Constructor Conundrum

Say I have two class hierarchies--Problem and Solver--which sketch like
this:

public abstract class AbstractProblem {
private AbstractSolver _solver;

// ...
}

public class ConcreteProblem extends AbstractProblem {
// ...
}

public abstract class AbstractSolver {
public void solve(/* ... */) {
// ...
}

// ...
}

public class ConcreteSolver extends AbstractSolver {
// lots of parameters not in AbstractSolver

// ...
}

ConcreteProblem needs to create a ConcreteSolver and hand it over to
AbstractProblem. The catch is: ConcreteSolver has several parameters
(variables controlling ConcreteProblem's behavior) ConcreteProblem may
want to change from their default values. Changing parameters is
accomplished through accessors.

How can I do this? I've tried...

public abstract class AbstractProblem {
private AbstractSolver _solver;

public AbstractProblem(AbstractSolver solver) {
_solver = solver;
}

// ...
}

public class ConcreteProblem extends AbstractProblem {
ConcreteSolver _cs = new ConcreteSolver();

public ConcreteProblem() {
super(_cs);
_cs.setParam(/* ... */);
}

// ...
}

....but Java won't let me access _cs before AbstractProblem's constructor
has been called.

I'd prefer to keep AbstractProblem's _solver private, since I can't give
subclasses access to it without giving everything else in the package
access to it, which I don't want to do. (Although I guess I could put
the Problem and Solver hierarchies into separate packages with nothing
else in them to approximate subtype-but-not-package access.)

I can't make all of ConcreteSolver's parameters settable via its
constructor, because I may want to change some defaults without changing
others, which I can't always do since Java lacks named parameters (as far
as I know).

Any ideas?
.