Multiway tree comparable interface issue

Question

I am creating a generic multiway tree which I KNOW will take only one of four types (Integer, Double, String and Character). I am having problems with the comparable interface and my insert function. Here is the relevant code I have written so far:

public class BTree<T extends Comparable<? super T>>
{
    private int m=0, height=0, numkeys=0;
    private BTreeNode<T> root;
    boolean ready=false;

class BTreeNode<T extends Comparable<? super T>>
{
    boolean leaf = true;
    int keyTally = 1;
    Comparable<T> keys[] = new Comparable[m-1];
    BTreeNode<T> references[] = new BTreeNode[m];
    BTreeNode( T key)
    {
        keys[0] = (Comparable<T>) key;
        for ( int i=0; i<m; i++)
            references[i] = null;
    }
}

public BTree( int c)
{
    root=null;
    m=c;
}

// Not finished - can't test due to other bug
public boolean insertElement( T elem)
{
    insert( elem, root, height);
    numkeys++;
    return true;
}

// And now the ploblem function

private BTreeNode<T> insert( T elem, BTreeNode<T> node, int hvar)
{
    int i;

    BTreeNode<T> temp = null;

    if (hvar == 0)
    {
        for (i = 0; i < node.keyTally; i++)
            if ( elem.compareTo(node.keys[i]) < 0)
                break;
    }
    else
    {
        for (i = 0; i < node.keyTally; i++)
        {
            if ((i+1 == node.keyTally) || elem.compareTo(node.keys[i]) < 0)
            {
                Node n = insert( elem, node.references[i++], hvar-1);
                if (n == null)
                    return null;

                temp.keys[0] = n.keys[0];
                temp.references[0] = n;
                break;
            }
        }
    }

    for (int j = node.keyTally; j > i; j--)
        node.keys[j] = node.keys[j-1];
    node.children[i] = temp;
    node.keyTally++;
    if (node.keyTally < m)
        return null;
    else
    {
        BTreeNode<T> newNode = null;
        newNode.keyTally = m/2;
        for( i=0; i<m/2; i++)
        newNode.references[i] = node.references[m/2+i];
        return newNode;
    }
    return null;
}

And here is my console output:

BTree.java:169: error: method compareTo in interface Comparable<T#2> cannot be applied
to given types;
if ( elem.compareTo(node.keys[i]) < 0)
         ^
required: CAP#1
found: Comparable<T#1>
reason: actual argument Comparable<T#1> cannot be converted to CAP#1 by method
invocation conversion
where T#1,T#2 are type-variables:
T#1 extends Comparable<? super T#1> declared in class BTree
T#2 extends Object declared in interface Comparable
where CAP#1 is a fresh type-variable:
CAP#1 extends Object super: T#1 from capture of ? super T#1

BTree.java:176: error: method compareTo in interface Comparable<T#2> cannot be
applied to given types;
if ((i+1 == node.keyTally) || elem.compareTo(node.keys[i]) < 0)
                                  ^
required: CAP#1
found: Comparable<T#1>
reason: actual argument Comparable<T#1> cannot be converted to CAP#1 by
method invocation conversion
where T#1,T#2 are type-variables:
T#1 extends Comparable<? super T#1> declared in class BTree
T#2 extends Object declared in interface Comparable
where CAP#1 is a fresh type-variable:
CAP#1 extends Object super: T#1 from capture of ? super T#1

BTree.java:178: error: cannot find symbol
Node n = insert( elem, node.references[i++], hvar-1);
^
symbol:   class Node
location: class BTree<T>
where T is a type-variable:
T extends Comparable<? super T> declared in class BTree

BTree.java:191: error: cannot find symbol
node.children[i] = temp;
    ^
symbol:   variable children
location: variable node of type BTree<T>.BTreeNode<T>
where T is a type-variable:
T extends Comparable<? super T> declared in class BTree

Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
4 errors

I don't understand what I am doing wrong with the comparable interface to make it complain about the compareTo method. Additionally, I was wondering what it will do if the string type is used? I assume it will just see which one comes first in order alphabetically. Thanks :)

Answer 1

This is because node.keys[i] is of type Comparable<T>, not T. Try the comparison the other way around.

Answer 2

I would suggest that you either change the type of keys from Comparable<T>[] to T[] (since that is what you want to use it as; you just have to be careful never to return it to the outside of the class); or you manually cast to T every time you get something out of it : (T)node.keys[i]