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[英]I am getting this runtime error in java: java.util.NoSuchElementException, and I'm not sure how to fix it
[英]I'm getting two NullPointerExceptions and I'm not sure how to fix
我正在制作一個包含向量的雙向鏈接的循環列表,但無法弄清楚如何修復正在獲取的兩個NullPointerExceptions。 這兩個都是我的add方法,一個add方法在列表的末尾插入一個對象,另一個在給定索引處插入該對象。 這是我有問題的代碼。
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Vector;
public class DList<T> implements ADTListInterface<T> {
private int size;
private int BUCKET;
private DListNode head;
private DListNode last;
public DList() {
head = new DListNode(null);
head.next = last;
head.prev = head;
BUCKET = 1;
}
public DList(int bucket){
if (bucket <= 0)
throw new IllegalArgumentException("Cannot have a bucket lower than 1");
this.BUCKET = bucket;
head = new DListNode(null);
head.next = last;
head.prev = head;
}
private class DListNode {
private Vector<T> item; //Line 29
private DListNode next; //Line 30
private DListNode prev;
public DListNode(T o) {
this(o, null, null);
}
public DListNode(T o, DListNode next, DListNode prev) {
item = new Vector<T>(BUCKET);
item.add(o);
this.next = next;
this.prev = prev;
}
public void add(T item) {
if (this.item.size() == this.item.capacity()) {
DListNode newNode = new DListNode(item, this.next, this);
this.next = newNode;
}
else {
insertAfter(item);
}
}
public void add(T item, int idx) {
if (this.item.size() == this.item.capacity()) {
DListNode newNode = new DListNode(this.item.elementAt(BUCKET), this.next, this);
this.next = newNode;
}
else {
this.item.add(idx, item);
}
}
public boolean remove(T o){
if (item.indexOf(o) != -1){
item.remove(o);
if (item.size() == 0){
this.prev.next = this.next;
this.next.prev = this.prev;
}
size--;
return true;
}
return false;
}
public void insertAfter(T item) {
next = new DListNode(item, next, prev);
next.next.prev = next;
}
public DListNode nth(int index) {
if (index == 1)
return this;
else if (index < 0 || next == null) {
throw new java.util.NoSuchElementException("No Such Element");
} else
return next.nth(index - 1);
}
}
@Override
public void add(T o) {
DListNode last = head;
while (last.item.size() >= BUCKET && last.next != head) {
last = last.next;
}
if (last.item.size() == BUCKET) { //Line 102
DListNode n = new DListNode(o, head, last);
head.prev.next = n;
head.prev = n;
size++;
} else {
last.item.add(o);
}
}
@Override
public void add(T o, int index) {
DListNode temp = head, testNext;
int count = 0;
while (count+1 <= index) {
count += temp.item.size();
temp = temp.next;
if (temp == head) {
throw new IndexOutOfBoundsException("Out Of Bounds!");
}
}
testNext = temp.next; //Line 126
temp.add(o, index);
if (testNext != temp.next)
size++;
}
}
我得到的兩個錯誤是:
DList $ DListNode.access $ 0(DList.java:29)在DList.add(DList.java:102)在DListNodeDriver.main(DListNodeDriver.java:7)在線程“ main”中的異常java.lang.NullPointerException
DList $ Drive.main(DListNodeDriver.java:6)的DList.add(DList.java:126)的DList $ DListNode.access $ 1(DList.java:30)的線程“ main”中的java.lang.NullPointerException異常
謝謝您的幫助。
主要方法
import java.util.Iterator;
public class DListNodeDriver {
public static void main(String[] args) {
DList<String> students = new DList<String>();
students.add("Other", 1);
students.add("New", 3);
Iterator<String> t = students.iterator();
while (t.hasNext()) {
String temp = t.next();
StdOut.println("Student: " + temp);
}
}
}
import java.util.Iterator;
public class DListNodeDriver {
public static void main(String[] args) {
DList<String> students = new DList<String>();
students.add("Other");
students.add("New");
Iterator<String> t = students.iterator();
while (t.hasNext()) {
String temp = t.next();
StdOut.println("Student: " + temp);
}
}
}
由於明顯的原因,您的temp
為空。 逐步執行:
head.next
為null
。 students.add("other",1);
-> temp = temp.next
在循環中,因此為null
。 類似的問題發生在students.add("other")
。
我會根據您的實現來做這樣的事情:
@Override
public void add(T o, int index) {
DListNode temp = head
int count = 0;
// < instead of <= because if index = size, your temp will be null otherwise.
// if index = 1 , you'll want to add after head. Watch out for index > size!
// It is an IndexOutOfBounds, index = size => add behind last element.
while (count+1 < index) {
count++;
if( temp.next == null ) { /*Handle that case */ }
else {temp = temp.next;}
//if (temp == head) {
// throw new IndexOutOfBoundsException("Out Of Bounds!");
//}
}
// temp should now be the element just before the index at which to insert.
// I'd do a temp.insertAfter( o );
/* DO YOUR ADDING HERE*/
}
您知道容器的最大大小,並且可以自行管理插入,因此我建議使用T []代替Vector。 您可能期望有更好的性能(無鎖定)和更明確的錯誤。
在下面的代碼示例中,用參數化參數T替換Object
http://fr.scribd.com/doc/81926911/37/The-Circular-Linked-List
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