public interface Function {
    public void calculate(long t);
}

public class ConstantFunction implements Function {
    private double constant;

    @Override
    public void calculate(long t) {
        // ...
    }
}

public class LinearFunction implements Function {
    private double slope;

    private double yIntercept;

    @Override
    public void calculate(long t) {
        // ...
    }
}

<myapp>
    <function type="ConstantFunction>
        <!-- ... -->
    </function>
    <function type="LinearFunction>
        <!-- ... -->
    </function>
</myapp>

XStream oxmapper = new XStream();
oxmapper.alias("myapp", MyApp.class);
oxmapper.alias("function", ???);

public class FunctionFactory implements Function {
    private double constant;
    private double slope;
    private double yIntercept;

    private Class<? extends Function> concreteClass;

    @Override
    public void calculate(long t) {
        // Do nothing. This class is a workaround to limitations with XStream.
        return;
    }
}

oxampper.alias("function", FunctionFactory.class);
oxmapper.aliasField("function", "type", "concreteClass");

XStream oxmapper = getConfiguredMapper();
MyApp app = oxmapper.fromXml("<myapp>...</myapp>");

FunctionFactory factory = app.getFunction();
Function concretion = factory.getConcreteClass();

app.setFunction(concretion);