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作爲第一年任務的一部分，我需要創建一個類，該類使用提供的鏈表實現來實現提供的堆棧接口，然後由另一個提供的類。我能夠使用我自己的LL輕鬆完成任務;然而，我被告知我需要使用提供的那個。java中的空指針異常需要幫助
Exception in thread "main" java.lang.NullPointerException
at StackLL.<init>(StackLL.java:21)
at StackTester.main(StackTester.java:91)
現在我每次嘗試運行它時都會收到空指針異常。我認爲這是由於測試人員在LL初始化之前嘗試獲取列表的大小而引起的，但似乎並非如此，我很難過。
關於我能做些什麼來解決這個錯誤，以便我可以交出剩下的任務？謝謝:)
所提供的鏈接列表實現
LinkedList.java
/**
* LinkedList - a simple linked list of ints
*/
public class LinkedList
{
Node head;
int count;
public LinkedList()
{
head = null;
count = 0;
}
/**
* Adds the given item to the start of the list
*/
public void addToStart (int item)
{
Node newNode = new Node();
newNode.value = item;
if (head != null)
newNode.next = head;
head = newNode;
count++;
}
/**
* Adds the given item to the end of the list
*/
public void addToEnd (int item)
{
if (size() == 0)
{
addToStart (item);
}
else
{
Node n = head;
while (n.next != null)
n = n.next;
n.next = new Node(item);
count++;
}
}
/**
* Remove and return the first item in the list
*/
public int removeFromStart()
{
if (size() == 0)
throw new EmptyListException();
int valtoReturn = head.value;
head = head.next;
count--;
return valtoReturn;
}
/**
* Remove and return the last item in the list
*/
public int removeFromEnd()
{
if (size() == 0)
throw new EmptyListException();
if (size() == 1)
return removeFromStart();
else
{
Node n = head;
while (n.next.next != null)
n = n.next;
int valtoReturn = n.next.value;
n.next = null;
count--;
return valtoReturn;
}
}
/**
* Return the number of items contained in this list
*/
public int size()
{
return count;
}
/**
* A basic node class
*/
private class Node
{
int value;
Node next;
Node()
{
}
Node (int value)
{
this.value = value;
}
}
// random testing code for the Linked List
public static void main (String [] args)
{
LinkedList l = new LinkedList();
l.addToStart (5);
int val = l.removeFromStart();
System.out.println (val == 5 ? "passed" : "failed");
System.out.println (l.size() == 0 ? "passed" : "failed");
for (int x = 0; x < 10; x++)
l.addToEnd (x);
System.out.println (l.size() == 10 ? "passed" : "failed");
while (l.size() > 0)
System.out.print (l.removeFromEnd() + " ");
System.out.println();
}
}
/**
* The exception class when a removal action is performed on
* an empty list.
*/
class EmptyListException extends RuntimeException
{
public EmptyListException()
{
super();
}
public EmptyListException (String s)
{
super(s);
}
}
我實現
StackLL.java
/**
* A linked list implementation of the Stack ADT.
*
*/
public class StackLL implements Stack
{
// The linked list that will contain the values in the stack
private LinkedList values;
public int size()
{
return values.size();
}
public boolean isEmpty()
{
if (values.size() <= 0) {
return true;
}
return false;
}
public void push(int element)
{
values.addToStart(element);
}
public int pop() throws StackEmptyException
{
if (values.size() == 0) {
throw new StackEmptyException();
}
else {
return values.removeFromStart();
}
}
public int peek() throws StackEmptyException
{
if (values.size() == 0) {
throw new StackEmptyException();
}
else { //This is a pretty silly way to do this, but I can't think of any other way without making my own linked list method.
int elementVal = values.removeFromStart();
values.addToStart(elementVal);
return elementVal;
}
}
}
提供的界面
Stack.java
/**
* Stack.java
*
* A specification of the Stack ADT
*
*/
public interface Stack
{
int size();
boolean isEmpty();
void push (int element);
int pop() throws StackEmptyException;
int peek() throws StackEmptyException;
}
class StackEmptyException extends Exception
{
public StackEmptyException()
{
super();
}
public StackEmptyException (String s)
{
super(s);
}
}
測試機
StackTester.java
/**
* StackTester.java
*
* Some test cases for a stack.
*/
public class StackTester
{
public static void testOne (Stack s)
{
try
{
if (s.size() != 0 || !s.isEmpty())
System.out.println("1: Failed size or isEmpty.");
s.push(1);
s.push(2);
if (s.size() != 2 || s.isEmpty())
System.out.println("2: Failed size or isEmpty.");
if (!(s.pop() == 2))
System.out.println("3: Failed pop");
if (!(s.peek() == 1))
System.out.println("4: Failed peek");
if (!(s.pop() == 1))
System.out.println("5: Failed pop");
if (s.size() != 0 || !s.isEmpty())
System.out.println("6: Failed size or isEmpty.");
}
catch (StackEmptyException e)
{
System.out.println(e);
}
}
public static void testTwo (Stack s)
{
try
{
for (int i = 0; i < 100; i++)
{
s.push(i);
}
if (s.size() != 100)
System.out.println("7: Failed size.");
for (int i = 99; i >= 0; i--)
{
if (!(s.pop() == i))
{
System.out.println("Failed pop for: " + i);
break;
}
}
}
catch (StackEmptyException e)
{
System.out.println("Failed testTwo.");
System.out.println(e);
}
}
public static void testThree (Stack s)
{
try {
while (!s.isEmpty())
s.pop();
}
catch (StackEmptyException e) {
System.out.println ("Failed empty stack test (popped on a non empty stack threw exception)");
}
try
{
s.pop();
System.out.println("Failed empty stack test.");
}
catch (StackEmptyException e)
{
/* If we get here, we
* passed the previous test.
*/
}
}
public static void main (String args[])
{
Stack s1 = new StackLL();
Stack s2 = new StackLL();
Stack s3 = new StackLL();
testOne(s1);
testTwo(s2);
testThree(s3);
}
}
在用，你會看到它發生就行了NullPointerException異常。這將幫助您和我們解決問題:) – Cristian 2010-10-30 05:49:19
http://techblog.bozho.net/?p=155 – Bozho 2010-10-30 05:53:02
感謝您的鏈接。不幸的是，通過列出的步驟使我對於實際出現的問題更加無知。計數應該初始化爲0，所以它不應該指向空值。我嘗試使用空指針異常來捕獲此案例;然而，我只是結束了另一個空指針錯誤。 – Pax 2010-10-30 06:44:59