Abstract class method - To instantiate child class object?

Question

I'm trying to create a matrix library (educational purpose) and have reached an obstacle I'm not sure how to approach with grace . Adding two matrices is a simple task, using a method get() on each each matrices' elements individually.

However, the syntax I've used is wrong. NetBeans claims it's expecting a class, but found a type parameter; to me, a type parameter is just a set with 1:1 mapping to the set of classes.

Why am I wrong here? I've never seen a type parameter be anything else than a class before, so shouldn't the following bit imply M to be a class?

M extends Matrix

public abstract class Matrix<T extends Number, M extends Matrix>
{
    private int rows, cols;
    public Matrix(int rows, int cols)
    {
        this.rows = rows;
        this.cols = cols;
    }

    public M plus(Matrix other)
    {
        // Do some maths using get() on implicit and explicit arguments.
        // Store result in a new matrix of the same type as the implicit argument,
        // using set() on a new matrix.
        M result = new M(2, 2); /* Example */
    }

    public abstract T get(int row, int col);
    public abstract void set(int row, int col, T val);
}

Answer 1

You cannot instantiate a type parameter M directly because you don't know its exact type.

---

I suggest thinking about creating the following method

public abstract <M extends Matrix> M plus(M other); 

and its implementation in the subclass.

Answer 2

From your code, I guess you want to extends some child class from Matrix and do calculation on them.

Change to

public abstract class Matrix<T extends number> {
  ...
  public abstract Matrix plus(Matrix other);
  ...
}

In each child class, add implementation of plus. Because of the construction function of child class is defined there.

Answer 3

I don't think your M is necessary.

If M is a subclass of Matrix , then just use Matrix in your definition.

public abstract class Matrix<T extends Number>
{
    private int rows, cols;
    public Matrix(int rows, int cols)
    {
        this.rows = rows;
        this.cols = cols;
    }

    public Matrix<T> plus(Matrix<T> other)
    {
    }

    public abstract T get(int row, int col);
    public abstract void set(int row, int col, T val);
}

Answer 4

The following code is wrong:

 M result = new M(2, 2);

M is not a class that you can instantiate.

Basically, you need to change your data structure a bit, because your Matrix class is abstract and can't be instantiated either!

I suggest you to change the return type of plus to Matrix and leave it abstract.