i'm trying to extend a class and i keep getting this error:

c:\codes\DiagonalToTridiagonal.java:7: cannot resolve symbol
symbol : constructor DiagonalMatrix ()
location: class dataStructures.DiagonalMatrix
public class DiagonalToTridiagonal extends DiagonalMatrix

the DiagonalMatrix class file is in the dataStructures folder, and i don't understand why there would be an error, or what the error is. when i extend from other classes, such as ArrayLinearList, i don't get the error. i seem to only get the error when i extend from a class that isn't implemented from an interface. so how do i extend from a class which isn't implemented from an interface? how can i extend DiagonalMatrix with DiagonalToTridiagonal without getting an error? i would greatly appreciate it if anyone can help; this problem is driving me crazy.

the following is my code:

package dataStructures;
import java.util.*;
import utilities.*;



public class DiagonalToTridiagonal extends DiagonalMatrix
{
    public TridiagonalMatrix transformToTridiagonal()
    {
    TridiagonalMatrix MyTridiagonalMatrix=new TridiagonalMatrix(this.rows, this.element[rows-1]);
    for(int i=0; i<this.rows; i=i++)
        {
        MyTridiagonalMatrix.element[rows-1+i]=this.element;
        MyTridiagonalMatrix.element[2*rows-1+i]=this.element;
        Computable x=(Computable) this.element;
        MyTridiagonalMatrix.element=x.add(this.element[i+1]);
        }
    MyTridiagonalMatrix.element[2*rows-2]=this.element[rows-1];
    return MyTridiagonalMatrix;
    }
}

It sure looks like the problem is that you didn't give DiagonalMatrix a default constructor, so you can't create it without parameters.