classes is working properly, and perhaps print out the "fields" with the get methods after you've assigned values to them with the set methods.