Башкирцев (Старовер) Станислав [email protected] JavaTalks OOD...
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Transcript of Башкирцев (Старовер) Станислав [email protected] JavaTalks OOD...
2
План презентации
• Intro
• Don’t Repeat Yourself
• Open Closed
• Single Responsibility
• Interface Segregation
• Inversion of Control
• Liskov’s Substitution
• Q/A
3
OOD Principles. What’s that?
• Recipes, best practices how to write• Clean, easy to understand code
• Maintainable (flexible, extendable) code
DRY
Don’t Repeat Yourself• Don’t duplicate code
5
Without DRY
st.executeQuery("select user.name, user.password from user where id=?");
If something changes?
st.executeQuery("select user.username, user.password from user where id=?");
With DRY
st.executeQuery("select user.name, user.password from user where id=?");
public User getUser(Long id) {…}
If something changes?
st.executeQuery("select user.username, user.password from user where id=?");
public User getUser(Long id) {…}
DRY
Don’t Repeat Yourself• Don’t duplicate code
• Names should be clear
Not clear names
public class Utils { public static Connection createConnection(String... params){...} public static Object[] getSortedArray(Object[] array){...} }
1. No one would guess to look sorting method in this class.2. Newbie always would write his own implementation.
Correct DRY
public class ArrayUtils { public static Object[] getSortedArray(Object[] array) {…} }
public class DatabaseUtils { public static Connection createConnection(String... params) {...} }
clear, well-defined class names
DRY
Don’t Repeat Yourself• Don’t duplicate code
• Names should be clear
• Location should be clear
Not clear location
package ru.jt.transformer.csv; public class ArrayUtils { public static Object[] getSortedArray(Object[] array) {...} }
No one would look for array utilities in such package
DRY. Pros & Cons
Pros:
• Changes impact local area
• Once written is not repeated
• No ”new” error prone solutions
Cons:
• Amount of classes grows
OCP
Open Closed Principle - code should be closed for modification, but open for extension.
OCP. Example
I want my clients to be able to see results of past games. Let’s
keep up with NHL & NBA games..
1. American customer
2. His thoughts
3. His ill imagination
OCP. Example
public class SportInfoParser implements SportInfoParser {
public SportInfo parseSportInfo(String[] sportInfoArray) { SportInfo sportInfo = null;
if ("nba".equals(sportInfoArray[0])) { NbaSportInfo nbaSportInfo = new NbaSportInfo(); Map<Long, Integer> scores = new HashMap<Long, Integer>(); scores.put(Long.parseLong(sportInfoArray[12]), Integer.parseInt(sportInfoArray[13])); nbaSportInfo.setScores(scores); sportInfo = nbaSportInfo;
} else if ("nhl".equals(sportInfoArray[0])) { NhlSportInfo nhlSportInfo = new NhlSportInfo(); nhlSportInfo.setSlapShotCount(1); } return sportInfo; } }
Base class
Creates specific objects according to...
OCP. Example
Great! A lot of new clients, a lot of money from ads. But.. Would be great if my cliends would be able to get info about MLB games too!
OCP. Example
public class SportInfoParser implements SportInfoParser { public SportInfo parseSportInfo(String[] sportInfoArray) { SportInfo sportInfo = null; if ("nba".equalsIgnoreCase(sportInfoArray[0])) { NbaSportInfo nbaSportInfo = new NbaSportInfo(); Map<Long, Integer> scores = new HashMap<Long, Integer>(); scores.put(Long.parseLong(sportInfoArray[12]), Integer.parseInt(sportInfoArray[13])); nbaSportInfo.setScores(scores); sportInfo = nbaSportInfo; } else if ("nhl".equalsIgnoreCase(sportInfoArray[0])) { NhlSportInfo nhlSportInfo = new NhlSportInfo(); nhlSportInfo.setSlapShotCount(1);
} else if(sportInfoArray[0].equalsIgnoreCase("mlb")){ MlbSportInfo mlbSportInfo = new MlbSportInfo(); mlbSportInfo.setHits(Integer.parseInt(sportInfoArray[1])); mlbSportInfo.setRuns(Integer.parseInt(sportInfoArray[2])); } return sportInfo; } }
New league was added
We are changing already working class!
OCP. Example
Why my clients see errors on every page?!! I pay you for work, not for errors! I
loose my clients, make the program work right!
OCP. Example
public class SportInfoParser implements SportInfoParser { public SportInfo parseSportInfo(String[] sportInfoArray) { SportInfo sportInfo = null; if ("nba".equalsIgnoreCase(sportInfoArray[0])) { NbaSportInfo nbaSportInfo = new NbaSportInfo(); Map<Long, Integer> scores = new HashMap<Long, Integer>(); scores.put(Long.parseLong(sportInfoArray[12]), Integer.parseInt(sportInfoArray[13])); nbaSportInfo.setScores(scores); sportInfo = nbaSportInfo; } else if ("nhl".equalsIgnoreCase(sportInfoArray[0])) { NhlSportInfo nhlSportInfo = new NhlSportInfo(); nhlSportInfo.setSlapShotCount(1);
} else if(sportInfoArray[0].equalsIgnoreCase("mlb")){ MlbSportInfo mlbSportInfo = new MlbSportInfo(); mlbSportInfo.setHits(Integer.parseInt(sportInfoArray[1])); mlbSportInfo.setRuns(Integer.parseInt(sportInfoArray[2])); } return sportInfo; } }
Element of array is compared with league name
– NPE is possible!
OCP. Moral
This example shows that changing code that already works is always bad idea.
Follow OCP to escape this!
OCP. Example
public class SportInfoParser implements SportInfoParser { private Map<String, SportInfoBuilder> builders; public SportInfoParserEnhanced(Map<String, SportInfoBuilder> builders) { this.builders = builders; } public SportInfo parseSportInfo(String[] sportInfoArray) { SportInfoBuilder builder = builders.get(sportInfoArray[0]); if(builder != null){ return builder.build(sportInfoArray); } return null; } }
MlbSportInfoBuilder
NbaSportInfoBuilder NhlSportInfoBuilder
public class NbaSportInfoBuilder implements SportInfoBuilder { public SportInfo build(String[] sportInfoArray) { NbaSportInfo nbaSportInfo = new NbaSportInfo(); Map<Long, Integer> scores = new HashMap<Long, Integer>(); scores.put(…, …); nbaSportInfo.setScores(scores); return nbaSportInfo; } }
OCP. Example
Great! You job satisfies me!But now I want WNBA to be
added
OCP. Example
public class SportInfoParser implements SportInfoParser { private Map<String, SportInfoBuilder> builders; public SportInfoParserEnhanced(Map<String, SportInfoBuilder> builders) { this.builders = builders; } public SportInfo parseSportInfo(String[] sportInfoArray) { SportInfoBuilder builder = builders.get(sportInfoArray[0]); if(builder != null){ return builder.build(sportInfoArray); } return null; } }
MlbSportInfoBuilder
NbaSportInfoBuilder NhlSportInfoBuilder
WnbaSportInfoBuilder
New league was added without changing single line of code!
OCP. How it works
OCP uses:• Delegation
• Inheritance
OCP. Pros & Cons
Pros:
• Adding new functionality without legacy code being changed
Cons:
• May complicate system because of amount of classes being grown
SRP
Single Responsibility Principle:
• Single class should have a single responsibility
• Class should have only one reason to change
SRP. Random thoughts
public class CurrencyConverter { public BigDecimal convert(Currency from, Currency to, BigDecimal amount) { // gets connection to some online service and asks it to convert currency // parses the answer and returns results } public BigDecimal getInflationIndex(Currency currency, Date from, Date to) { // gets connection to some online service to get data about // currency inflation for specified period } }
Hm.. Strange that inflation is counted in CurrencyConverter..
Hm.. What if format of currency service changes?
What if the format of inflation service changes?
We’ll have to change this class in both cases!
It’s not intuitive! It’s overloaded!
We have to do something!
SRP. Separate Responsibilities
public class CurrencyConverter { public BigDecimal convert(Currency from, Currency to, BigDecimal amount) { // gets connection to some online service and asks it to convert currency // parses the answer and returns results } }
public class InflationIndexCounter { public BigDecimal getInflationIndex(Currency currency, Date from, Date to) { // gets connection to some online service to get data about // currency inflation for specified period } }
Hm.. What if format of currency service changes? We
change CurrencyConverter!
Hm.. What if format of inflation service changes? We change InflationIndexCounter!
SRP & DRY
Again two responsibilities:Authentication & getting user
from database
public class UserAuthenticator { public boolean authenticate(String username, String password){ User user = getUser(username); return user.getPassword().equals(password); } private User getUser(String username){ st.executeQuery("select user.name, user.password from user where id=?"); // something's here return user; } }
DRY violation!
SRP. Delegating responsibilities
public class UserAuthenticator { private UserDetailsService userDetailsService; public UserAuthenticator(UserDetailsService service) { userDetailsService = service; } public boolean authenticate(String username, String password){ User user = userDetailsService.getUser(username); return user.getPassword().equals(password); } }
Now we don’t work directly with database!
If we would want to use ORM, UserAuthenticator
won’t change!
SRP. Pros & Cons
Pros:
• Helps to follow DRY
• Lowers chances to change single class
• Class names correspond to what they do
Cons:
• May complecate system by adding too much new classes
ISP
Interface Segregation Principle – client code shouldn’t be obligated to depend on interfaces it doesn’t use.
ISP. Example
interface Person { void goToWork(); void withdrawSalary(); void eat(); }
Base interface of the person.
All these methods are useful for current
implementation of person.
ISP. Extra methods
interface Person { void goToWork(); void withdrawSalary(); void eat(); }
But we’re writting new module that considers a person only as a human being. So we need only
one method eat()
public class PersonImpl implements Person { public void goToWork() { throw new UnsupportedOperationException(); } public void withdrawSalary() { throw new UnsupportedOperationException(); } public void eat() { //some real implementation } }
So our new implementation has two
extra methods.
ISP. What these methods do?!
ISP. Fat/Polluted Interfaces
interface Person { void goToWork(); void withdrawSalary(); void eat(); }
It’s fat
It’s POLLUTED
ISP. Interface Separating
public interface Person { void eat(); }
public interface Worker { void goToWork(); void withdrawSalary(); }
We separated Person into Person & Worker.
Two conceptually different interfaces.
ISP. No extra methods
public class PersonImpl implements Person { public void eat() { //some real implementation } }
Now we have only needed methods.
ISP. Legacy code
What if we have ready implementation, but we
don’t want to use fat interface?
public class FatPersonImpl implements FatPerson { public void goToWork() { //some real implementation } public void withdrawSalary() { //some real implementation } public void eat() { //some real implementation } }
ISP. Use Adapters
public class PersonAdapter implements Person { private FatPerson fatPerson; public PersonAdapter(FatPerson fatPerson) { this.fatPerson = fatPerson; } public void eat() { fatPerson.eat(); } }
Thin interface
Fat interface
ISP. Pros & Cons
Pros:
• No need to implement unnecessary methods
• Client code sees only what it should see
Cons:
• Adding additional interfaces
IoCP
Invertion of Control Principle says:
• Code to abstraction, not to implementation
• Objects that use other objects, shouldn’t create latter ones
IoCP. Coding to implementationpublic interface HtmlParser { HtmlDocument parseUrl(String url); }
public class Crawler { public void saveHtmlDocument() { DomBasedHtmlParser parser = new DomBasedHtmlParser(); HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } public void save(HtmlDocument htmlDocument, String pageName) { // logic of saving } }
public class DomBasedHtmlParser implements HtmlParser { public HtmlDocument parseUrl(String url) { // getting html page as stream //parsing it with DOM parser //creating HtmlDocument return htmlDocuments; } }
IoCP. How to test Crawler?
public class Crawler { public void saveHtmlDocument() { DomBasedHtmlParser parser = new DomBasedHtmlParser(); HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } public void save(HtmlDocument htmlDocument, String pageName) { // logic of saving } }
It’s impossible to write unit test for Crawler, because you cannot
mock parser.
IoCP. Let’s injectpublic class Crawler { private DomBasedHtmlParser parser; public Crawler(DomBasedHtmlParser parser) { this.parser = parser; } public void saveHtmlDocument() { HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } public void save(HtmlDocument htmlDocument, String pageName) { // logic of saving } }
Crawler crawler = new Crawler(someMockParser);
Now you can specify parser through
constructor. You can inject dummy object
while testing.
IoCP. Again doesn’t work
Your parser doesn’t work with HTML that
isn’t a valid XML!
IoCP. New Implementation
HtmlParser
DomBasedHtmlParser
EnhancedHtmlParser
IoCP. But how do we replace?
public class Crawler { private DomBasedHtmlParser parser; public Crawler(DomBasedHtmlParser parser) { this.parser = parser; } public void saveHtmlDocument() { HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } public void save(HtmlDocument htmlDocument, String pageName) { // logic of saving } }
We cannot specify another implementaion!
IoCP. Let’s code to interface
public class Crawler { private HtmlParser parser; public Crawler(HtmlParser parser) { this.parser = parser; } public void saveHtmlDocument() { HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } public void save(HtmlDocument htmlDocument, String pageName) { // logic of saving } }
Now we use interface, so we can specify enhanced
implementation of parser.
IoCP. Another look
How do we inject objects if we work on framework/library? We cannot use IoC Containers, our clients don’t allow us extra dependencies, they don’t want to depend on Spring or Guice. We need leightweight decision.
IoCP. Let’s use Factories
public class Crawler{ private HtmlParser parser = ParserFactory.getHtmlParser(); public void saveHtmlDocument() { HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } }
Let’s use Factories! Hm.. But we cannot write unit tests again!
IoCP. Let’s mix
public class Crawler{ private HtmlParser parser = ParserFactory.getHtmlParser(); public void saveHtmlDocument() { HtmlDocument document = parser.parseUrl("http://javatalks.ru"); save(document, "jt-index"); } public void setParser(HtmlParser parser) { this.parser = parser; } }
We have both: setter and factory in the same class.
Now we have default implementation and possibility to change
default behavior.
IoCP. Pros & Cons
Pros:
• Classes don’t depend on concrete implementation
• Allows easily change implementation
• Allows write good unit tests
Cons:
• Creating additional interfaces
• Creating Factories/depending on IoC containers
LSP
Liskov’s Substitution Principle – derived types must be completely substitutable for their base types.
LSP declares how to use inheritance correctly.
LSP. java.util.List
List list = new ArrayList();
List list = new LinkedList();
list.get(1);
Would be strange if these implementation would do
different things here
LSP. Emu doesn’t fly!
class Bird extends Animal { @Override //walk is overriden from Animal public void walk() {...} @Override //makeOffspring() is overriden from Animal public void makeOffspring() {...}; //was added public void fly() {...} }
class Emu extend Bird { public void makeOffspring() {...} }
But emu doesn’t fly!
LSP. Emu indeed doesn’t fly
class Bird extends Animal { @Override //walk is overriden from Animal public void walk() {...} @Override //makeOffspring() is overriden from Animal public void makeOffspring() {...} } class FlyingBird extends Bird {
public void fly() {...} }
class Emu extends Bird { @Override public void makeOffspring(){..} }
Simply birds.Flying birds.
Emu is simply a bird.It doesn’t have extra
methods.
LSP. Array example
interface ArraySorter { Object[] parse(Object []args); }
class DefaultArraySorter implements ArraySorter { public Object[] sort(Object []array){ Object[] result = array.clone(); ... } }
Default implementation. It’s a temporal class that
uses unefficient approach. But it does it’s work
correctly.
LSP. Array example
interface ArraySorter { Object[] parse(Object []args); }
At last we wrote enhanced implementation that’s
really efficient.
class QuickArraySorter implements ArraySorter { public Object[] sort(Object []array){ Object[] result = array; ... } }
LSP. Again bugs!
Your system always throws errors! It’s a
chaos!!
LSP. Array example
class QuickArraySorter implements ArraySorter { public Object[] sort(Object []array){ Object[] result = array; ... } }
It sorts the original array! We have problems with
synchronization.
Implementation does its work, but it has side
effects. It doesn’t satisfy interface contract!
We cannot simply replace one implementation with
another one, because they differ!
We should correct its behaviour to copy original
array and work with its copy.
LSP & equals()
public class Point { private int x; private int y; @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof Point)) return false; Point point = (Point) o; if (x != point.x) return false; if (y != point.y) return false; return true; } }
public class ColoredPoint extends Point { private int color; @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof ColoredPoint)) return false; if (!super.equals(o)) return false; ColoredPoint that = (ColoredPoint) o; if (color != that.color) return false; return true; } }
Colored point extends simple point and adds new
field – color.
But it works only with ColoredPoint!
LSP & equals
Point point = new Point(1, 1); ColoredPoint coloredPoint = new ColoredPoint(1, 1, 1); System.out.println(point.equals(coloredPoint)); System.out.println(coloredPoint.equals(point));
This will print:true false
This violates equals() contract!
There is no correct dicision in this situation!
LSP & equals()
public class ColoredPoint { private Point point; private int color; }
The only correct way here is to use delegation
instead of inheritance.
LSP & exceptions
List list = new OurSuperPuperImplementation(); list.iterator().next();
What if this method in our implementation threw IOException?
How would we know about that? We work
with interface List, not with implementation!
That’s why Java doesn’t allow us to throw any checked
exception that are not declared in base class.
LSP. Pros & Cons
Pros:
• Allows not to think about concrete implementation, but code to abstraction
• Unambiguously defines contract all implementers should follow
• Allows to interchange implementation correctly, without side effects
The End
Q/A