使用多态

Question

我在使用多态性向对象添加一些字符串时遇到了麻烦。经历的第一个对象获取字符串，但是我传递字符串的第二个对象由于某种原因没有获得它。对不起，如果我发布了太多代码，我不太清楚哪些部分对于找出问题很重要。

这是代码

public class Publication {

public static void main(String[] args) {


    List <Book> list = new ArrayList <Book>();

    Book bookOne = new Periodical("Harry Potter", "House", "2000", "Fantasy");
    bookOne.setPubFreq("Yearly");
    bookOne.setEditor("Dave");

    Book bookTwo = new Book("Game of Thrones", "Lanister", "2000", "Fantasy");
    bookTwo.setPubFreq("Yearly");
    bookTwo.setEditor("George");

    System.out.print(bookOne);
    System.out.print(bookTwo);
}

使用上面的主要方法，我试图传递字符串，第一个打印就像这样

Book Title               :Harry Potter
Publisher                :House
Publication date         :2000
Subject                  :Fantasy
Publication Frequency    :Yearly
Editor                   :Dave

然而，由于某种原因，本书不包括出版频率或编辑。

以下是代码的其余部分，我怀疑它与摘要有关。

public class Periodical extends Book {

  private String pubFreq, editor;
  //CONSTRUCTOR
  public Periodical(String title, String publisher, String publicationDate, String subject) {
    super(title, publisher, publicationDate, subject);
  }

  //PUBLICATION FREQUENCY
  public String getPubFreq() {
    return pubFreq;
  }

  public void setPubFreq(String bookPubFreq) {
      this.pubFreq = bookPubFreq;
    }
    //EDITOR
  public String getEditor() {
    return editor;
  }

  public void setEditor(String bookEditor) {
    this.editor = bookEditor;
  }

  public String toString() {
    StringBuffer strBuf = new StringBuffer(super.toString());
    strBuf.append("\nPublication Frequency    :");
    strBuf.append(pubFreq);
    strBuf.append("\nEditor                   :");
    strBuf.append(editor);

    return strBuf.toString();
  }



}

public class Book {

  private String title, publisher, publicationDate, subject;
  //CONSTRUCTOR
  Book() {
    setTitle("");
    setPublisher("");
    setPublicationDate("");
    setSubject("");
  }

  Book(String title, String publisher, String publicationDate, String subject) {
    setTitle(title);
    setPublisher(publisher);
    setPublicationDate(publicationDate);
    setSubject(subject);
  }

  //TITLE
  public String getTitle() {
    return title;
  }

  public void setTitle(String bookTitle) {
      this.title = bookTitle;
    }
    //PUBLISHER
  public String getPublisher() {
    return publisher;
  }

  public void setPublisher(String bookPublisher) {
      this.publisher = bookPublisher;
    }
    //PUBLICATION DATE

  public String getPublicationDate() {
    return publicationDate;
  }

  public void setPublicationDate(String bookPublicationDate) {
      this.publicationDate = bookPublicationDate;
    }
    //SUBJECT

  public String getSubject() {
    return subject;
  }

  public void setSubject(String bookSubject) {
    this.subject = bookSubject;
  }

  public String toString() {
      return ("Book Title               :" + title + "\n" +
        "Publisher                :" + publisher + "\n" +
        "Publication date         :" + publicationDate + "\n" +
        "Subject                  :" + subject);
    }
    //ABSTRACT
  public void setPubFreq(String string) {

  }

  public void setEditor(String string) {

  }


}

Answer 1

您没有在Book类中将setEditor和setPubFreq声明为abstract。如果你想要它们是抽象的，你必须像这样声明它们：

public abstract void setPubFreq(String string);
public abstract void setEditor(String string);

您还必须将整个类声明为抽象 - 然后它将无法实例化。如果这不是你的意图，你可以使用一个声明这两个方法的接口，让你的Book和Periodical类实现它，或者制作另一个Book扩展的抽象类，只有这两个方法，声明为abstract。 / p>

但我想你可能只是想这样做：

Book bookTwo = new Periodical("Game of Thrones", "Lanister", "2000", "Fantasy");

而不是：

Book bookTwo = new Book("Game of Thrones", "Lanister", "2000", "Fantasy");

就像你使用bookOne一样。