Question

我正在处理的项目现在涉及我从文本文件中读取单词并将它们加载到数组中（最终是二叉树，但稍后将完成）。我必须将单词和单词的频率（最初为1）加载到数组中，因此我将两个变量都打包到对象WordNode中。我能够将单词加载到数组中，但是当我尝试检查单词是否已经存在于数组中时，事情就会崩溃。如果是，我必须将频率增加1.我编写了一个新的equals()方法来覆盖默认方法，以便比较单词而不是变量引用。但是，我现在收到错误Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LWordNode;。为什么会这样？我以为只有WordNode个对象被传递给equals()。

主要方法：

public class Driver {
/////////////// fields ///////////////
public static ArrayUnorderedList<WordNode> wordArray = new ArrayUnorderedList<WordNode>();
public static LinkedBinarySearchTree<WordNode> wordTree = new LinkedBinarySearchTree<WordNode>();   //tree to hold words

/////////////// methods ///////////////
public static void main(String[] args) throws Exception {
    //ask for filename       
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    System.out.println("Enter the name of the file to read from: ");
    Reader file = new FileReader(reader.readLine());

    //read file
    Scanner input = new Scanner(file);

    while(input.hasNext()) {
        //get words from file
        String word = input.next();

        //remove non-word characters and convert to lowercase
        word = word.replaceAll("\\W", "");
        word = word.toLowerCase();

        //create node
        WordNode newWord = new WordNode(word);

        //if word is already in array
        if(wordArray.contains(newWord)) {
            System.out.println("Word is already in array");

            //increment frequency by 1
            int index = wordArray.find(newWord);
            wordArray.list[index].setFrequency(wordArray.list[index].getFrequency() + 1);
            System.out.println("(" + newWord.getFrequency() + ") " + newWord.getWord());
        } else {
            System.out.println("Word is not yet in array");

            //add word to tree
            System.out.println("(" + newWord.getFrequency() + ") " + newWord.getWord());
            wordArray.addToRear(newWord);
        }
    }

    //insert into tree

    //perform traversals on tree
}

WordNode类：

public class WordNode {
   protected String word;
   protected WordNode left, right;
   protected int frequency;

   /**
    * Creates a new node with the specified data.
    * @param obj the element that will become a part of the new node
    */
   WordNode(String obj) {
      word = obj;
      left = null;
      right = null;
      frequency = 1;
   }

   /**
    * Method to check for string equivalence.
    * @return true if words are the same
    */
   public boolean equals(Object obj) {
       WordNode tempWord = (WordNode)obj;
       return(tempWord.getWord().equals(this.word));
   }

   /**
    * Gets the word.
    * @return the word
    */
   public String getWord() {
      return word;
   }

   /**
    * Sets the word.
    * @param word the word to set
    */
   public void setWord(String word) {
      this.word = word;
   }

   /**
    * Gets the left.
    * @return the left
    */
   public WordNode getLeft() {
      return left;
   }

   /**
    * Sets the left.
    * @param left the left to set
    */
   public void setLeft(WordNode left) {
      this.left = left;
   }

   /**
    * Gets the right.
    * @return the right
    */
   public WordNode getRight() {
      return right;
   }

   /**
    * Sets the right.
    * @param right the right to set
    */
   public void setRight(WordNode right) {
      this.right = right;
   }

   /**
    * Gets the frequency.
    * @return the frequency
    */
   public int getFrequency() {
      return frequency;
   }

   /**
    * Sets the frequency.
    * @param frequency the frequency to set
    */
   public void setFrequency(int frequency) {
      this.frequency = frequency;
   }
}

ArrayList类中的一些方法：

/**
* Returns true if this list contains the specified element.
* @param target the element that the list is searched for
* @return true if the target is in the list, false if otherwise 
*/
public boolean contains(T target) {
    return (find(target) != NOT_FOUND);
}

/**
* Returns the array index of the specified element, or the
* constant NOT_FOUND if it is not found.
* @param target the element that the list will be searched for
* @return the integer index into the array containing the target element, or the NOT_FOUND constant
*/
public int find(T target) {
    int scan = 0, result = NOT_FOUND;
    boolean found = false;

    if (!isEmpty()) {
       while (!found && scan < rear) {
          if (target.equals(list[scan])) {
              found = true;
          } else {
             scan++;
          }
       }
    }

    if (found) {
       result = scan;
    }

    return result;
}

Answer 1

一个可能的原因是您的equals方法：

public boolean equals(Object obj) {
    //You need to check that obj is a WordNode
    //For example: if (obj instanceof WordNode) {...
    WordNode tempWord = (WordNode)obj; 
    return(tempWord.getWord().equals(this.word));
}

如果obj不是WordNode，

将抛出异常。

您应该查看this discussion about how one should override equals and hashcode in Java。

Answer 2

使用Generics，Java erases the type information并在运行时执行强制转换。这意味着，如果您的代码的任何部分包含T[]，则会将其编译为Object[]。

根据对wordArray.list[index].setFrequency()和wordArray.list[index].getFrequency()的引用，您似乎使用T[]作为ArrayUnorderedList的list字段的类型。这不是Generics的正确使用。在访问数组的索引之前，Java将尝试将Object[] list强制转换为WordNode[]。将一个数组类型转换为另一个数组类型将始终产生ClassCastException。

不要直接访问list字段，而应考虑在ArrayUnorderedList中实现public T get(int index)方法。在方法签名中使用泛型类型T是在泛型集合中实现对象访问的正确方法。这里，隐式转换在数组访问之后发生。

或者，请考虑使用java.util.ArrayList或java.util.HashSet类。两者都是通用集合。

Answer 3

除了equals方法之外，不应该使用WordNode参数调用。

您必须验证参数对象的类型

public boolean equals(Object obj) {
   if(!(obj instanceof WordNone)){
     return false;
   }
   // rest of equals method
}

Answer 4

假设在equals中传递的内容始终是同一个对象的实例，这绝不是安全的。你需要做一些检查，如

public boolean equals(Object obj) {
    if (this == obj) {
        return true;
    }
    if (obj == null) {
        return false;
    }
    if (getClass() != obj.getClass()) {
        return false;
    }
    WordNode otherWord = (WordNode) obj;

    return(otherWord.getWord().equals(this.word));
}

无论如何，为什么不使用Set呢？这是您可以用于案例的最佳数据结构。

Answer 5

既然您只关心一个单词及其频率，为什么不使用Map或Set呢？密钥是单词，值是频率。这样你就可以简单地调用Map.contains（word），看看它是否已经存在，然后做你想做的任何事情。

Java类强制转换异常

5 个答案: