Java: Nie popełniaj tych błędów!

JAVA: NIE POPEŁNIAJ TYCH BŁĘDÓW!
@dpokusa
Daniel Pokusa

Not all those who wander are lost.

public String reverse(String msg) {
}
Napisz metodę, która odwróci ciąg znaków. (Jeśli na wejściu
dostaniemy “ABCD” to zwróci “DCBA” itd):

public String reverse(String msg) {
return new StringBuilder(msg)
.reverse()
.toString();
}

public static String reverse(String message) {
String ret = "";
for (int i = 0; i < message.length(); i++) {
int indexBegin = message.length() - 2 - i;
int indexEnd = indexBegin + 1;
ret += message.substring(indexBegin, indexEnd);
}
return ret;
}

class Book {
private String name;
private String isbn;
private List<String> authors;
public Book(String name, String isbn, List<String> authors) {
this.name = name;
this.isbn = isbn;
this.authors = authors;
}
// getters
}

final class Book {
private String isbn;
private Collection<String> authors;
public Book(String name,
String isbn,
Collection<String> authors) {
this.name = name;
this.isbn = isbn;
}
// getters
}

final class Book {
final private String name;
final private String isbn;
final private Collection<String> authors;
String isbn,
this.name = name;
this.isbn = isbn;
}
// getters
}

List<String> author = new ArrayList<>();
author.add("J.R.R. Tolkien");
Book lotr = new Book(
"Lord of the rings",
"9780007117116",
author
);
lotr.getAuthors().add("Daniel Pokusa"); // HAHA! Hacked!

final class Book {
private final String name;
private final String isbn;
private final Collection<String> authors;
String isbn,
this.name = name;
this.isbn = isbn;
this.authors = new ArrayList<>(authors);
}
public Collection<String> getAuthors() {
return Collections.unmodifiableCollection(authors);
}
// other getters

final class Book {
private final String name;
private final String isbn;
private final Collection<String> authors;
String isbn,
this.name = name;
this.isbn = isbn;
this.authors = new ArrayList<>(authors);
}
public Collection<String> getAuthors() {
return unmodifiableCollection(authors);
}
// other getters

“Short cuts make delays, but inns make longer ones.”
– Frodo

public BigDecimal retrieve(String currency) {
RestTemplate restTemplate = new RestTemplate();
NBPApiResponse result = restTemplate.getForObject(
url,
NBPApiResponse.class
);
BigDecimal currentRate = result.rates[0].mid;
log.info("Current rate for {} is {}",
currency,
currentRate
);
return currentRate;
}

public BigDecimal retrieve(RestTemplate restTemplate,
String currency) {
url,
);
BigDecimal currentRate = result.rates[0].mid;
log.debug("Current rate for {} is {}",
currency,
currentRate
);
return currentRate;
}

String currency) {
url,
);
BigDecimal currentRate = result.getCurrentRate();
log.debug("Current rate for {} is {}",
currency,
currentRate
);
return currentRate;
}

String currency) {
url,
);
return result.getCurrentRate();
}
?

Even the smallest person can change the course of the future.
– Galadriel

class Person {
private String surname;
private String nip;
private LocalDate birthdate;
// Constructors, getters, ...
}

public int getAge(LocalDate birthdate);
public int getAge(Person p);

public int getAge(LocalDate birthdate) {
return Period.between(birthdate, now()).getYears();
}

return Period.between(birthdate, now()).getYears();
}
LocalDate.now()

return between(birthdate, now()).getYears();
}

public int getAge(LocalDate date);
public int getAge();

public boolean shouldShowHappyBirthdaySignFor(Person person);
public boolean shouldShowHappyBirthdaySignFor(
LocalDate birthdayDate);

All we have to decide is what to do with the time that is given to us.
- Gandalf

public boolean isSuperhero(Person p, World w);
public boolean shouldBeSuperhero(Person p, World w);

public int calculateFriendsQuantityFor(Person person);
public boolean isPopular(Person person) {
return calculateFriendsQuantityFor(person)
> MIN_FRIENDS_QUANTITY_FOR_POPULAR_PERSONS;
}

public int calculateFriendsQuantityFor(Person person);
public boolean isPopular(Person person) {
return calculateFriendsQuantityFor(person)
> MIN_FRIENDS_QUANTITY_FOR_POPULAR_PERSONS;
}
public static final int
MIN_FRIENDS_QUANTITY_FOR_POPULAR_PERSONS = 50;

● get
● retrieve
● load
● calculate
● For
● Into
● Of
● By

I will take it! I will take the Ring to Mordor. Though… I do not know the way.
- Frodo

public class ShoppingBag {
public Collection<Item> items = new ArrayList<>();
public static final BigDecimal MULTI_DISCOUNT = new BigDecimal(0.2); // 20%
public static final BigDecimal OVERAL_DISCOUNT = new BigDecimal(0.1); // 10%
public Receipt calculate() {
Receipt r = new Receipt();
for (Item item : items) {
r.addPosition(item);
int promotions = (int) (item.getQuantity() / 2);
if (promotions > 0) {
r.addPosition(new Item("More than one promotion!", item.getPrice().multiply(MULTI_DISCOUNT).negate(),
promotions));
}
}
if (r.getFinalPrice().compareTo(new BigDecimal(100)) > 0) {
r.addPosition(new Item("Overall discount", r.getFinalPrice().multiply(OVERAL_DISCOUNT).negate(), 1));
}
saveReceipt(r);
return r;
}
// Other methods
}

promotions));
}
}
if (r.getFinalPrice().compareTo(new BigDecimal(100)) > 0) {
}
saveReceipt(r);
return r;
}

promotions));
}
}
if (isOverallDiscountAwarded(r.getFinalPrice())) {
}
saveReceipt(r);
return r;
}

int promotions = getNumberOfMultiDiscountsPromotionsFor(item.getQuantity());
promotions));
}
}
}
saveReceipt(r);
return r;
}

promotions));
}
}
}
return r;
}

promotions));
}
}
r.addPosition(getOveralDiscountFor(r.getFinalPrice()));
}
return r;
}

r.addPosition(getMultiDiscountFor(item.getPrice(), promotions)));
}
}
r.addPosition(getOverallDiscountFor(r.getFinalPrice()));
}
return r;
}

return ReceiptBuilder.for(items)
.addItemDiscount(priceItemDiscount)
.addOverallDiscount(priceOverallDiscount)
.build();
}

.build();
}
private void calculateItems() {
receipt.addPosition(item);
for (ItemDiscount discount : itemDiscounts) {
Optional<Item> itemDiscount = discount.calculateFor(item);
if (itemDiscount.isPresent()) {
receipt.addPosition(itemDiscount.get());
}
}
}
}

.build();
}
private void applyOverallDiscounts() {
for (OverallDiscount discount : overallDiscounts) {
Optional<Item> discountItem = discount.calculateFor(receipt);
if (discountItem.isPresent()) {
receipt.addPosition(discountItem.get());
}
}
}

.build();
}
public Receipt build() {
calculateItemsWithDiscounts();
applyOverallDiscounts();
return receipt;
}

public Receipt calculate(List<ItemDiscount> itemDiscounts, List<OveralDiscount> overallDiscounts) {
.addItemDiscounts(itemDiscounts)
.addOverallDiscounts(overallDiscount)
.build();
}

Courage is found in unlikely places
- Gildor

● NIE ignoruj podstaw języka

● Zrozum istotę klas niemodyfikowalnych i klas wartości

● Znaj przynajmniej podstawy GC

● Pamiętaj o regule Demeter

● Pamiętaj o regule Demeter
● Nazewnictwo jest bardzo ważne

●Pamiętaj o regule Demeter
● SOLID, ale zacznij od porządnego zrozumienia SRP :)

● Jeśli korzystasz z jakiegoś frameworka- spróbuj zrozumieć jego
podstawowe mechanizmy
● Pisz testy.
● Programuj tak jakby Code Review mał zrobić Ci socjopata, który zna
Twój adres domowy :)

● Pisz testy.

software-empathy.pl
@dpokusa
Danielpokusa.pl

SPREADIT.PL
@dpokusa
18.11.2017, Gliwice

Java: Nie popełniaj tych błędów!

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