3-8作业问题

package com.Imooc;


public class Department {
  private int departmentid;
  private String departmentname;
  private Staff[] myStaff;
  private int staffNum;
public Department(){
	
}
public Department(int departmentid,String departmentname){
	this.departmentid =departmentid;
	this.departmentname =departmentname;
	
}
public int getDepartmentid() {
	return departmentid;
}
public String getDepartmentname() {
	return departmentname;
}
public void setDepartmentid(int departmentid) {
	this.departmentid = departmentid;
}
public void setDepartmentname(String departmentname) {
	this.departmentname = departmentname;
}
public Staff[] getMyStaff() {
	if(this.myStaff==null)
		this.myStaff =new Staff[20];
	return myStaff;
}
public int getStaffNum() {
	return staffNum;
}
public void setMyStaff(Staff[] myStaff) {
	
	this.myStaff = myStaff;
}
public void setPostNu(int staffNum) {
	this.staffNum = staffNum;
}
public void  addStaff(Staff sta){
	for(int i=0;i<this.getMyStaff().length;i++){
		if(this.getMyStaff()[i]==null){
			this.getMyStaff()[i]=sta;
			this.staffNum=i+1;
			return;
		}
	}
}  
}

package com.Imooc;

public class Staff {
//成员属性：员工姓名，工号，年龄，性别，所属部门，职务信息
private String staffname;
private String staffid;
private int staffage;
private String staffsex;
private Department staffdepartment;
private Post staffpost;

//无参构造方法
public Staff(){
	
}
//带参构造方法，实现对属性的全部赋值
public Staff(String staffname,String staffid,int staffage,String staffsex){
	this.staffname=staffname;
	this.staffid =staffid;
	this.setStaffage(staffage);
	this.setStaffsex(staffsex);
	
}
public String getStaffname() {
	return staffname;
}
public String getStaffid() {
	return staffid;
}
public int getStaffage() {
	
	return staffage;
}
public String getStaffsex() {
	return staffsex;
}
public Department getStaffdepartment() {
	return staffdepartment;
}
public Post getStaffpost() {
	return staffpost;
}
public void setStaffname(String staffname) {
	this.staffname = staffname;
}
public void setStaffid(String staffid) {
	this.staffid = staffid;
}
//设置年龄限制，必须在18-65之间，否则强制默认为18
public void setStaffage(int staffage) {
	if(staffage>=18&&staffage<=65){
		this.staffage = staffage;
	}
	else{
		this.staffage=18;
	}
	
}
//设定性别只能是“男”或“女”，反之则设置默认为“男”
public void setStaffsex(String staffsex) {
	if(staffsex.equals("女")){
	this.staffsex = staffsex;
	}
	else{
		this.staffsex = "男";
	}
}
public void setStaffdepartment(Department staffdepartment) {
	this.staffdepartment = staffdepartment;
}
public void setStaffpost(Post staffpost) {
	this.staffpost = staffpost;
}
public String info(Department staffdepartment, Post staffpost){
	String str="姓名:"+this.getStaffname()+"\n工号"+this.getStaffid()+"\n年龄"+this.getStaffage()+"\n性别"+this.getStaffsex()+"\n职务:"+this.getStaffdepartment().getDepartmentname()+this.getStaffpost().getPostname();
	return str;
	
}
}

package com.Imooc;

public class Post {
private int postid;
private String postname;
public Post(){
	
}
public Post(int postid,String postname){
this.postid=postid;
this.postname=postname;
}
public int getPostid() {
	return postid;
}
public String getPostname() {
	return postname;
}
public void setPostid(int postid) {
	this.postid = postid;
}
public void setPostname(String postname) {
	this.postname = postname;
}


}

package com.Imooc;

public class Test {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
Department dep1=new Department(1,"人事部");
Department dep2=new Department(2,"市场部");
Post po1=new Post(1,"经理");
Post po2=new Post(2,"助理");
Post po3=new Post(3,"职员");
Staff sta1=new Staff("张铭","S001",29,"男");
Staff sta2=new Staff("李艾爱","S002",21,"女");
Staff sta3=new Staff("孙超","S004",29,"男");
Staff sta4=new Staff("张美美","S005",26,"女");
Staff sta5=new Staff("蓝迪","S006",37,"男");
Staff sta6=new Staff("米莉","S007",24,"女");
dep1.addStaff(sta1);
dep1.addStaff(sta2);
dep1.addStaff(sta3);
dep2.addStaff(sta4);
dep2.addStaff(sta5);
dep2.addStaff(sta6);
System.out.println(sta1.info(dep1,po1));
System.out.println("=================================");
System.out.println(sta1.info(dep1,po2));
System.out.println("=================================");
System.out.println(sta1.info(dep1,po3));
System.out.println("=================================");
System.out.println(sta1.info(dep2,po3));
System.out.println("=================================");
System.out.println(sta1.info(dep2,po1));
System.out.println("=================================");
System.out.println(sta1.info(dep2,po3));
System.out.println("=================================");
System.out.println("人事部总共有"+dep1.getStaffNum()+"名员工");
System.out.println("市场部总共有"+dep2.getStaffNum()+"名员工");
	}

}

Exception in thread "main" java.lang.NullPointerException
	at com.Imooc.Staff.info(Staff.java:75)
	at com.Imooc.Test.main(Test.java:24)

为什么会出现空指针错误，哪里出错了？该改成什么样？