Java通用堆棧錯誤

Question

因此，我已經用Java創建了DoubleQueue。它用於我的Data Structures和Algorithms類，我們試圖證明DoubleQueue可以用於實現Stacks和Queues。 運作方式如下：

它使用數組來存儲項目，其頭部指向數組中的前一項，而尾部指向數組的后項。 該陣列是圓形的，當容量達到90％時它將擴展。 現在，我認為擴張正在大驚小怪。 當我將一堆整數添加到堆棧中，然后全部刪除時，它們以相反的順序顯示良好。 但是，我試圖通過推String.charAt來反轉字符串，然后彈出所有字符並打印它們。 對於足夠短以至於不需要擴展的字符串，此方法很好用。 但是，一旦需要擴展，我就會開始得到一個奇怪的輸出，其中包含混亂的字符（順序不正確，缺少字符）以及空值。

我認為這與我轉換數組的方式有關：

 /**
  * Expands the capacity of the storage array that holds the collection of items
  * within the DoubleQueue *** only if expansion is needed ***
  * @return True if the array was expanded; False otherwise
  */
 @SuppressWarnings({"unchecked"})
 public boolean expandArray() {

  // First, see if need to expand
  if( (double) size / storage.length >= CAPACITY_EXPAND ) {

   // Create a temporary pointer to the storage array
   E[] temp = storage;

   // First out what type of expansion we're doing and create a new, larger array
   if( expansionRule == EXPANSION_DOUBLE )
    storage = ( E[] )new Object[ 2 * temp.length ];
   else 
    storage = ( E[] )new Object[ temp.length + INCREASE_CONSTANT ];

   int i = 0; // Counter for storage
   int j = tail; // Counter for temp

   // Copy temp to storage
   while( j != head ) {
    storage[ i ] = temp[ j ];
    if( j == temp.length - 1 ) j = 0; // If we have to wrap around the array
    else j++;
    i++;
   }

   storage[ i ] = temp[ j ];

   return true;
  }

  return false;
 }

這是完整的代碼：

public class DoubleQueue<E> {

 /** CONSTANTS **/
 private final int INCREASE_CONSTANT = 10; // The amount to increase by if expansion rule is increase by constant
 private final double CAPACITY_EXPAND = 0.9; // The size:capacity ratio at which the storage should be expanded
 public static final char EXPANSION_DOUBLE = 'd'; // Parameter for expansionRule()
 public static final char EXPANSION_CONSTANT = 'c'; // Parameter for expansionRule()


 /** VARIABLES **/
 private char expansionRule; // Stores the current expansion rule
 private int head; // Keeps track of the index of the first element
 private int tail; // Keeps track of the index of the last element
 private int size; // Keeps track of the number of items in storage
 private E storage[]; // Holds the collection of items


 /** CONSTRUCTORS **/
 /**
  * Creates an empty DoubleQueue with a given capacity.  Initializes variables
  * and sets the default expansion rule.
  * @param capacity The initial capacity of the DoubleQueue
  */
 @SuppressWarnings({ "unchecked" })
 public DoubleQueue( int capacity ) {

  size = 0;
  head = -1;
  tail = -1;
  storage = ( E[] )new Object[ capacity ];

  setExpansionRule( EXPANSION_DOUBLE );

 }

 /**
  * Creates an empty DoubleQueue of initial capacity 10.
  */
 public DoubleQueue() {

  this( 10 );
 }


 /** METHODS **/
 /**
  * Checks to see if the DoubleQueue is currently empty
  * @return True if the DoubleQueue is empty; False otherwise
  */
 public boolean isEmpty() {

  return size == 0;
 }

 /**
  * Adds an item at the beginning of the DoubleQueue
  * @param item The item to be added to the DoubleQueue
  * @return True if the item was successfully added
  */
 public boolean addFirst( E item ) {

  // First, perform expansion if neccessary
  expandArray();

  // Put the head and tail pointers in the right spots
  if( isEmpty() ) {
   head = 0;
   tail = 0;
  } else if( head == storage.length - 1 ) head = 0;
  else head++;

  // Add the item
  storage[ head ] = item;

  // Increment the size
  size++;

  return true;
 }

 /**
  * Adds an item at the end of the DoubleQueue
  * @param item The item to be added to the DoubleQueue
  * @return True if the item was successfully added
  */
 public boolean addLast( E item ) {

   // First, perform expansion if neccessary
   expandArray();

   // Put the head and tail pointers in the right place
   if( isEmpty() ) {
     head = 0;
     tail = 0;
   } else if( tail == 0 ) tail = storage.length - 1;
   else tail--;

   // Add the item
   storage[ tail ] = item;

   // Increment the size
   size++;

   return true;
 }

 /**
  * Expands the capacity of the storage array that holds the collection of items
  * within the DoubleQueue *** only if expansion is needed ***
  * @return True if the array was expanded; False otherwise
  */
 @SuppressWarnings({"unchecked"})
 public boolean expandArray() {

  // First, see if need to expand
  if( (double) size / storage.length >= CAPACITY_EXPAND ) {

   // Create a temporary pointer to the storage array
   E[] temp = storage;

   // First out what type of expansion we're doing and create a new, larger array
   if( expansionRule == EXPANSION_DOUBLE )
    storage = ( E[] )new Object[ 2 * temp.length ];
   else 
    storage = ( E[] )new Object[ temp.length + INCREASE_CONSTANT ];

   int i = 0; // Counter for storage
   int j = tail; // Counter for temp

   // Copy temp to storage
   while( j != head ) {
    storage[ i ] = temp[ j ];
    if( j == temp.length - 1 ) j = 0; // If we have to wrap around the array
    else j++;
    i++;
   }

   storage[ i ] = temp[ j ];

   return true;
  }

  return false;
 }

 /**
  * Gets the element at the front of the DoubleQueue & removes it
  * @throws EmptyDoubleQueueException if the queue is empty and there is no item to remove
  * @return The item at the front of the DoubleQueue
  */
 public E removeFirst() throws EmptyDoubleQueueException {

  // If the DoubleQueue is empty we obviously can't get an element from it..
  if( size == 0 ) throw new EmptyDoubleQueueException();

  // Create a temporary record of the item and remove it from storage
  E temp = storage[ head ];
  storage[ head ] = null;

  size--; // Decrease the size of the DoubleQueue

  if( size == 0 ) { // If we're taking the last element
   head = -1;
   tail = -1;
  } else if ( head == 0 ) head = storage.length - 1; // Wrap around the array
  else head--;

  return temp;
 }

 /**
  * Gets the item at the back of the DoubleQueue & removes it
  * @throws EmptyDoubleQueueException if the queue is empty and there is no item to return
  * @return The item at the back of the doubleQueue
  */
 public E removeLast() throws EmptyDoubleQueueException {

  // If the DoubleQueue is empty we obviously can't get an element from it..
  if( size == 0 ) throw new EmptyDoubleQueueException();

  // Create a temporary record of the item and remove it from storage
  E temp = storage[ tail ];
  storage[ tail ] = null;

  size--; // Decrease the size of the DoubleQueue

  if( size == 0 ) { // If we're taking the last element
   head = -1;
   tail = -1;
  } else if ( tail == storage.length - 1 ) tail = 0; // Wrap around the array
  else tail++;

  return temp;
 }

 /**
  * Gets the item at the front of the DoubleQueue
  * @return The item at the front of the DoubleQueue; null if list is empty
  */
 public E peekFirst() {

  if( size == 0 ) return null; // Nothing to return
  return storage[ head ];
 }

 /**
  * Gets the item at the back of the DoubleQueue
  * @return The item at the back of the DoubleQueue; null if list is empty
  */
 public E peekLast() {

  if( size == 0 ) return null; // Nothing to return
  return storage[  tail ];
 }

 /**
  * Trims the size of the underlying collection to the exact number of items within
  * the collection
  */
 @SuppressWarnings({"unchecked"})
 public void truncate() {

  E[] temp = storage; // Create a temporary pointer to storage
  storage = ( E[] )new Object[ storage.length ]; // Create a new storage array that is the size of the # of elements in storage

  int i = 0; // Pointer to iterate storage
  int j = 0; // Pointer to iterate temp

  // Copy values from temp to storage
  while( j != head ) {
   storage[ i ] = temp[ j ];
   i++;
   if( j == temp.length - 1 ) j = 0; // Wrap around array
   else j++;
  }
 }

 /**
  * Sets the type of expansion to be used when the underlying array nears capacity
  * @param rule The type of expansion to be used in the future
  * @throws InvalidInputException when the input provided does not correspond to an expansion type
  * @return True if the expansion rule was set successfully
  */
 public boolean setExpansionRule( char rule ) {

  // Check to make sure the rule provided was valid
  if( rule != EXPANSION_CONSTANT && rule != EXPANSION_DOUBLE )
   throw new InvalidInputException();

  // Set the rule
  expansionRule = rule;
  return true;
 }

 /**
  * Returns the current capacity of the array that holds the collection of
  * items currently in the DoubleQueue.  This method is used for testing
  * purposes only and WOULD NOT BE INCLUDED in a public release of DoubleQueue.
  * @return The capcity of the storage array
  */
 public int getCapacity() {

  return storage.length;
 }

 /**
  * Gets the number of elements currently in the DoubleQueue
  * @return The number of elements currently in  the DoubleQueue
  */
 public int getSize() {

  return size;
 }
}

這是驅動程序：

public class Driver {


  public static void main( String[] args ) {

    // First, check the isEmpty() method
    System.out.println( "Check isEmpty() method:" );
    DoubleQueue<Integer> dq = new DoubleQueue<Integer>();
    if( dq.isEmpty() ) System.out.println( "isEmpty() method returns true when DoubleQueue is empty." );
    else System.out.println( "ERROR: isEmpty() method returns false when DoubleQueue is empty." );

    // Add something and check the isEmpty() method again
    dq.addFirst( 1 );
    if( dq.isEmpty() ) System.out.println( "ERROR: isEmpty() method returns true when DoubleQueue is not empty." );
    else System.out.println( "isEmpty() method returns false when DoubleQueue is not empty." );
    System.out.println();

    // Test expansion using default expansion ( double )
    System.out.println( "Check expansion by doubling (should double in capacity when the 10th element is added and again when the 19th element is added):" );
    dq = new DoubleQueue<Integer>();
    System.out.println( "Capacity before adding elements: " + dq.getCapacity() );

    for( int i = 1; i <= 20; i++ ) {

      dq.addFirst( i );
      System.out.println( "Added element #" + i + ". Capacity: " + dq.getCapacity() + "." );
    }

    // Test expansion using constant expansion
    System.out.println();
    System.out.println( "Check expansion by constant (should add ten to capacity when the 10th and 19th elements are added)." );
    dq = new DoubleQueue<Integer>();
    dq.setExpansionRule( DoubleQueue.EXPANSION_CONSTANT );
    System.out.println( "Capacity before adding elements: " + dq.getCapacity() );

    for( int i = 1; i <= 20; i++ ) {

      dq.addFirst( i );
      System.out.println( "Added element #" + i + ". Capacity: " + dq.getCapacity() + ". " );
    }

    // Test functionality as a STACK
    DoubleQueue<Character> stack = new DoubleQueue<Character>();
    String toBeReversed = "reht olleh";
    System.out.println();
    System.out.println( "String to be reversed: " + toBeReversed );

    // Add string to be reversed letter by letter into strck
    for( int i = 0; i < toBeReversed.length(); i++ ) {

      char j = toBeReversed.charAt( i );
      stack.addLast( j );
      System.out.println( "Added letter " + j );
    }

    System.out.println( stack.getCapacity() );
    System.out.println( stack.getSize() );

    // Print stack
    while( !stack.isEmpty() ) {

      try {

        System.out.print( stack.removeLast() );
      } catch( EmptyDoubleQueueException e ) {


      }
    }

    /**
    // This first bit tests the DoubleQueue as a stack, to reverse a strign
    DoubleQueue<Character> dq1 = new DoubleQueue<Character>();
    String toBeReversed = "hello";

    for( int i = 0; i < toBeReversed.length(); i++ ) {

      dq1.addLast( toBeReversed.charAt( i ) );
    }

    while( !dq1.isEmpty() ) {

      try {
        System.out.print( dq1.removeFirst() );
      } catch( EmptyDoubleQueueException e ) {
        // Do nothing
      }
    }**/

  }
}

Answer 1

調用expand之后，您的head和tail索引似乎是錯誤的。

您的expand方法返回[e, l, l, o, , t, h, e, r, null, null, null, null, null, null, null, null, null, null, null]后，您將具有以下數組

然后將您的head和tail分別設置為0和1。 然后在索引1處插入“ h”，結果是：

[e, h, l, o, , t, h, e, r, null, null, null, null, null, null, null, null, null, null, null]

這顯然是錯誤的。

由於這是家庭作業，因此我將讓您嘗試從這里得出答案。