警告：unchecked调用compareTo（T）作为原始类型java.lang.Comparable的成员

Question

I am creating a class that implements a generic Set using a binary search tree. I use the "compareTo" method in a couple of my methods, and I keep getting the stated warning regardless of what I do. Any help is appreciated!

// 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);
    }

  }

}

EDIT : includes SimpleSet interface:

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();

}

Answer 1

The signature on your method uses the raw Comparable interface without the generic. There appears to be a requirement in your set implementation that the data types implement Comparable , so since you're now using generics, you should make the change comprehensively.

You didn't post the class declaration for SimpleSet , so the Comparable restriction on E may already be there. If not, you need to change your class declaration to:

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

This tells clients of your class that only types implementing Comparable are permitted as generic type parameters for your set implementation. You didn't post the code for BTNode , but it probably needs to be parameterized on E as well ( BTNode<E> ).

Now, since you're only permitting objects of type E to be added to the set, you should change your adder method to reflect that:

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

And so on, for subtreeContains , etc. The point of Java generics is that you replace all those casts everywhere (to Comparable , in your case) with type placeholders ( E for element) that limit at compile time what can be added and so do away with most need for explicit casting. Generics are a powerful but complicated feature, and I recommend reading the official tutorial .

Answer 2

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);
}