注意在Certificate中添加了两个MCA。运行结果如下：
First Name: Manoj Last Name: Kumar Salary: 4000 Certificate: PMP Certificate: MCA Certificate: MBA First Name: Dilip Last Name: Kumar Salary: 3000 Certificate: BCA Certificate: BA ====================== First Name: Manoj Last Name: Kumar Salary: 5000 Certificate: PMP Certificate: MCA Certificate: MBA
MCA只出现了一次！注意Certificate中hashCode和equals的作用。