nullpointerexception无法理解为什么在使用java设置月份时出现null指针异常。本地日期
我正在创建一个使用2个类的程序,其中一个类创建一个使用java构造的具有姓名和生日的作者。localdate和创建引用author类的书籍的第二个。使用demo类,我试图设置作者的生日,但是当我使用Month.JULY
设置月份时,我得到了一个空指针异常,我不知道为什么。这是我的密码:
public class Book {
private String name;
private Author author;
private String ISBN;
private double price;
public Book(String name, Author author, double price, String ISBN) {
this.name = name;
this.author = author;
this.price = price;
this.ISBN = ISBN;
}
public String getName() {
return name;
}
public Author getAuthor() {
return author;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
public String getISBN() {
return ISBN;
}
public void setISBN(String ISBN) {
this.ISBN = ISBN;
}
public String toString() {
return name + " by " + author + ISBN + price;
}
}
import java.time.LocalDate;
import java.time.Month;
public class Author {
private String firstname;
private String lastname;
private int year;
private Month month;
private int dayOfMonth;
LocalDate birthday = LocalDate.of( year, month, dayOfMonth);
public Author(String firstname, String lastname, int year, Month month, int dayOfMonth) {
this.firstname= firstname;
this.lastname= lastname;
this.year = year;
this.month = month;
this.dayOfMonth=dayOfMonth;
}
public String getFirstName() {
return firstname;
}
/** Returns the gender */
public String getLastName() {
return lastname;
}
/** Returns the email */
public int getYear() {
return year;
}
public Month getMonth() {
return month;
}
public int getdayOfMOnth() {
return dayOfMonth;
}
/** Returns a self-descriptive String */
public String toString() {
return firstname + " " + lastname + "(birthday:" + birthday + ")";
}
}
import java.time.Month;
import java.time.localDate:
//I tried with and without java.time.localDate
public class testBook {
public static void main(String[] args) {
Author jkr = new Author("JK","Rowling",1965,Month.JULY,31);
Book HPSS = new Book("Harry Potter and the Sorcerer's Stone", jkr, 11.99, "B017V4IMVQ" );
System.out.println(HPSS);
System.out.println(jkr);
}
}
# 1 楼答案
你有台词吗
现在year和dayOfMonth是int,并用0初始化,但month是month Cals的对象,在创建Author类的实例时为null。这会导致一个空指针
因此,如果您尝试在构造函数中移动该行,如下所示:
您将获得如下输出:
# 2 楼答案
在我看来,您希望将变量'birth'的初始化向下移动到Author的构造函数中