我在编写链接序列类的toString（）方法时遇到问题。 我似乎无法弄清楚我错过了什么。任何帮助将不胜感激。现在实现的那个在调用addAfter（）方法时不会显示列表。
这里是代码＆＃34;
public class DoubleLinkSeq implements Cloneable {
private int manyNodes; // keeps track of the number of nodes in the list
private IntNode head; // reference to the first node
private IntNode tail; // reference to the last node
private IntNode cursor; // refers to the node with the current element (or null)
private IntNode precursor; // refers to the node before the current element (or null)
/**
* Constructor for the IntLinkedSeq class
* Initialize an empty sequence
*/
public DoubleLinkSeq() {
head = null;
tail = null;
cursor = null;
precursor = head;
manyNodes = 0;
}
/**
* Adds a new element to this sequence before the current element
*
* @param element - the new element that is being added
*/
public void addBefore(int element) {
if (isCurrent()) { // if there is a current element
if (cursor == head) { // where cursor is the head
precursor = new IntNode(element, cursor); // add new node
head = precursor; // move head;
} else { // if cursor is not the head
precursor.addNodeAfter(element); // add new node
cursor = precursor.getLink(); // move cursor
}
}
else{ // if no current element
if(head == null){
head = new IntNode(element, null);
cursor = head; // move cursor
precursor = head; // move precursor
tail = head; // move tail
}
else{ // otherwise, add the node after the precursor, before cursor
precursor.addNodeAfter(element); // add after precursor
}
}
manyNodes++; // update the invariant
}
/**
* Adds a new element to this sequence after the current element
*
* @param element - the new element that is being added
* @exception OutOfMemoryError
* Indicates insufficient memory for a new node
*/
public void addAfter(int element) {
if (isCurrent()) {
cursor.addNodeAfter(element);
precursor = cursor;
cursor = cursor.getLink();
}
else{
if(tail == null){
tail = new IntNode(element, null);
cursor = tail;
precursor = tail;
head = tail;
}
else{
tail.addNodeAfter(element);
precursor = tail;
tail = tail.getLink();
cursor = tail;
}
manyNodes++;
}
}
/**
* Place the contents of another sequence at the end of this sequence
* @param addend - A sequence that will be placed at the end of this sequence
*/
public void addAll(DoubleLinkSeq addend){
IntNode[] copy;
if(addend == null){
throw new IllegalArgumentException("addend is null");
}
if(addend.size() > 0){
copy = IntNode.listCopyWithTail(addend.head);
tail.getLink().setLink(copy[0]); // set what used to be the last node to the start of the new list
copy[1].setLink(null); // set link of last item to null
tail.setLink(copy[0]); // set tail link to the first element of the copy
manyNodes += addend.size(); // update the invariant
}
}
/**
* Determine the number of elements in this sequence
* @return
*/
public int size() {
return manyNodes;
}
/**
* Accessor method to determine whether this sequence has a specified
* current element that can be retrieved with the getCurrent method
*
* @return True if there is a current element, otherwise false
*/
public boolean isCurrent() {
if (cursor == null) {
return false;
} else {
return true;
}
}
/**
* Accessor method to determine the current element of the sequence Note:
* isCurrent must return true
*
* @return The current element of the sequence
*/
public int getCurrent() {
if(!isCurrent()){
throw new IllegalStateException("isCurrent() is null: there is no current element");
}
// return the integer value of data at cursor
return cursor.getData();
}
/**
* Move forward, so that the current element is now the next element in this sequence
* Note: isCurrent() must return true
*/
public void advance() {
if (isCurrent()) {
precursor = cursor; // move the precursor to where the cursor is
cursor = cursor.getLink(); // advance the current element forward to the next link
} else {
// at end of sequence, do nothing
}
}
/**
* Generate a copy of this sequence
* @return A copy of this sequence. Subsequent changes to the copy will not affect the original and vice versa.
* @throws CloneNotSupportedException
*/
@Override
public Object clone() throws CloneNotSupportedException{
DoubleLinkSeq answer;
try{
answer = (DoubleLinkSeq) super.clone();
}
catch(CloneNotSupportedException e){
throw new RuntimeException("This class does not implement Cloneable.");
}
answer.head = IntNode.listCopy(head);
return answer;
}
public static DoubleLinkSeq concatenation(DoubleLinkSeq s1, DoubleLinkSeq s2){
if( (s1 == null) || (s2 == null) ){
throw new IllegalArgumentException("s1 or s2 is null");
}
DoubleLinkSeq answer = new DoubleLinkSeq();
answer.addAll(s1); // add all from the first sequence
answer.addAll(s2); // add all from the second sequence
// Return the union IntLinkedSeq object
return answer;
}
/**
* Remove the current element from this sequence
* Note: if there is no current element, removeCurrent must not be called
*/
public void removeCurrent(){
if(!isCurrent()){
throw new IllegalStateException("isCurrent() is null");
}
if(tail == head){ // only one node case
head = null;
tail = null;
precursor = head;
manyNodes--; // update invariant
return;
}
if(cursor == tail){ // if cursor is at the last node
tail = precursor; // remove the last node
tail.setLink(null);
cursor = tail; // move cursor
precursor = head; // move precursor to beginning
while(precursor.getLink() != cursor){
// search for a link for precursor
if(precursor.getLink() == null){
break;
}
precursor = precursor.getLink();
}
manyNodes--; // update invariant
return;
}
if(cursor == head){ // if cursor is at the first node
head = head.getLink();
cursor = head;
precursor = head;
manyNodes--; // update invariant
return;
}
// regular case
cursor = cursor.getLink();
precursor.setLink(cursor);
manyNodes--; // update invariant
}
/**
* Set the current element at the front of this sequence
*/
public void start(){
if(head == null){ // if there are no elements in the sequence
cursor = null; // no cursor
}
cursor = head; // move cursor to the first node
precursor = head;
}
public String toString() {
String result = "";
if (head == null)
return manyNodes + "";
else
cursor = head;
while(cursor.getLink() != null){
cursor = cursor.getLink();
result += cursor.getLink() + ", ";
}
return "List: " + result;
}
}
答案 0 :(得分：0)
尝试在你的else语句周围添加花括号，这样看起来像这样：
<select class="AllVmList chosen" multiple id="select4${index}" >
<c:forEach var="vm" items="${vmList}">
<option><c:out value="${vm}"/></option>
</c:forEach>
</select>