java - 创建实例时出现空指针异常

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

我有一个异常，我不知道如何解决它。

Person a = new Person();
Scanner in = new Scanner(System.in);
System.out.println("Please type name: ");
String name = in.nextLine();

它说第一行有一个异常。以下代码是默认构造函数。

Person(){
         this.name = "No_Name";
         this.surname = "No_Surname";
         this.number = "No_Number";
         this.personalCode = "No_personalCode";
         this.dateOfBirth.set(Calendar.DAY_OF_MONTH, 1);
         this.dateOfBirth.set(Calendar.MONTH, Calendar.JANUARY);
         this.dateOfBirth.set(Calendar.YEAR, 1900);
     }

类(class)人员完整代码:

import java.util.Calendar;
public class Person {
     String name;
     String surname;
     private String number;
     private String personalCode;
     private Calendar dateOfBirth;



     Person(){
         this.name = "No_Name";
         this.surname = "No_Surname";
         this.number = "No_Number";
         this.personalCode = "No_personalCode";
         this.dateOfBirth.set(Calendar.DAY_OF_MONTH, 1);
         this.dateOfBirth.set(Calendar.MONTH, Calendar.JANUARY);
         this.dateOfBirth.set(Calendar.YEAR, 1900);
     }
     Person(String name,String surname,String number,String personalCode,Calendar dateOfBirth){
         this.name = name;
         this.surname = surname;
         this.number = number;
         this.personalCode = personalCode;
         this.dateOfBirth = (Calendar) dateOfBirth.clone();

     }

     Person setName(Person a, String name) {
         a.name = name;
         return a;
     }
     Person setSurname(Person a, String surname) {
         a.surname = surname;
         return a;
     }
     Person setNumber(Person a, String number) {
         a.number = number;
         return a;
     }
    Person setpersonalCode(Person a, String personalCode) {
        a.personalCode = personalCode;
        return a;
    }

    String getName(Person a) {
        return a.name;
    }
    String getSurname(Person a) {
        return a.surname;
    }
    String getNumber(Person a) {
        return a.number;
    }
    String getPersonalCode(Person a) {
        return a.personalCode;
    }
    Calendar getDateOfBirth(Person a) {
        return a.dateOfBirth;
    }
}

Answer 1

您正在对空字段 (dateOfBirth) 调用 set()。

可能的解决方法:

Date date = new GregorianCalendar(1900, Calendar.FEBRUARY, 1).getTime();
this.dateOfBirth = date;