使用NO ARG构造函数添加到数组中

Question

我似乎在如何使用no-arg构造函数添加到数组上。我在这里缺少什么？

public class Book {
private String title;
private String author;
private int pages;
private double price;
public static int numBooks = 0;

public Book(String title, String author, int pages, double price) {
    super();
    this.title = title;
    this.author = author;
    this.pages = pages;
    this.price = price;
    numBooks++;
}
public Book() {
    super();
    numBooks++;
}
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 int getPages() {
    return pages;
}
public void setPages(int pages) {
    this.pages = pages;
}
public double getPrice() {
    return price;
}
public void setPrice(double price) {
    this.price = price;
}
@Override
public String toString() {
    return "book [title=" + title + ", author=" + author + ", pages=" + pages + ", price=" + price + "]";
}
public static int getNumBooks(){ 
    return numBooks;
}

这是我遇到很多麻烦的地方。我的主要位置，下面有我的No arg构造函数，我无法弄清楚如何在MAIN中添加该数组。任何朝着正确方向的建议或指针都将很棒。

}

public class TestBook {
private String title;
private String author;
private int pages;
private double price;
public TestBook(String title, String author, int pages, double price){
    this.title = title;
    this.author = author;
    this.pages = pages;
    this.price = price;
}
public static void main(String[] args) {
    Book[] bookArray = new Book[6]; //This is the array that I am trying to add the last two books too 
    Book bookArray0 = new Book("Java Proramming", "Liang", 1320, 145.00);
    Book bookArray1 = new Book("Horton Hears a Who", "Dr. Seuss", 72, 19.99);
    Book bookArray2 = new Book("The Hobbit", "Tolkien", 320, 9.25);
    Book bookArray3 = new Book("Born a Crime", "Noah", 304, 17.33);
    Book bookArray4 = new Book();
    Book bookArray5 = new Book();
    bookArray[0] = bookArray0;
    bookArray[1] = bookArray1;
    bookArray[2] = bookArray2;
    bookArray[3] = bookArray3;
    bookArray[4] = bookArray4;
    bookArray[5] = bookArray5;

    for(Book d : bookArray)
        System.out.println(d);

}
public TestBook() { //This is the no-arg that I am trying to use to add to the array above.
    Book[] bookArray = new Book[6];
    Book bookArray4 = new Book("The Town", "Chuck Hogan", 477, 14.99);
    Book bookArray5 = new Book("The Pretender ", "Sombody", 400, 24.99);
    bookArray[4] = bookArray4;
    bookArray[5] = bookArray5;
    for(Book b : bookArray)
        System.out.println(b);
    }
public void finishArray(){
Book lastBook = new Book();
lastBook.setTitle(title);
lastBook.setAuthor(author);
lastBook.setPages(pages);
lastBook.setPrice(price);
}

}

Answer 1

您的收入为null，因为您没有将数据设置为这些对象。这就是settitle和setauthor的目的。所以

之类的东西

  bookArray4.setTitle("The Town");
  bookArray4.setAuthor("Somebody");

当您使用传递信息的构造函数时，这些内容已设置在其中。但是对于没有参数的构造函数，没有传递数据。因此，您使用"设置器"方法设置数据

Answer 2

as sean reaves 指出，您需要将固定器用于使用默认构造函数创建的书籍。

书

public class Book {
    private String title;
    private String author;
    private int pages;
    private double price;
    private static int numBooks = 0;
    public Book(String title, String author, int pages, double price) {
        this();
        this.title = title;
        this.author = author;
        this.pages = pages;
        this.price = price;
    }
    public Book() {
        numBooks++;
    }
    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 int getPages() {
        return pages;
    }
    public void setPages(int pages) {
        this.pages = pages;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    @Override
    public String toString() {
        return "book [title=" + this.title + ", author=" + this.author + ", pages=" + this.pages + ", price=" + this.price + "]";
    }
    public static int getNumBooks() {
        return numBooks;
    }
}

我对您的代码进行了一些小修改，只是为了清理一点。

MAIN

import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        // All books
        Book[] books = new Book[] { 
            new Book("Java Proramming", "Liang", 1320, 145.00),
            new Book("Horton Hears a Who", "Dr. Seuss", 72, 19.99), 
            new Book("The Hobbit", "Tolkien", 320, 9.25),
            new Book("Born a Crime", "Noah", 304, 17.33), 
            new Book(), 
            new Book() 
        };
        // Use setters to avoid null values
        books[5].setAuthor("Not Null Author");
        books[5].setTitle("Not Null Title");
        // Print
        Arrays.asList(books).forEach(System.out::println);
    }
}

您也可以将price和pages属性的固定器拨打。

输出

book [title=Java Proramming, author=Liang, pages=1320, price=145.0]
book [title=Horton Hears a Who, author=Dr. Seuss, pages=72, price=19.99]
book [title=The Hobbit, author=Tolkien, pages=320, price=9.25]
book [title=Born a Crime, author=Noah, pages=304, price=17.33]
book [title=null, author=null, pages=0, price=0.0]
book [title=Not Null Title, author=Not Null Author, pages=0, price=0.0]