Binary Tree implementation using Comparable in Java

Question

I am trying to implement a Binary Tree class.Here's the insert method.When I compile it I get an error as "uses unchecked or unsafe operations.Recompile with -Xlint:unchecked for details".
Can some on please tell me how I can correct this?

public class BinaryNode{
    Comparable element;
    BinaryNode left;
    BinaryNode right;
    BinaryNode(Comparable theElement,BinaryNode lt,BinaryNode rt){
        element=theElement;
        left=lt;
        right=rt;       
    }
    BinaryNode(Comparable theElement){
        this(theElement,null,null);
    }

}  
public class BinaryTr  {
    private BinaryNode root;
    BinaryTr(){
        root=null;
    }
    public void insert(Comparable x){
        root=insert(root,x);
    }
    public BinaryNode insert(BinaryNode current,Comparable x){
        if (current==null)
            current=new BinaryNode(x);
        else{
            if(current.element.compareTo(x)>0)
                current.left=insert(current.left,x);
        else if (current.element.compareTo(x)<0)
            current.right=insert(current.right,x);
        else{
            System.out.println("Duplicates not allowed");
        }
    }
    return current;
}

public static void main (String args[]){
    BinaryTr t=new BinaryTr();
    t.insert(5);
            t.insert(4);
            t.insert(5);
    }
}