class Collection{
    private $_members=array();

    public  function addItem($obj,$key=null)
    {
        if($key)
        {
            if(isset($this->_members[$key]))
            {
                throw  new exception("Key \"$key\" already in use!");
            }
            else
            {
                $this->_members[$key]=$obj;
            }
        }
        else
        {
            $this->_members[]=$obj;
        }
    }

    public function removeItem($key)
    {
        if(isset($this->_members[$key]))
        {
            unset($this->_members[$key]);
        }
        else
        {
            throw new exception("Invalid Key \"$key\"!");
        }
    }
    public function getItem($key)
    {
        if(isset($this->_members[$key]))
        {
            return $this->_members[$key];
        }
        else
        {
            throw new  exception("Invalid Key \"$key\"!");
        }
    }

    public function Keys()
    {
        return array_keys($this->_members);
    }

    public function legth()
    {
        return sizeof($this->_members);
    }

    public function exists($key)
    {
        return (isset($this->_members[$key]));
    }
}

class  Course
{
    private $_id;
    private $_courseCode;
    private $_name;

  public function __construct($id,$courseCode,$name)
    {
        $this->_id=$id;
        $this->_courseCode=$courseCode;
        $this->_name=$name;
    }

    public function getName()
    {
        return $this->_name;
    }

    public function getID()
    {
        return $this->_id;
    }

    public function getCourseCode()
    {
        return $this->_courseCode;
    }

    public function __toString()
    {
        return $this->_name;
    }
}

class Collection {
  private $_members = array();    //collection members
  private $_onload;               //holder for callback function
  private $_isLoaded = false;     //flag that indicates whether the callback
                                  //has been invoked
  public function addItem($obj, $key = null) {
    $this->_checkCallback();      //_checkCallback is defined a little later

    if($key) {
      if(isset($this->_members[$key])) {
        throw new KeyInUseException("Key \"$key\" already in use!");
      } else {
        $this->_members[$key] = $obj;
      }
    } else {
      $this->_members[] = $obj;
    }
  }
  public function removeItem($key) {
    $this->_checkCallback();

    if(isset($this->_members[$key])) {
      unset($this->_members[$key]);
    } else {
      throw new KeyInvalidException("Invalid key \"$key\"!");
    } 
  }

  public function getItem($key) {
    $this->_checkCallback();

    if(isset($this->_members[$key])) {
      return $this->_members[$key];
    } else {
      throw new KeyInvalidException("Invalid key \"$key\"!");
    }
  }
  public function keys() {
    $this->_checkCallback();
    return array_keys($this->_members);
  }
  public function length() {
    $this->_checkCallback();
    return sizeof($this->_members);
  }
  public function exists($key) {
    $this->_checkCallback();
    return (isset($this->_members[$key]));
  }
  /**
   * Use this method to define a function to be
   * invoked prior to accessing the collection. 
   * The function should take a collection as a
   * its sole parameter.
   */
  public function setLoadCallback($functionName, $objOrClass = null) {
    if($objOrClass) {
      $callback = array($objOrClass, $functionName);
    } else {
      $callback = $functionName;
    }

    //make sure the function/method is valid
    if(!is_callable($callback, false, $callableName)) {
      throw new Exception("$callableName is not callable " .
                          "as a parameter to onload");
      return false;
    }

    $this->_onload = $callback;
  }

  /**
   * Check to see if a callback has been defined and if so,
   * whether or not it has already been called.  If not,
   * invoke the callback function.
   */
  private function _checkCallback() {
    if(isset($this->_onload) && !$this->_isLoaded) {
      $this->_isLoaded = true;
      call_user_func($this->_onload, $this);
    }
  }
}

class CourseCollection extends Collection {
 public function addItem(Course $obj,$key=null) {
        parent::addItem($obj,$key);
    }
}
class Student{
    private $_id;
    private $_name;
    public $course;

    public  function __construct($id,$name)
    {
        $this->_id=$id;
        $this->_name=$name;
        $this->course=new CourseCollection();
        $this->course->setLoadCallback('loadCourses',$this);
    }

    public function getName()
    {
        return $this->_name;
    }

    public function getID()
    {
        return $this->_id;
    }

    public function __toString()
    {
        return $this->_name;
    }
    public function loadCourses(Collection $col)
    {
        $col->addItem(new Course(1, "001", "语文"),1);
        $col->addItem(new Course(2, "002", "数学"),2);
    }
}