循環リンクリストを使用して、よく知られているヨセフスの問題を実装することになっているJavaのコードに取り組んでいます。ヨセフス問題に関する情報は次のとおりです。http：//en.wikipedia.org/wiki/Josephus_problem
ジョセフスクラスを作成するために与えられた学生クラスとドライバークラスがあります。
これが学生クラスです：http：//pastebin.com/4YgSA7CM
これがDriverクラスです：http：//pastebin.com/Nb08Dtqk
これらのクラスはどちらも変更できません。
ゼロから始めて、ヨセフス問題を効果的に使用する循環リンクリストを使用するヨセフスクラスを作成する必要がありました。
これが、コンパイラエラーのない完成したJosephusクラスです。
/** Implementation of Josephus problem. The Josephus problem
is named after the historian Flavius Josephus. For more
information on this problem visit:
http://en.wikipedia.org/wiki/Josephus_problem
*/
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.ArrayList;
public class Josephus <E> {
private Node<E> head;
private int count; // number of elements in the list
/** Constructs an empty Josephus circle */
// Complexity O(1)
public Josephus() {
head = null;
count = 0;
}
/** Constructs an Josephus circle by adding the
elements from an arraylist
@param array The ArrayList of items of type E
*/
// Complexity: O(n)
public Josephus(ArrayList<E> array) {
head = null;
for (int i = 0; i < array.size(); i++)
add(array.get(i));
}
/** Inserts the specified element in the list at the
last position
@param dataItem the element to add
*/
// Complexity O(1)
@SuppressWarnings({ "unchecked" })
public void add(E dataItem) {
Node <E> node = new Node <E> (dataItem, null, null);
if (count == 0) // list is empty
head = node.previous= node ;
else
head.previous.next = node;
node.previous = head.previous;
head.previous = node;
count++;
}
// To be completed by the student
/** Inserts the specified element in the list at the
end. This method has the same behavior as add(E)
@param dataItem the element to add at the end
*/
// Complexity O(1)
public void addLast(E dataItem) {
add(dataItem);
}
/** Inserts the element at the beginning of the list
@param dataItem The element to be added
*/
// Complexity O(1)
public void addFirst(E dataItem) {
Node<E> node = new Node <E>(dataItem, null, null);
// To be completed by the student
if (head == null) // list is empty
head = head.previous = node;
else {
node.next = head;
head.previous = node;
head = node;
}
count++;
}
/** removes the element from the beginning of the list
@return The element that was remvoed
@throws NoSuchElementException if the list is empty
*/
// Complexity O(1)
public E removeFirst() {
// To be completed by the student
if (head != null) {
E item = head.data;
if (head == head.previous) // list has only one element
head = head.previous = null;
else { // list has more than 1 element
head = head.next;
head.previous = null;
}
count--;
return item;
}
else throw new NoSuchElementException();
}
/** removes the element from the end of the list
@return The element that was remvoed
@throws NoSuchElementException if the list is empty
*/
// Complexity O(1)
public E removeLast() {
// to be completed by the student
if (head.previous != null) {
E item = head.previous.data;
if (head == head.previous) // list has only one item
head = head.previous = null;
else { // list has more than one element
head.previous = (head.previous).previous;
head.previous.next = null;
}
count--;
return item;
}
else throw new NoSuchElementException();
}
/** returns a reference to the element at
position index
@param index The index of the element being sought
@return A reference to the element at position index
@throws IndexOutOfBoundsException if index is out of range
*/
// Complexity O(n)
public E get(int index) {
if ((index < 0) || (index >= count))
throw new IndexOutOfBoundsException(Integer.toString(index));
Node<E> temp = head;
for (int i = 0; i < index; i++)
temp = temp.next;
return temp.data;
}
/** Sets the element at position index to reference
anEntry.
@param index The position of the element that is to
be set
@param anEntry The new value at position index
@return the element that was previously at position index
@throws IndexOutOfBoundsException if index is out of range
*/
// Complexity O(n)
public E set(int index, E anEntry) {
if ((index < 0) || (index >= count))
throw new IndexOutOfBoundsException(Integer.toString(index));
Node<E> temp = head;
for (int i = 0; i < index; i++)
temp = temp.next;
E result = temp.data;
temp.data = anEntry;
return result;
}
/** Inserts the specified element in the list at a
given index
@param index The position at which the new element
has to be inserted
@param anEntry The element to add
@throws IndexOutOfBoundsException if index is out of range
*/
// Complexity O(n)
public void add(int index, E anEntry) {
// To be completed by the student
if ((index < 0) || (index > count))
throw new IndexOutOfBoundsException(Integer.toString(index));
if (index == 0) addFirst(anEntry);
else if (index == count) addLast(anEntry);
else {
Node <E> node = head;
int i = 0;
while(node!=null && i<index){
i++;
node = node.next;
}
Node<E> newNode = new Node <E> (anEntry, node, node.next);
node.next.previous = newNode;
node.next = newNode;
count++;
}
}
/** searches for target and returns the position of the
first occurrence, or -1 if it is not in the list
@param target The element we are searching for
@return The position of target if found; -1 if not found
*/
// Complexity O(n)
public int indexOf(E target) {
Node<E> temp = head;
for (int i = 0; i < count; i++) {
if (temp.data.equals(target)) return i;
temp = temp.next;
}
return -1;
}
/** removes the element at position index
@param index The index of the element to be removed
@return The element that was removed
@throws IndexOutOfBoundsException if index is invalid
*/
// Complexity O(n)
public E remove(int index) {
// to be completed by the student
if ((index < 0) || (index >= count))
throw new IndexOutOfBoundsException(Integer.toString(index));
Node<E> temp = head;
for(int i =0;i<index; i++)
temp = temp.next;
E result = temp.data;
temp.next = temp.previous;
return result;
}
/** sets the start position for the Josephus game
@param index The starting position
@throws IndexOutOfBoundsException if index is invalid
*/
// Complexity O(n)
public void setStartPosition(int index) {
if ((index < 0) || (index >= count))
throw new IndexOutOfBoundsException(Integer.toString(index));
for (int i = 0; i < index; i++)
head = head.next;
}
/* This private utility method is used in startJosephus
method.
Complexity O(1)
*/
private void removeAfter(Node<E> node) {
node.next = node.next.next;
node.next.previous = node;
count--;
}
/** simulates the Josephus game by killing every other person
until the winner is the only one left.
@return The survivor of the game
*/
public E startJosephus() {
E item =head.data;
if(head.next != null){
if(head == head.previous)
return item;
else
while(count>1);
removeAfter(head);
head =head.next;
}
return item;
}
/** Returns a list-iterator of the elements in this list
(in proper sequence), starting at the beginning
of the list.
*/
public ListIterator <E> iterator() {
return new myIterator();
}
/** @return The number of elements in the list
*/
public int size() {
return count;
}
// this is an inner clss implementing the ListIterator
// interface.
// visit http://docs.oracle.com/javase/1.4.2/docs/api/java/util/ListIterator.html
// for a complete list of methods in ListIterator
private class myIterator implements ListIterator <E> {
private Node<E> forward = head;
private Node<E> backward = head;
private boolean firstTime = true;
/** checks if a current item E is the last
in the collection
*/
public boolean hasNext() {
return (forward != null);
}
/** returns the next item in the collection if
there is one. If there is no more items
throws NoSuchElementException
*/
public E next() {
if (forward == null) throw
new NoSuchElementException();
else {
E item = forward.data;
forward = forward.next;
if (forward == head) forward = null;
return item;
}
}
/** checks if a current item is the first
in the collection
*/
public boolean hasPrevious() {
return (backward != null);
}
/** returns the previous item in the collection if
there is one. If there is no more items
throws NoSuchElementException
*/
public E previous() {
if (backward == null) throw
new NoSuchElementException();
else {
if (firstTime) {
backward = backward.previous;
firstTime = false;
}
E item = backward.data;
backward = backward.previous;
if (backward == head.previous) backward = null;
return item;
}
}
/* this operation is not supported */
public void add(E obj) {
throw new UnsupportedOperationException();
}
/* this operation is not supported */
public void set(E obj) {
throw new UnsupportedOperationException();
}
/* this operation is not supported */
public int previousIndex() {
throw new UnsupportedOperationException();
}
/* this operation is not supported */
public int nextIndex() {
throw new UnsupportedOperationException();
}
/* this operation is not supported */
public void remove() {
throw new UnsupportedOperationException();
}
}
private static class Node <E> {
private E data;
private Node<E> next;
private Node<E> previous;
/** constructor Creates an empty node with both next and
previous fields set to be null
@param dataItem - item to be inserted
*/
private Node(E dataItem) {
data= dataItem;
previous = next = null;
}
/** creates a new node that references another node
@param dataItem The data stored
@param previousNodeRef The node previous to this node
@param nextNodeRef The node next to this node
*/
private Node(E dataItem, Node<E> previousNodeRef, Node <E> nextNodeRef ) {
data = dataItem;
previous = previousNodeRef;
next = nextNodeRef;
}
}
}
私のstartJosephusメソッドは私が信じる主な問題です。しかし、完全にはわかりません。上記のコード内のstartJosephusメソッドは次のとおりです。
/** simulates the Josephus game by killing every other person
until the winner is the only one left.
@return The survivor of the game
*/
public E startJosephus() {
E item =head.data;
if(head.next != null){
if(head == head.previous)
return item;
else
while(count>1);
removeAfter(head);
head =head.next;
}
return item;
}
これが私のJosephusクラスを実行するときに実行されているものです：http：//pastebin.com/5GnChgYd
出力が生成することになっているものは次のとおりです：http：//pastebin.com/Qr5dCZJp
また、この出力を生成するために使用される2つの入力ファイルは次のとおりです。
StudentList1.txt： http: //pastebin.com/ysjevQ8u
StudentList2.txt： http: //pastebin.com/r2YeppNm
私が得ている出力と私が得ているはずの出力に基づくと、ヨセフスの問題は始まっておらず、殺し屋をシミュレートしていないようです。しかし、コードの何が問題なのかわかりません。循環リンクリストであるため、私のコードにテールを付けることはできません。私がここで間違っていることについて何か考えはありますか？すべてのPastebinリンクについて申し訳ありませんが、ここで提示しているすべてのコードを整理するためのより良い方法のように思えました。あなたの考えを聞いてみたいです。
EDIT:
There are 21 persons in this list
The game starts with McElroy,Breanna at starting position
The killing spree begins......
The sole survivor at the end of this gruesome game is McElroy,Breanna
Exception in thread "main" java.lang.NullPointerException
at Josephus$Node.access$5(Josephus.java:383)
at Josephus.add(Josephus.java:49)
at Josephus.addLast(Josephus.java:66)
at Driver.main(Driver.java:96)
これは、無限ループの問題を修正した後に発生する新しいランタイムエラーです。助言がありますか？？？？これらすべてのヌルポインタ例外とは何ですか