import java.util.function.Function;

class FunctionCompose<T, R> {
    public final Function<T, R> function;

    public FunctionCompose(Function<T, R> function) {
        this.function = function;
    }

    public <U> FunctionCompose<T, U> map(Function<? super R, ? extends U> mapper) {
        return new FunctionCompose<>(function.andThen(mapper));
    }

    //code flatMap

    public R apply(T smth) {
        return function.apply(smth);
    }
}

I am stuck at writing flatMap, where it is suppose to take in another Function which provides another FunctionCompose as an output. The code is suppose to work when
FunctionCompose of type <String, Integer> flatMap with (Integer -> FunctionCompose<Integer, Integer>)

Thanks in advance