java - 这段代码如何违反 Demeter 法则？

Question

以下代码打破了 Law of Demeter :

public class Student extends Person {
  private Grades grades;

  public Student() {
  }

  /** Must never return null; throw an appropriately named exception, instead. */
  private synchronized Grades getGrades() throws GradesException {
    if( this.grades == null ) {
      this.grades = createGrades();
    }

    return this.grades;
  }

  /** Create a new instance of grades for this student. */
  protected Grades createGrades() throws GradesException {
    // Reads the grades from the database, if needed.
    //
    return new Grades();
  }

  /** Answers if this student was graded by a teacher with the given name. */
  public boolean isTeacher( int year, String name ) throws GradesException, TeacherException {
    // The method only knows about Teacher instances.
    //
    return getTeacher( year ).nameEquals( name );
  }

  private Grades getGradesForYear( int year ) throws GradesException {
    // The method only knows about Grades instances.
    //
    return getGrades().getForYear( year );
  }

  private Teacher getTeacher( int year ) throws GradesException, TeacherException {
    // This method knows about Grades and Teacher instances. A mistake?
    //
    return getGradesForYear( year ).getTeacher();
  }
}

public class Teacher extends Person {
  public Teacher() {
  }

  /**
   * This method will take into consideration first name,
   * last name, middle initial, case sensitivity, and
   * eventually it could answer true to wild cards and
   * regular expressions.
   */
  public boolean nameEquals( String name ) {
    return getName().equalsIgnoreCase( name );
  }

  /** Never returns null. */
  private synchronized String getName() {
    if( this.name == null ) {
      this.name == "";
    }

    return this.name;
  }
}

问题

1. LoD 是如何被破坏的？
2. 破坏 LoD 的代码在哪里？
3. 应如何编写代码以维护 LoD？

Answer 1

我认为这里有两个问题:

1. Grades 逻辑与 Student 混合太多。它应该在 Grades 类中完成
2. Teacher 的逻辑放在Student 中。

结论:Student对Teacher和Grades的内部结构和逻辑了解太多，破坏了LoD