Question

Java运行错误：Exception in thread "main" java.lang.NullPointerException

Eclipse中运行错误
功能：建立Employee类（姓名，年薪，受雇时间），MyDate类（年月日），测试类
代码：

package abc.cba;
class MyDate {
private int day;
private int month;
private int year;
public void setDay (int day) {this.day = day;}
public void setMonth (int month) {this.month = month;}
public void setYear (int year) {this.year = year;}
public int getDay () {return this.day;}
public int getMonth () {return this.month;}
public int getYear () {return this.year;}
}
class Employee {
private String name;
private double annualSalary;
private MyDate employeedDate;
public void setName (String name) {this.name = name;}
public void setAnnualSalary (double annualSalary) {this.annualSalary = annualSalary;}
public void setEmployeedDate (int year_1, int month_1, int day_1) {
employeedDate.setYear(year_1);
employeedDate.setMonth(month_1);
employeedDate.setDay(day_1);
}
public String getName () {return this.name;}
public double getAnnualSalary () {return this.annualSalary;}
void tellAll () {
System.out.println("雇员姓名：" + name);
System.out.println("雇员年薪：" + annualSalary);
System.out.println("受雇时间："+employeedDate.getYear()+"年"+employeedDate.getMonth()+"月"+employeedDate.getDay()+"日");
}
}
public class TestEmployee {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Employee emp1 = new Employee ();
emp1.setName("小明");
emp1.setAnnualSalary(100000);
emp1.setEmployeedDate(2012, 2, 1);
emp1.tellAll();
}
}
运行错误：

Exception in thread "main" java.lang.NullPointerException
at abc.cba.Employee.setEmployeedDate(TestEmployee.java:24)
at abc.cba.TestEmployee.main(TestEmployee.java:50)
求解。

Answer 1

两个错误：
1.你的label数组个数不一样，第一个错越界了，改为：JLabel[]
label=new
JLabel[256];
2.label[i]要进行实例化，在
label[i].setBounds((i/32)*150,(i%32)*25,21,6);之前，增加一句：label[i]=new
JLabel();

Answer 2

private MyDate employeedDate没有实例化，应该private MyDate employeedDate=new MyDate();

Answer 3

空指针 setEmployeedDate 这个方法中的 employeedDate对象为空 没有初始化

Answer 4

EmployeedDate是对象，应该使用new EmployeedDate来创建类实例，然后给它的成员变量赋值。

在Employee的构造函数中，初始化该对象，或在变量声明时初始化该对象。如：
private MyDate employeedDate = new EmployeedDate();