I'm having trouble ironing out language features in 1.5 to perform some
operations and have them be strongly typed.  I'd like to implement
strongly-typed classes derived from a simple base class.  It seems like
the result would be a combination of covariant return types, plus a
proper use of <T extends Number>, but I can't seem to get it right.

public abstract class Number {
    public abstract Number add(Number number);
}

The derived classes should override add(n), but be strongly typed.
Something that acts like what follows:

public class RealNumber extends Number {
    private final double real;
    public RealNumber(double real) {
        this.real = real;
    }
    public RealNumber add(RealNumber rhs) {
        return new RealNumber(this.real + rhs.real);
    }
}

But, obviously,  you'd want to be able to do operations in a proper
generic way, like on a Vector<T extends Number>.

Another example, for complex numbers:

public class ComplexNumber extends Number {
    private final double real;
    private final double imaginary;
    public RealNumber(double real, double imaginary) {
        this.real = real;
        this.imaginary = real;
    }
    public ComplexNumber add(ComplexNumber rhs) {
        return new ComplexNumber(this.real + rhs.real, this.imaginary +
rhs.imaginary);
    }
}