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我正在创建一个使用二叉搜索树实现泛型 Set 的类。我在我的几个方法中使用了“compareTo”方法，无论我做什么，我都会不断收到声明的警告。任何帮助表示赞赏！

// Allow short name access to following classes
import csc143.data_structures.*;

public class MySet<E> implements SimpleSet<E> {

  // the root of the "tree" that structures the set
  private BTNode root;
  // the current number of elements in the set
  private int numElems;

  public MySet() {
    root = null;

    numElems = 0;
  }

  /**
   * Add an element to the set. 
   * 
   * @param e The element to be added to the set.
   * @return  <tt>true</tt> If this operation updated the contents of the set.
   */
  public boolean add(E e) {
    try {
      root = addToSubtree(root, (Comparable) e);
      return true;
    } catch(DuplicateAdded exc) {
      // duplicate trying to be added
      return false;
    }

  }

  // This helper method adds the element "e" to tree rooted at r. Returns
  // (possibly new) tree containing "e", or throws DuplicateAdded exception
  // if "e" already exists in tree.
  private BTNode addToSubtree(BTNode r, Comparable elem)
    throws DuplicateAdded {
    if(r == null) {
      return new BTNode(elem);
    }

    int compare = elem.compareTo(r.item);
    // element already in tree
    if(compare == 0) {
      throw new DuplicateAdded("Element is already in set");
    }
    if(compare < 0) {
      r.left = addToSubtree(r.left, elem);
    } else {  // compare > 0
      r.right = addToSubtree(r.right, elem);
    }

    // element has been added
    return r;
  }

  /**
   * Remove all elements from this set.
   */
  public void clear() {
    root = null;

    numElems = 0;
  }

  /**
   * Checks for the existance of the specified value within the set.
   * 
   * @param e The value sought.
   * @return  <tt>true</tt> If the value exists in the set.
   */
  public boolean contains(E e) {
    return subtreeContains(root, (Comparable) e);
  }

  // This helper method returns whether element "elem" is in
  // (sub-)tree with root "r".
  private boolean subtreeContains(BTNode r, Comparable elem) {
    if(r == null) {
      return false;
    } else {
      int compare = elem.compareTo(r.item);
      // found element
      if(compare == 0){
        return true;
      } else if(compare < 0) {
        return subtreeContains(r.left, elem);
      } else {  // compare > 0
        return subtreeContains(r.right, elem);
      }

    }

  }

  /**
   * Check for the existance of elements in the set.
   * 
   * @return  <tt>true</tt> If there are no elements in the set.
   */
  public boolean isEmpty() {
    return root == null;
  }

  /**
   * Return the number of elements in the set.
   * 
   * @return The number of elements in the set.
   */
  public int size() {
    return numElems;
  }

  /**
   * Returns a String representation of the contents of the set.
   * 
   * @return  The String representation of the set.
   */
  public String toString() {

  }

  // this inner class creates the node that compose the binary tree structure
  class BTNode<E> {

    /**
     * The item stored in the node.
     */
    public E item;

    /**
     * The node to the left of "this" node.
     */
    public BTNode left;

    /**
     * The node to the right of "this" node.
     */
    public BTNode right;

    /**
     * Constructs the BTNode object (three parameters).
     * 
     * @param item The item to be stored in the node.
     * @param left The node to the left of "this" node.
     * @param right The node to the right of "this" node.
     */
    @SuppressWarnings("unchecked")
    public BTNode(Object item, BTNode left, BTNode right) {
      // bind to references
      this.item = (E) item;
      this.left = left;
      this.right = right;
    }

    /**
     * Constructs the BTNode (one parameter).
     * 
     * @param The item to be stored in the node.
     */
    public BTNode(Object item) {
      // call three parameter constructor
      this(item, null, null);
    }

  }

}

编辑：包括 SimpleSet 接口：

package csc143.data_structures;

public interface SimpleSet<E> {

  /**
   * Add an element to the set. 
   * 
   * @param e The element to be added to the set.
   * @return  <tt>true</tt> If this operation updated the contents of the set.
   */
  public boolean add(E e);

  /**
   * Remove all elements from this set.
   */
  public void clear();

  /**
   * Checks for the existance of the specified value within the set.
   * 
   * @param e The value sought.
   * @return  <tt>true</tt> If the value exists in the set.
   */
  public boolean contains(E e);

  /**
   * Check for the existance of elements in the set.
   * 
   * @return  <tt>true</tt> If there are no elements in the set.
   */
  public boolean isEmpty();

  /**
   * Return the number of elements in the set.
   * 
   * @return The number of elements in the set.
   */
  public int size();

  /**
   * Returns a String representation of the contents of the set.
   * 
   * @return  The String representation of the set.
   */
  public String toString();

}
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2 回答 2

2

Comparable您的方法上的签名使用没有泛型的原始接口。您的集合实现中似乎有数据类型实现的要求Comparable，因此由于您现在使用的是泛型，因此您应该全面进行更改。

您没有发布 的​​类声明SimpleSet，因此对 的Comparable限制E可能已经存在。如果没有，您需要将您的类声明更改为：

public class MySet<E extends Comparable<? super E>> implements SimpleSet<E>

这告诉您的类的客户，只有实现的类型Comparable才允许作为您的集合实现的泛型类型参数。您没有发布 的​​代码BTNode，但它可能也需要参数化E（BTNode<E>）。

现在，由于您只允许将类型E的对象添加到集合中，因此您应该更改 adder 方法以反映这一点：

private BTNode<E> addToSubtree(BTNode<E> r, E elem) throws DuplicateAdded

依此类推，forsubtreeContains等。Java 泛型的要点是，您可以Comparable用类型占位符（在您的情况下为 to ）替换所有这些强制转换，这些占位符E在编译时限制了可以添加的内容，因此取消了大多数需要显式转换。泛型是一个强大但复杂的特性，推荐阅读官方教程。

于 2013-08-09T09:15:41.333 回答
0

When you do elem.compareTo(r.item);

elem is of type Comparable

r is of type BTNode

That's is why the warning, compiler is complaining because elem can be something else and not a BTNode instance.

Ideally your private BTNode addToSubtree(BTNode r, Comparable elem) should take both params of same type. Or you can band-aid fix it with:

if(elem instanceof BTNode){
  elem.compareTo(r.item);
}
于 2013-08-09T09:19:50.847 回答