Не принимает метод scanFileSystem и getHiddenClassObjectByKey.

public HiddenClass getHiddenClassObjectByKey(String key) {
        try {
            for (Class clazz : hiddenClasses) {
                if (clazz.getSimpleName().toLowerCase().startsWith(key)) {
                    Constructor[] constructors = clazz.getDeclaredConstructors();
                    Constructor constructor = constructors[0];
//                    constructor.setAccessible(true);
                    return ((HiddenClass) constructor.newInstance());
                }
            }
        } catch (Exception e) {
        }
        return null;
 }

if(clazz.getSimpleName().toLowerCase().startsWith(key.toLowerCase())){
                    Constructor[] constructors = clazz.getDeclaredConstructors();
                    for(Constructor con : constructors){
                        if(con.getParameterTypes().length == 0){
                            con.setAccessible(true);
                            return (HiddenClass)con.newInstance(null);
                        }
                    }
                }

Class clazz = new MyClassLoader().load(f.toPath(), "com.javarush.task.task36.task3606.data.second"); //Loading class from path

public static class MyClassLoader extends ClassLoader {
        public Class<?> load(Path path, String packageName) {
            try {
                byte[] b = Files.readAllBytes(path);
                return defineClass(null, b, 0, b.length);
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
    }

package com.javarush.task.task36.task3606;

import java.io.IOException;
import java.lang.reflect.Constructor;
import java.nio.file.*;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.ArrayList;
import java.util.List;

/*
Осваиваем ClassLoader и Reflection
*/
public class Solution {
    private List<Class> hiddenClasses = new ArrayList<>();
    private String packageName;

    public Solution(String packageName) {
        this.packageName = packageName;
    }

    public static void main(String[] args) throws ClassNotFoundException {
        Solution solution = new Solution(Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath() + "com/javarush/task/task36/task3606/data/second");
        solution.scanFileSystem();
        System.out.println(solution.getHiddenClassObjectByKey("hiddenclassimplse"));
        System.out.println(solution.getHiddenClassObjectByKey("hiddenclassimplf"));
        System.out.println(solution.getHiddenClassObjectByKey("packa"));
    }

    public void scanFileSystem() throws ClassNotFoundException {
        MyFileVisitor myFileVisitor = new MyFileVisitor();
        try {
            Files.walkFileTree(Paths.get(packageName.replaceFirst("/", "")), myFileVisitor);
        } catch (IOException e) {
            e.printStackTrace();
        }

        MyClassLoader myClassLoader = new MyClassLoader();

        for (Path classPath : myFileVisitor.getPathToFiles()) {
            try {
                String fileName = classPath.getFileName().toString();
                String simpleNameOfClass = ("com.javarush.task.task36.task3606.data.second." + fileName.substring(0, fileName.length() - 1 - 5));
                myClassLoader.load(classPath, simpleNameOfClass);
                hiddenClasses.add(Class.forName(simpleNameOfClass));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

public HiddenClass getHiddenClassObjectByKey(String key) {
        try {
            for (Class clazz : hiddenClasses) {
                if (clazz.getSimpleName().toLowerCase().startsWith(key.toLowerCase())) {
                    Constructor[] constructors = clazz.getDeclaredConstructors();
                    for (Constructor constructor : constructors) {
                        if (constructor.getParameterCount() == 0){
                            constructor.setAccessible(true);
                            return ((HiddenClass) constructor.newInstance());
                        }
                    }
                }
            }
        } catch (Exception e) {
        }
        return null;
    }

    public class MyClassLoader extends ClassLoader{
        public Class<?> load(Path classPath, String simpleNameOfClass) throws IOException {
            byte[] bytes = Files.readAllBytes(classPath);
            return defineClass(simpleNameOfClass, bytes, 0, bytes.length);
        }
    }

    public class MyFileVisitor extends SimpleFileVisitor{
        private ArrayList<Path> pathToFiles = new ArrayList<>();

        @Override
        public FileVisitResult visitFile(Object file, BasicFileAttributes attrs) throws IOException {
            if (((Path) file).toAbsolutePath().toString().endsWith(".class")) {
                pathToFiles.add(((Path) file));
            }
            return FileVisitResult.CONTINUE;
        }

        public ArrayList<Path> getPathToFiles() {
            return pathToFiles;
        }
    }
}

Solution.class.getPackage().getName() + ".data.second." + fileName.substring(0, fileName.length() - 1 - 5)

MyClassLoader myClassLoader = new MyClassLoader();

        for (Path classPath : myFileVisitor.getPathToFiles()) {
            try {
                String fileName = classPath.getFileName().toString();
                String fileSeparator = File.separator + File.separator;
                String newPackageName = packageName.replaceFirst("/", "").replaceAll("/", fileSeparator);
                String newClassPath = Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath().replaceFirst("/", "").replaceAll("/", fileSeparator);
                String pack = newPackageName.replaceAll(fileSeparator, ".").replaceAll(newClassPath.replaceAll(fileSeparator, "."), "");
                String simpleNameOfClass = pack + "." + fileName.substring(0, fileName.length() - 1 - 5);
                myClassLoader.load(classPath, simpleNameOfClass);
                hiddenClasses.add(Class.forName(simpleNameOfClass,true, myClassLoader));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

public class Solution {
    private List<Class> hiddenClasses = new ArrayList<>();
    private String packageName;

    public Solution(String packageName) {
        this.packageName = packageName;
    }
}

Solution solution = new Solution(Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath() + "com/javarush/task/task36/task3606/data/second");

public static void main(String[] args) {
    printParameter(5);
}

static void printParameter(int parameter) {
    System.out.println(5);
}

Solution solution = new Solution(Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath() + "ЛюбойДругойПакет");

String simpleNameOfClass = ("com.javarush.task.task36.task3606.data.second." + fileName.substring(0, fileName.length() - 1 - 5));

Solution.class.getPackage().getName() + ".data.second." + fileName.substring(0, fileName.length() - 1 - 5)

MyClassLoader myClassLoader = new MyClassLoader();

        for (Path classPath : myFileVisitor.getPathToFiles()) {
            try {
                String fileName = classPath.getFileName().toString();
                String fileSeparator = File.separator + File.separator;
                String newPackageName = packageName.replaceFirst("/", "").replaceAll("/", fileSeparator);
                String newClassPath = Solution.class.getProtectionDomain().getCodeSource().getLocation().getPath().replaceFirst("/", "").replaceAll("/", fileSeparator);
                String pack = newPackageName.replaceAll(fileSeparator, ".").replaceAll(newClassPath.replaceAll(fileSeparator, "."), "");
                String simpleNameOfClass = pack + "." + fileName.substring(0, fileName.length() - 1 - 5);
                myClassLoader.load(classPath, simpleNameOfClass);
                hiddenClasses.add(Class.forName(simpleNameOfClass,true, myClassLoader));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }