Question

为什么会出现Exception in thread "main" java.lang.NullPointerException 的错误？

package pac;

import java.util.*;

public class CloneTest {
public static void main(String[] args) {
try {
Employee original = new Employee("zhang san", 3000);
original.setHireDay(2000, 1, 1);
Employee clone = original.clone();
clone.raiseSalary(10);
clone.setHireDay(2001, 2, 24);
System.out.println("original:" + original);
System.out.println("clone:" + clone);
} catch (CloneNotSupportedException e) {
e.printStackTrace();
}
}
}

class Employee implements Cloneable {
Employee(String name, double salary) {
this.name = name;
this.salary = salary;
}

public Date setHireDay(int year, int month, int day) {
GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
Date date = calendar.getTime();
return date;
}
public double raiseSalary(double d){
return salary+salary*d/100;
}
public String toString(){
return "name:"+name+"salary:"+salary+"hireDay"+hireDate;
}
protected Employee clone()throws CloneNotSupportedException{
//call Object.clone();
Employee cloned=(Employee)super.clone();
//clone mutable fields
cloned.hireDate=(Date)hireDate.clone();
return cloned;
}
private String name = "";
private double salary = 0;
private Date hireDate;

}

Answer 1

cloned.hireDate=(Date)hireDate.clone();

这句话里面的hireDate你就没有初始化，当然会抛出空指针异常啦。

你setHireDay（）里面有初始化你在最后定义的private Date hireDate;吗？？？？

Answer 2

重写后clone的方法签名是 public Object clone() throws CloneNotSupportedException{..}

另外 NPE 一般是没有初始化变量，你在出错的哪里看看哪个变量值是null。

Answer 3

空指针是因为hireDate没有值，
cloned.hireDate=(Date)hireDate.clone();会抛异常

Answer 4

hireDate没有初始值

Answer 5

一般包空指针的异常 你找到报错那一行， 一般都是.前面的东西为空。倒着查一下就能看到哪里为空了 。