java问题请教
public class PTList
{
// Instance members,
// hd is the object at the head of the list
// tl is the tail of the list
// null indicates the end of list and an empty list.
private Object hd;
private PTList tl;

// The constructor is private - PTList objects can only be created by cons.
private PTList() {}

/**
* Return the tail of the list.
* @return The tail of this list.
*/
public PTList tail()
{
return null ; // replace this code
}

/**
* Return the head of the list.
* @return The Object at the head of this list.
*/
public Object head()
{
return null ; // replace this code
}


/**
* Create a new PTList object with hd as the value at the head of the list
* and tl as the tail of the list.
* @param hd The value at the head of the PTList.
* @param tl The tail of the new PTList.
* @return A new PTList object.
*/
public static PTList cons(Object hd, PTList tl)
{
//return null ; // replace this code
PTList newptList=new PTList();
newptList.hd=hd;
newptList.t1=t1;
return newptList;
}


/**
* Returns a list consisting of the elements of PTList a joined to PTList b.
* For example
* <br>if a is: 1->2->3->null and b is: 4->5->null
* <br> then append(a,b) is 1->2->3->4->5->null
* @param a The first part of the new PTList.
* @param b The second part of the new PTList.
* @return The new PTList.
*/
public static PTList append(PTList a, PTList b)
{
return null ; // replace this code
}


/**
* Returns a string consisting of the form "" + {@link PTList#head head()} + "->" + {@link PTList#tail tail()}.
* An empty list is represented by null.
* @return The correctly formatted String.
*/
// Do not modify this method.
public String toString()
{
return "" + hd + "->" + tl;
}
}

请问这样一个类到底是什么意思？我们的作业要求完成这个类。在其他地方调用该类的方法，传的参数是一系列数字，但是搞不明白这个Object对象的属性到底能存什么。能存一个链表？各位大虾帮助，谢谢了~~

------解决方案--------------------
PTList，list of points? 当其是一个简单链表来实现吧

观此构成 1->2->3->null
PTlist成员需要有value，有next

private int value;
private PTList next;

在cons方法中构造链表，当使用时就能从头遍历到尾
------解决方案--------------------
至于head是Object的，或是为了增加弹性，或是为了增添迷惑，或...未知
Object可以是PTList