建立排序的单链表

    排序的单链表是指，各结点按data域的值递增或递减顺序链接。结点data域的对象必须是可比较大小的，即已经实现了java.lang.Comparable接口的compareTo()方法。此方法的API描述为：

compareTo

int compareTo(T o)

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception iff y.compareTo(x) throws an exception.)

The implementor must also ensure that the relation is transitive: (x.compareTo(y)>0 && y.compareTo(z)>0) implies x.compareTo(z)>0.

Finally, the implementor must ensure that x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z.

It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is "Note: this class has a natural ordering that is inconsistent with equals."

In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive.

Parameters:

o - the object to be compared.

Returns:

a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

Throws:

ClassCastException - if the specified object's type prevents it from being compared to this object.

        本例继承带头结点的单链表类HSLinkedList（点击打开链接），覆盖父类的add(element)方法，插入可比较大小的对象。

        算法描述：从空单链表开始，逐个插入结点建立排序的单链表。每插入一个结点，首先要从单链表的第0个结点开始，将待插入元素element依次与当前结点的data值比较大小，以确定插入位置并进行插入操作。

          程序如下：

package linearList;

public class SortedHSLinkedList<E> extends HSLinkedList<E>{
	
	public SortedHSLinkedList(){
		super();
	}
	
	/*
	 * 根据指定对象的大小，插入到指定的位置
	 */
	public boolean add(E element){
		if(element==null||!(element instanceof Comparable)){
			return false;//不能插入null或者非comparable对象
		}
		Comparable cmp = (Comparable)element;
		Node<E> front = this.head;
		Node<E> p = front.next;//front是p的前驱结点
		while(p!=null&&cmp.compareTo(p.data)>0){//比较大小，如果要插入的元素大于p值
			front = p;
			p = p.next;//往后行动front和p指针一次
		}
		front.next = new Node<E>(element,p);//插入，把新插入的结点的引用赋值给front的域引用
		if(p==null){
			this.rear = front.next;//尾插入，修改尾指针
		}
		this.n++;
		return true;
	}
	
	public static void main(String[] args) {
		SortedHSLinkedList<Integer> list = new SortedHSLinkedList<Integer>();
		int n = 10;
		System.out.println("insert: ");
		for(int i=0;i<n;i++){
			int k = (int)(Math.random()*100);
			list.add(new Integer(k));
			System.out.print(k+" ");
		}
		System.out.println("\nlist:"+list.toString());
	}

}

执行结果如下：

insert:
Exception in thread "main" java.lang.NullPointerException
    at linearList.SortedHSLinkedList.add(SortedHSLinkedList.java:18)
    at linearList.SortedHSLinkedList.main(SortedHSLinkedList.java:37)

对于这个结果我有点始料不及，根据报错的位置

Node<E> p = front.next;//front是p的前驱结点

再次分析了一下源代码，也没找到错误，，看来还得发到网上看看大家有什么想法？