EffectiveJavaExercises.java

// 1. refactor the code so it's at the same level of abstraction (SLAP).
int from = 8000;
int to = 9000;
IntStream
        .rangeClosed(from, to)
        .mapToObj(Port::new)
        .filter(Port::isFree)
        .findFirst();

class Port {
        // throws IOException on a network connection failure.
        boolean isFree() throws IOException {
                ...
        }
}


// 2 Refactor code to a higher (and single) level of abstraction
// 2 Refactor code to a higher (and single) level of abstraction
class Registration {
  
  private final RegistrationForm form;
  private final Repository repo;

  Registration(RegistrationForm form, Repository repo) {
    this.form = form;
    this.repo = repo;
  }

  void complete() {
    var credentials = validCredentials()
    var user = newUser(credentials);
    DomainEvents.publish(new RegistrationCompleted(user));
  }

  private Credentials validCredentials(){
    if (!form.isValid()) {
      throw new RegistrationException(MISSING_CREDENTIALS);
    }
    var username = new Username(form.username());
    var isUsernameTaken = username.satisfies(new IsTaken(repo));
    if (isUsernameTaken) {
      throw new RegistrationException(USERNAME_TAKEN);
    }
    var password = new Password(form.password());
    var isWeakPassword = password.satisfies(new IsWeak(repo));
    if (isWeakPassword) {
      throw new RegistrationException(WEAK_PASSWORD);
    }

    return new Credentials(username, password);
  }

}

class Credentials {
    public Username username;
    public Password password;

    public Credentials(Username username, Password password){
        this.username = username;
        this.password = password;
    }
}


// 3. eliminate getters and setters and turn Member into an object
member.offers().add(offer);
member.activeOffers().increase();
        
        
// 4. Find missing domain objects and reify (thingify) them.     
borrower.loans().apply(new Loan(amount, term));
        

// 5. Find a missing domain objects and reify (thingify) it.
class MortgageRiskService {
    
  Risk calculate(MortageApplication mortgageApplication) {
    ...
  }
  
  boolean isTolerable(Risk risk) {
    ...
  }

  boolean areEquivalent(Risk oneRisk, Risk otherRisk) {
    ...
  }
}        
    

// 6. Find a missing domain objects and reify (thingify) it.
class Bankruptcy {
    Probability calculate(Business business){ ... }
    boolean isHigh(Probability probability){...}
}     
        

// 7. Replace a procedural design (and an agent noun CsvParser) with an object. Also, make sure the new design doesn't violate CQS (queries = nouns, commands = verbs).
class CsvFile<T extends Line> { 
    Collection<T> lines;

    CsvFile(File file) { ... } 

    Collection<T> lines() { ... } 
} 

        
// 8. Replace a procedural design (and an agent noun Pinger) with an object
interface Pinger { 
    void ping(); 
}

        

// 9. Replace a procedural design (and an agent noun MoneyFormatter) with an object
interface Money {
  String format();
}
        

// 10. Turn XmlMarshaller into a class Xml
// + Make the class generic (decouple it from Invoice)
// + Use type inference
// + Use try-with-resources block 
class Xml<T> {
    byte[] marshall(T object) { 
        try(ByteArrayOutputStream outStream = new ByteArrayOutputStream()) {
                JaxbMarshaller jaxbMarshaller = new JaxbMarshaller(object.getClass()); 
                jaxbMarshaller.marshallObject(object, outStream); 
                return outStream.toByteArray();
        }
    } 
}     

// 11. /validate/ method is a query (returns the result), but it sounds like an action. Fix it!
interface Input {
  boolean isValid();
}


        
// 12. make Permissions "optional", ditch a setter and provide a domain-specific command.
class User {
    
    private Permissions permissions = Permissions.DEFAULT;
    
    void grant(Permissions permissions) {
        this.permissions = permissions
    }
}
        
        
// 13. Because of CQS, a naming conflict might arise. Fix it! (no getters & setters allowed).
user.banDetails() // returns user's ban and the corresponding information (if any)
user.ban() // bans a user
        
        
        
// 14. Can you spot an object that pretends as a service? Fix it!
interface BlacklistRequest { 
     String getEmail(); 
     String getIpAddress();
     boolean isBlocked(); 
}     
        
        
// 15. Turn this procedure into an object "AuthenticationToken"
class AuthenticationToken {
      static from(String username, String password) throws UserDoesNotExistException;     
}
        
        
// 16. fix naming issues
interface Suite { 
    interface Test { 
        void print();
        boolean isSuccessful()
    } 
    void run(); 
    Collection<Suite.Test> tests(); 
} 

suite.run()
for (Suite.Test test : suite.tests()) { 
    if (!test.isSuccessful() ) {
        // pretty printing 
        test.print();
    }
}       

        
// 17. Can you SLAP (Single Level of Abstraction Principle) it?
boolean destroyButtonAvailable = 
    widgets
        .stream()
        .filter(Widget::isButton)
        .filter(Button::isForArmagedon)
        .findAny()
        .isPresent();


// 18.
// implement the /fullName/ method so that it: 
// 1. returned "firstName lastName" if nickname is missing 
// 2. returned "firstName <nickname> lastName" if nickname is present.
// For example: "Robert Martin" or "Robert <Uncle Bob> Martin"
class User {
   private final Optional<String> nickName;
   private final String firstName;
   private final String lastName;
    
   String fullName() {
     return firstName 
     + " " 
     + nickName.map(nickName -> "<"+nickName+"> ").orElse("") 
     + lastName;
   }
}

      
// 19.
// from variable names, omit words that are deductable from the context
void openBankAccount() {
var withdrawalLimits = WithdrawalLimits.defaults(env);
var holder = new AccountHolder(...);
var account = new BankAccount(holder, withdrawalLimits);
account.open();
account.deposit(bonus());
accountRepository.save(account);                
}



// 20.
// calling a logic (such as remote system call) in a constructor not always a good idea. Do you know how to fix that?
class SecurePassword  {
    
    private final String rawPassword;
    
    private SecurePassword(String rawPassword) {
        this.rawPassword = rawPassword;
    }
        
    static from(Vault vault) {
        return new SecurePassword(vault.verySecurePassword());
    }
    
    public String raw() {
        return rawPassword;
    }    
    
}