Question

我的HashTable出了问题。虽然代码编译并运行得很好，但我没有得到我想要的输出。

final HashTable的表格大小应为10; Sue（453）和Bobby（170）应该在索引1中，Robert（348）在索引2中，Mark（231）在3中，George（15）在6中。相反，当我运行我的程序时，我的HashTable看起来完全不同，因为它的大小为22，Bobby有两个值（一个应该被删除）所以我不确定我做错了什么。

我怀疑我在"put"方法中搞砸了，但是我无法解决那里可能存在的问题以及为什么在尝试删除第一个Bobby时比较失败的原因值而不是将其添加到旧值之上。

public class HashTable <K, V>
{
   private HashItem[] table;
   private int count;
   private double loadFactor;

   private static final int DEFAULT_CAPACITY = 5;
   private static final double DEFAULT_LOAD_FACTOR = 0.75;

private class HashItem <K, V>
{
   private int hash;
   private K key;
   private V value;
   private HashItem <K, V> next;

   public HashItem(int hashIn, K keyIn, V valueIn, HashItem<K, V> nextIn)
   {
    hash = hashIn;
    key = keyIn;
    value = valueIn;
    next = nextIn;
    }
}

public HashTable(int initialCapacity, double loadFactorIn)
{
   if(initialCapacity <= 0)
   {
   throw new  
            IllegalArgumentException("Capacity must be > 0.");
   }
   if(loadFactorIn < 0)
   {
   throw new IllegalArgumentException("Load factor must be > 0.");
   }
   loadFactor = loadFactorIn;
   table = new HashItem[initialCapacity];
}

public HashTable()
{
   this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
}

public int size()
{
   return count;
}

private void rehash()
{
   HashItem[] oldTable = table;

   //create new table
   int capacity = oldTable.length * 2 + 1;
   table = new HashItem[capacity];

   //get elements at each old table index
   for(int i = 0; i< oldTable.length; i++)
   {
      HashItem<K, V> item = oldTable[i];
      //add the element from old table and its linked elements
      while(item != null)
      {
      put(item.key, item.value);
      item = item.next;
      }
   }
}

public V put(K keyIn, V valueIn)
{
   if (valueIn == null)
   {
   throw new IllegalArgumentException("Value cannot be null");
   }

   int hash = Math.abs(keyIn.hashCode());
   int index = hash % table.length;

   // get hash item at target location(if any)
   HashItem<K , V> current = table[index];
   // iterate through linked nodes at the location (if any)
   while(current != null)
   {
      //if an item with the same hash & key is there, replace
      if(hash == current.hash && current.key.equals(current.hash))
      {
       V oldItem = current.value;
       current.value = valueIn;
       return oldItem;
      }
     current = current.next;
   }      

   int threshold = (int) (table.length * loadFactor);
   if(size() >= threshold)
   {
      rehash();
      index = hash % table.length;
   }

   current = table[index];
   table[index] = new HashItem <K, V>(hash, keyIn, valueIn, current);
   count++;

   return valueIn;
}

public V get(Object keyIn)
{
   int hash = Math.abs(keyIn.hashCode());
   int index = hash % table.length;

   HashItem<K, V> item = table[index];
   while(item != null)
   {
      if(hash == item.hash && item.key.equals(keyIn))
      {
         return item.value;
      }
      item = item.next;
   }   
return null;
}

public boolean remove(Object keyIn)
 {
   int hash = Math.abs(keyIn.hashCode());
   int index = hash % table.length;

   HashItem<K, V> item = table[index];
   HashItem<K, V> previous = null;
   while(item != null)
   {
      if(item.key.equals(keyIn))
      {
      //if it is not in root node, replace links
         if(previous != null)
         {
            previous.next = item.next;
         }  
      //if it was the root, move next item in the chain down
      else{
            table[index] = item.next;
          }
      count--;
      return true;
   }        
   previous = item;
   item = item.next;
   } 
  return false;
 }

 public void makeEmpty()
   {
      table = new HashItem[table.length];
      count = 0;
   }  

public static void main(String [] args)
 {
   HashTable<String, Integer> purchases = new HashTable <String, Integer>();

   String names[] = {"Yuan", "Bobby", "Kevin"};

   purchases.put(names[0], 654);
   purchases.put(names[1], 341);
   purchases.put(names[2], 70);

   purchases.put(names[1], 170);

   System.out.println("Yuan: " + purchases.get(names[0]));
   System.out.println("Bobby: " + purchases.get(names[1]));
   System.out.println("Kevin: " + purchases.get(names[2]));

   purchases.remove(names[0]);
   purchases.remove(names[2]);

   purchases.put("Robert" , 348);
   purchases.put("Sue", 453);
   purchases.put("Mark", 231);
   purchases.put("George", 15);
 }    
}

Answer 1

浏览代码。问题似乎与你的Rehash方法有关。当你在rehash（）中再次调用put时，put方法不知道调用是来自用户作为插入还是重新散列。即使调用不正确的rehashing，count变量也会递增。

Kindle使用调试器来帮助解决其他问题。快速谷歌搜索调试程序将有所帮助。

编辑：内置方法current.key.equals（current.hash）不应该这个比较更像 current.key.equals（keyIn）..原文可能永远不会是真的，这就是替换不起作用的原因。

希望这有帮助

HashTable不输出正确的数据

1 个答案: