说得再清楚点儿就好了

哦，多谢！是类的继承。
源码：
class StudentTest
{
public static void main(String[] args)
{
Student aStudent = new Student();
aStudent.setName("张楠");
aStudent.setStudentNum("20030408");
System.out.println("学生的姓名是：" + aStudent.getName() + "，学号是：" + aStudent.getStudentNum());
}
}

class People
{

private String name;
public String getName(){
return name;
}

public void setName(String strName)
{
name = strName;
}
}

class Student extends People
{

private String studentNum;
public String getStudentNum()
{
return studentNum;
}

public void setStudentNum(String strStudentNum)
{
studentNum = strStudentNum;
}

}