将对象推入线程中Java异常的数组中;“主要”;java.lang.NullPointerException
我正在学习Java,我坚持这个例子 这是主要原因将对象推入线程中Java异常的数组中;“主要”;java.lang.NullPointerException,java,nullpointerexception,Java,Nullpointerexception,我正在学习Java,我坚持这个例子 这是主要原因 package bookstore; public class BookStore { public static void main(String[] args) { // create a Book object Book Java = new Book(); Book Python = new Book(); Book PHP = new Book();
package bookstore;
public class BookStore {
public static void main(String[] args) {
// create a Book object
Book Java = new Book();
Book Python = new Book();
Book PHP = new Book();
Java.name = "Thinking In Java";
Java.author = "Ahmad Ajmi";
Java.canBorrow = true;
Java.canReturned = true;
Java.pagesNumber = 400;
Java.publisher = "Oriely";
String publish = Java.publishedDate(2013,4,5);
System.out.println(publish);
// Library.booksNumber works fine
System.out.println("You have " + Library.booksNumber + " Books");
// The problem is here
System.out.println(Library.books);
}
}
图书班
package bookstore;
public class Book {
// Defining book variables
String name;
String author;
String publisher;
int pagesNumber;
int publisheddate;
boolean canBorrow;
boolean canReturned;
public String publishedDate(int year, int month, int day) {
return "" + year + " - " + month + " - " + day;
}
Book () {
// Keep track of books created
Library.booksNumber += 1;
// Here I want to push the book name into array
for (int i = 0; i < Library.booksNumber; i++) {
Library.books[i] = name;
}
}
}
当我运行时,我得到了这个错误
Exception in thread "main" java.lang.NullPointerException
at bookstore.Book.<init>(Book.java:23)
at bookstore.BookStore.main(BookStore.java:9)
Java Result: 1
线程“main”java.lang.NullPointerException中的异常
bookstore.Book.(Book.java:23)
在bookstore.bookstore.main(bookstore.java:9)
Java结果:1
我想将图书名称推送到库中。图书数组您需要初始化
图书
数组。在
书中试试这个
Library.books = new String[Library.booksNumber];
for (int i = 0; i < Library.booksNumber; i++) {
Library.books[i] = name;
}
Library.books=新字符串[Library.booksNumber];
for(int i=0;i
Library.books
未初始化。这是(int i=0;iLibrary.books = new String[Library.booksNumber];
for (int i = 0; i < Library.booksNumber; i++) {
Library.books[i] = name;
}