笔记

We may wish to instantiate additional objects related to the Student object:
初始化与对象相关的一些额外对象：
public class Student() {
    // Every Student maintains a handle on his/her own individual Transcript object.
    private Transcript transcript;
    public Student() {
        // Create a new Transcript object for this new Student.
        transcript = new Transcript();
        // etc.
    }
}
读取数据库来初始化对象属性：
public class Student {
    // Attributes.
    String studentId;
    String name;
    double gpa;
    // etc.
    // Constructor.
    public Student(String id) {
        studentId = id;
        // Pseudocode.
        use studentId as a primary key to retrieve data from the Student table of a
        relational database;
        if (studentId found in Student table) {
           retrieve all data in the Student record;
            name = name retrieved from database;
            gpa = value retrieved from database;
        // etc.
        }
    }
// etc.
}

和其他已存在的对象交流：
public class Student {
    // Details omitted.
    // Constructor.
    public Student(String major) {
        // Alert the student's designated major department that a new student has
        // joined the university.
        // Pseudocode.
        majorDept.notify(about this student ...);
    // etc.
    }
    // etc.
}

好习惯：如果需要有参数的构造函数，最好同时显示声明一个无参构造函数。
容易出现的bug：如果给构造函数加上void编译会通过！不过会被当作方法而不是构造函数！
当有多个构造函数，而且都有共同的初始化内容时，就会出现很多重复的代码，比如构造一个新学生，我们会做：
1 通知登记办公室学生的存在
2 给学生创建学生成绩报告单
重复引起以后修改必须修改多处，如果使用this 会得到改善

public class Student {
 // Attribute details omitted.
 // Constructor #1.
 public Student() {
// Assign default values to selected attributes ... details omitted.
// Do the things common to all three constructors in this first
// constructor ...
// Pseudocode.
alert the registrar's office of this student's existence
// Create a transcript for this student.
transcript = new Transcript();
}

 // Constructor #2.
 public Student(String s) {
  // ... then, REUSE the code of the first constructor within the second!
  this();
  // Then, do whatever else extra is necessary for constructor #2.
  this.setSsn(s);
 }

 // Constructor #3.
 public Student(String s, String n, int i) {
  // ... and REUSE the code of the first constructor within the third!
  this();
  // Then, do whatever else extra is necessary for constructor #3.
  this.setSsn(s);
  this.setName(n);
  this.setAge(i);
 }
 // etc.
}