java - 继承和私有(private)/公共(public)变量

Question

所以我有三个类:books、booksOut 和 Allbooks。 booksout 和 Allbooks 是扩展/是书籍的子类。但是我有一个问题/问题:

我在 toString 方法中收到来自 allBooks 的错误。说 bookID 等(即 books 中的所有变量)都设置为私有(private)。我被告知你应该总是尽量避免公共(public)变量，所以我犹豫要不要这样做。还有别的办法吗？或者将它们公开是处理此问题的最简单/最佳方法？

代码如下:

书籍

public class books {

private int bookID;
private String title;
private String author;


public books() {
}

public books(int bookID, String title, String author) {
    this.bookID = bookID;
    this.title = title;
    this.author = author;
}

public int getBookID() {
    return bookID;
}

public void setBookID(int bookID) {
    this.bookID = bookID;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public String getAuthor() {
    return author;
}

public void setAuthor(String author) {
    this.author = author;
}


public String addSpaces(String s, int w) {
    String spc = "";
    for (int i = 1; i <= (w - s.length()); i++) {
        spc = spc + " ";
    }
    return spc;
}

public class AllBooks extends books{
private String genre;
private String status;
private String Location;
private String condition;

所有书籍

public AllBooks(int bookID, String title, String author, String genre, String status, String Location, String condition ) {
    super(bookID, title, author);
    this.genre = genre;
    this.status = status;
    this.Location = Location;
    this.condition = condition;
}

public String getGenre() {
    return genre;
}

public void setGenre(String genre) {
    this.genre = genre;
}

public String getStatus() {
    return status;
}

public void setStatus(String status) {
    this.status = status;
}

public String getLocation() {
    return Location;
}

public void setLocation(String Location) {
    this.Location = Location;
}

public String getCondition() {
    return condition;
}

public void setCondition(String condition) {
    this.condition = condition;
}



    @Override
public String toString() {
    String stg = "";
    stg = stg + bookID + '\t' + title + addSpaces(title, 30) + author + addSpaces(author, 30) + genre + addSpaces(genre, 15) + status + addSpaces(status, 5) + Location + addSpaces(Location, 20) + condition;
    return stg;
}

    public String toString(int i){
    String stg = "";
    stg += bookID + "#" + title + "#" + author + "#" + genre + "#" + status + "#" + Location + "#" + condition + "#";
    return stg;
}

附言

如果这是一个愚蠢的问题，我很抱歉。这是一个学校项目，应该在我假期后完成，我现在正在休假，因此我不问老师的原因。我确实在网上查过答案，但是我遇到的教程并没有太多提及礼仪。感谢您的帮助，和/或抱歉浪费您的时间。

Answer 1

使用现有的公共(public)访问器。例如，在子类中使用 getBookID() 而不是尝试直接访问父类(super class)的私有(private)字段。

@Override
public String toString() {
    String stg = getBookID() + '\t' + getTitle() 
        + addSpaces(title, 30) + getAuthor() 
        + addSpaces(author, 30) + getGenre() 
        + addSpaces(genre, 15) + getStatus() 
        + addSpaces(status, 5) + getLocation() 
        + addSpaces(Location, 20) + getCondition();
    return stg;
}

另一种方法是使字段 protected 。但是，这允许任意数量的子类直接耦合到父类(super class)的表示。封装方法背后的数据通常更可取。

顺便说一句，您可能会发现 String.format()有用。有关语法，请参阅 Format String Syntax .