[英]How to delete a specific entry in double linked list?
它向我顯示“線程“主”中的異常java.lang.NullPointerException”,並且錯誤出現在“ instead.prev.next =相反。next;”中 這里。
這是我寫的全部代碼
import javax.xml.soap.Node;
public class LinkedSet<T> {
private class DLNode {
public T data;
public DLNode prev, next;
// Can add constructors and other methods
public DLNode(T dataValue, DLNode PREV, DLNode NEXT) {
data = dataValue;
prev = PREV;
next = NEXT;
}
}
private DLNode head;
private int numberOfEntries;
private boolean initialized = false;
public LinkedSet() {
head = null;
numberOfEntries = 0;
initialized = true;
}
// Add at beginning of linked list
public boolean add(T newEntry) {
checkInitialization();
DLNode doubleLinked = new DLNode (newEntry, null,head);
head = doubleLinked;
numberOfEntries++;
return true;
}
public T[] toArray() {
@SuppressWarnings("unchecked")
T[] result = (T[]) new Object[numberOfEntries];
int index = 0;
DLNode currentNode = head;
while((index < numberOfEntries)&& (currentNode != null)) {
result [index] = currentNode.data;
currentNode = currentNode.next;
index ++;
}
return result;
}
public boolean isEmpty() {
return numberOfEntries == 0;
}
public int getCurrentSize() {
return numberOfEntries;
}
public boolean contains(T anEntry) {
checkInitialization();
int index = 0;
DLNode currentNode = head;
if(isEmpty()) {
return false;
}
else {
while ((currentNode.next.data!= null) && index > numberOfEntries) {
if (currentNode.data.equals(anEntry))
{
return true;}
else {
index ++;
currentNode = currentNode.next;
}
}
}
return false;
}
public void clear() {
head = null;
numberOfEntries = 0;
}
// remove in-place. That is, do not rearrange elements in the array.
public DLNode find (T anEntry) {
DLNode currentNode = head;
while (currentNode != null) {
if (currentNode.data.equals(anEntry)) {
return currentNode;
}
currentNode = currentNode.next;
}return null;
}
public boolean remove(T anEntry) {
DLNode instead = find(anEntry);
if (isEmpty()) {
return false;
}
else {
if(instead ==null) {
return false;
}
else {
if (numberOfEntries ==1) {
clear();
}
else {
instead.prev.next = instead.next;
if (instead.next != null) {
instead.next.prev = instead.prev;
}
numberOfEntries--;
}
}return true;
}
}
private void checkInitialization() {
if (!initialized) {
throw new SecurityException("Uninitialized object used " + "to call an ArrayBag method.");
}
}
// Don't change this.
public int getCapacity() {
return numberOfEntries;
}
public static void main(String[] args) {
LinkedSet <String> abag = new LinkedSet<>();
abag.add("A");
abag.add("B");
abag.add("C");
abag.add("D");
displayBag(abag);
System.out.println(abag.isEmpty());
System.out.println(abag.getCurrentSize());
System.out.println(abag.contains("A"));
abag.clear();
displayBag(abag);
abag.add("A");
abag.add("B");
abag.add("C");
abag.add("D");
displayBag(abag);
System.out.println(abag.remove("C"));
}
private static void displayBag(LinkedSet<String> aBag) {
System.out.println("The bag contains the following string(s):");
Object[] bagArray = aBag.toArray();
for (int index = 0; index < bagArray.length; index++) {
System.out.print(bagArray[index] + " ");
} // end for
System.out.println();
}
要求我們不能更改此數組中元素的順序。 但是我不知道為什么是“ instead.prev.next =相反.next;”。 是錯的。
在add
方法中,您執行DLNode doubleLinked = new DLNode(newEntry, null, head);
,因此,您沒有提供對先前條目的引用。 這就是為什么在remove
,當您執行instead.prev.next
instead.prev
時的原因,因為instead.prev
為null
,您會得到NPE。
使用調試器很容易檢測到。
這是您的add
方法的外觀:
public boolean add(T newEntry) {
checkInitialization();
DLNode doubleLinked = new DLNode(newEntry, null, head);
//code I added
if (head != null) {
head.prev = doubleLinked;
}
head = doubleLinked;
numberOfEntries++;
return true;
}
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