java 封装 为什么构造方法那不用this.setName this.setAge(age)也是正确的？！只用this.name 就可以?

// 学生类 public class student{  private string name; // 姓名  private int age;  // 年龄  // 构造方法  public student(){   this.name = null;   this.age = 0;  }  public student(string name){   this.name = name;   this.age = 0;  }  public student(int age){   this.name = null;   this.age = age;  }  public student(string name, int age){   this.name = name;   this.age = age;  }  // 一般方法  public string getname() {   return name;  }  public void setname(string name) {   this.name = name;  }  public int getage() {   return age;  }  public void setage(int age) {   this.age = age;  }    public static void main(string args[]){   student s1 = new student();   student s2 = new student("zhang san");   student s3 = new student(30);   student s4 = new student("li si", 28);  } }