老师，为啥我输出的值是Null

老师，为什么输出员工信息时，部门信息会是Null，检查了好几次还是没找到问题点，但是能在部门类的方法体中能输出相应的值(也就是已经赋值成功了，但输出员工信息还是Null)； 

另外想知道 == 和 equals的区别在哪，在有些循环结构中 equals("y")和== y的效果不同。

package com.imooc.company;

//职务类

public class Post {

//成员属性:职务编号,职务名称

}

package com.imooc.company;

//部门类

public class Branch {

//成员属性：部门编号,部门名称,员工数组,员工人数

private String braNo;//部门编号

private String braNa;//部门名称

private Staff[] braPes;//员工数组

private Staff braPe;//员工类

private int braNu;//员工人数

//无参构造

public Branch(){

}

//带参构造 内容有：部门编号,名称

public Branch(String braNo,String braNa){

this.setNo(braNo);

this.setNa(braNa);

}

//带参构造 内容有：部门编号,名称,员工数组,员工人数

public Branch(String braNo,String braNa,Staff[] braPes,int braNu){

this.setNo(braNo);

this.setNa(braNa);

this.setPes(braPes);

this.setNum(braNu);

}

//set,get方法

//部门编号

public void setNo(String braNo){

this.braNo=braNo;

}

public String getNo(){

return this.braNo;

}

//部门名称

public void setNa(String braNa){

this.braNa=braNa;

}

public String getNa(){

return this.braNa;

}

//员工数组

public void setPes(Staff[] braPes){

this.braPes=braPes;

}

//如果员工数组内部无空间时，将定义空间

public Staff[] getPes(){

if(this.braPes == null)

this.braPes=new Staff[10];

return this.braPes;

}

//计数

public void setNum(int braNu){

this.braNu=braNu;

}

public int getNum(){

return this.braNu;

}

//员工类

public void setPe(Staff braPe){

this.braPe=braPe;

}

public Staff getPe(){

if(this.braPe == null)

this.braPe=new Staff();

return this.braPe;

}

//统计员工

public void infoNum(Staff bra){

for(int i=0;i<this.braPes.length;i++){

if(this.getPes()[i] == null){

braPe.setBra(this);

this.getPes()[i]=bra;

this.braNu=i+1;

return;

}

}

}

public String infoS(){

String str="ss:"+this.getNa()+"sa:"+this.getNum();

return str;

}

}

package com.imooc.company;

//员工类

public class Staff {

//成员属性：姓名,工号,性别,年龄,所属部门,职务信息

private String name;//姓名

private String workNo;//工号

private String sex;//性别

private int age;//年龄

private Branch workBra;//所属部门

private Post workPost;//职务信息

//无参构造

public Staff(){

}

//带参构造 内容有:员工姓名,工号,性别,年龄

public Staff(String name,String workNo,String sex,int age){

this.setName(name);

this.setNo(workNo);

this.setSex(sex);

this.setAge(age);

}

//带参构造 内容有:员工姓名,工号,性别,年龄.部门,职位

public Staff(String name,String workNo,String sex,int age,Branch workBra,Post workPost){

this.setName(name);

this.setNo(workNo);

this.setSex(sex);

this.setAge(age);

this.setBra(workBra);

this.setPost(workPost);

}

//set/get方法

//名字

public void setName(String name){

this.name=name;

}

public String getName(){

return this.name;

}

//工号

public void setNo(String workNo){

this.workNo=workNo;

}

public String getNo(){

return this.workNo;

}

//性别

//性别只能为男女，否则为男

public void setSex(String sex){

if(sex!="男" && sex!="女")

this.sex="男";

else

this.sex=sex;

}

public String getSex(){

return this.sex;

}

//年龄

//年龄为18-65岁之间，否则为18岁

public void setAge(int age){

if(age<18 || age>65)

this.age=18;

else

this.age=age;

}

public int getAge(){

return this.age;

}

//所属部门

public void setBra(Branch workBra){

this.workBra=workBra;

}

public Branch getBra(){

if(this.workBra == null)

this.workBra=new Branch();

return this.workBra;

}

//职务信息

public void setPost(Post workPost){

this.workPost=workPost;

}

public Post getPost(){

if(this.workPost == null)

this.workPost=new Post();

return this.workPost;

}

//显示员工信息

public String infoMa(Branch workBra){

String str="姓名:"+this.name+"\n工号:"+this.workNo+"\n性别:"+this.sex+"\n年龄:"+this.age+"\n职务:"+this.getBra().getNa()+""+this.getPost();

return str;

}

}

package com.imooc.company;

//测试用

public class WorkTest {

public static void main(String[] args) {

//员工类赋值

Staff st=new Staff("张铭","S001","男",29);

Branch br=new Branch("人事部","D001");

br.setPe(st);

//员工类1赋值

Staff st1=new Staff();

st1.setName("李艾爱");

st1.setNo("S002");

st1.setSex("女");

st1.setAge(21);

//员工类2赋值

Staff st2=new Staff();

st2.setName("孙超");

st2.setNo("S003");

st2.setSex("男");

st2.setAge(29);

//员工类3赋值

Staff st3=new Staff();

st3.setName("张美美");

st3.setNo("S004");

st3.setSex("女");

st3.setAge(26);

//员工类4赋值

Staff st4=new Staff();

st4.setName("蓝迪");

st4.setNo("S005");

st4.setSex("男");

st4.setAge(37);

//员工类5赋值

Staff st5=new Staff();

st5.setName("米莉");

st5.setNo("S006");

st5.setSex("女");

st5.setAge(24);

System.out.println(st2.infoMa(br));

System.out.println(br.infoS());

System.out.println();

}

}