Error compareTo Key

Question

Save people, I have the following problem, I am implementing a code that returns me the median of a sequence, using max-heap and min-heap. Even before completing the code the compiler returned me an error:

method compareTo in interface java.lang.Comparable<T> cannot be applied to gives type; required: Key found java.lang.Comparable reason actual argument java.lang.Comparable cannot be converted to Key by method invocation conversion

In the case both max-heap and min-heap return a type Key (left.max() method). Also in the assignment v = left.max(); the compiler is barking, I have hunted the error, but I did not find, if you can help me thank.

package median;

public class MedianPQ <Key extends Comparable<Key>>{
  private MaxPq left;
  private MinPq right;

  public MedianPQ(int N, int M) {
    left = new MaxPq(N);
    right = new MinPq(M);
  }

  private boolean less( Key k) {
    return k.compareTo(left.max()) < 0;
  }

  public void insert(Key k) {
    Key v;
    if( left.isEmpty() && right.isEmpty()) v = k;
    else {
      if( less(k) && (left.size() <= right.size() + 1)) {
        left.insert(k);
        v = left.max();
      }

    }
  }

Example of the class max-heap (the class min-heap is essentially the same):

public class MaxPq <Key extends Comparable <Key>>{
  private Key[] pq;
  private int N = 0;

  public MaxPq(int maxN) {
    pq = (Key[]) new Comparable[maxN + 1];
  }

  public boolean isEmpty() {
    return N == 0;
  }

  public int size() {
    return N;
  }

  public void insert( Key v) {
    pq[++N] = v;
    swin(N);
  }

  private boolean less( int i, int j) {
    return pq[i].compareTo(pq[j]) < 0;
  }

  private void exch(int i, int j) {
    Key t = pq[i]; pq[i] = pq[j]; pq[j] = t;
  }

  public void swin( int k) {
    while ( k > 1 && less(k/2, k)) {
      exch(k/2, k);
      k = k/2;
    }
  }

  public void sink( int k) {
    while( 2*k <= N) {
      int j = 2*k;
      if( j < N && less(j, j+1)) j++;
      if( !less( k, j)) break;
      exch(k, j);
      k = j;
    }
  }

  public Key delMax() {
    Key max = pq[1];
    exch(1, N--);
    pq[N+1] = null;
    sink(1);
    return max;
  }

  public Key max () {
    return pq[1];
  }
}