java clean code examples

Duplication.java

import java.util.Arrays;
import java.util.List;
import java.util.function.BinaryOperator;

public class Duplication {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        Integer product = Math2.sum(numbers);
        System.out.println(product);
    }
}

//with duplication
class Math1 {
    public static Integer sum(List<Integer> numbers) {
        Integer sum = 0;
        for (Integer n : numbers) {
            sum += n;
        }
        return sum;
    }

    public static Integer multiply(List<Integer> numbers) {
        Integer product = 1;
        for (Integer n : numbers) {
            product *= n;
        }
        return product;
    }
}


//after refactor with lambda
class Math2 {

    public static Integer reduce(List<Integer> numbers, Integer unit, BinaryOperator<Integer> operator) {
        Integer result = unit;
        for (Integer n : numbers) {
            result = operator.apply(result, n);
        }
        return result;
    }

    public static Integer sum(List<Integer> numbers) {
        return reduce(numbers, 0, (x, y) -> x + y);
    }

    public static Integer multiply(List<Integer> numbers) {
        return reduce(numbers, 1, (x, y) -> x * y);
    }
}

NestedIfElse.java

import java.util.logging.Logger;

class Page {
    public String name;
}

class Registry {
    public int deleteReference(String name) {
        return 0;
    }
}

class ConfigKeys {

    public int deleteKey(String name) {
        return 0;
    }
}

//with nested if-else
class NestedIfElse1 {


    private static final int E_OK = 1;
    private static final int E_ERROR = 2;
    private Registry registry;
    private ConfigKeys configKeys;
    private Logger logger;

    public int delete(Page page) {
        if (deletePage(page) == E_OK) {
            if (registry.deleteReference(page.name) == E_OK) {
                if (configKeys.deleteKey(page.name) == E_OK) {
                    logger.info("page deleted");
                } else {
                    logger.info("configKey not deleted");
                }
            } else {
                logger.info("deleteReference from registry failed");
            }
        } else {
            logger.info("delete failed");
            return E_ERROR;
        }
        return E_OK;
    }

    private int deletePage(Page page) {
        return 0;
    }
}


//after refactor with exception
class NestedIfElse2 {

    private Logger logger;
    private Registry registry;
    private ConfigKeys configKeys;

    public void delete(Page page) {
        try {
            deletePage(page);
            registry.deleteReference(page.name);
            configKeys.deleteKey(page.name);
        } catch (Exception e) {
            logger.info(e.getMessage());
        }
    }

    private void deletePage(Page page) {

    }
}


public class NestedIfElse {

}

SideEffects.java

public class SideEffects {

}

class User {

    private Session session;

    public Session getSession() {
        return session;
    }

    public void setSession(Session session) {
        this.session = session;
    }

    public boolean passwordMatched(String password) {
        return false;
    }
}

class Session {
}

//with side effects
class SessionService1 {

    public Session login(User user, String password) {
        if (user.getSession() != null) {
            return user.getSession();
        } else if (user.passwordMatched(password)) {
            Session session = new Session();
            user.setSession(session);
            return session;
        }
        return null;
    }
}


//after refactor with CQRS
class SessionService2 {

    public boolean isLogin(User user) {
        return user.getSession() != null;
    }

    public void login(User user, String password) {
        if (user.passwordMatched(password)) {
            user.setSession(new Session());
        }
    }
}

Functions must only do what the name suggests and nothing else. isLogin is a function that handles a validation action not login