content: package com.itbulls.learnit.javacore.oop.exam.templates.onlineshop.services.impl;
  
  import com.itbulls.learnit.javacore.oop.exam.onlineshop.enteties.User;
  import com.itbulls.learnit.javacore.oop.exam.onlineshop.services.UserManagementService;
  
  public class DefaultUserManagementService implements UserManagementService {
  	
  	private static final String NOT_UNIQUE_EMAIL_ERROR_MESSAGE = "This email is already used by another user. Please, use another email";
  	private static final String EMPTY_EMAIL_ERROR_MESSAGE = "You have to input email to register. Please, try one more time";
  	private static final String NO_ERROR_MESSAGE = "";
  	
  	private static final int DEFAULT_USERS_CAPACITY = 10;
  	
  	private static DefaultUserManagementService instance;
  	
  	// <write your code here>
  
  	private DefaultUserManagementService() {
  	}
  	
  	@Override
  	public String registerUser(User user) {
  		// <write your code here>
  		return null;
  	}
  
  	public static UserManagementService getInstance() {
  		if (instance == null) {
  			instance = new DefaultUserManagementService();
  		}
  		return instance;
  	}
  
  	
  	@Override
  	public User[] getUsers() {
  		// <write your code here>
  		return null;
  	}
  
  	@Override
  	public User getUserByEmail(String userEmail) {
  		// <write your code here>
  		return null;
  	}
  	
  	void clearServiceState() {
  		// <write your code here>
  	}
  }
  