# Design Patterns

categorization
- by purpose
  - creational
  - structural
  - behavioral
- by scope
  - class — relationships between classes and their subclasses
  - object
index
- creational
  - class — Factory Method
  - object — Abstract Factory, Builder, Prototype, Singleton
- structural
  - class and object — Adapter
  - object — Bridge, Composite, Decorator, Facade, Flyweight, Proxy
- behavioral
  - class — Interpreter, Template Method
  - object — Chain of Responsibility, Command, Iterator, Mediator, Memento, Observer, State, Strategy, Visitor
inheritance and composition
- inheritance — white-box reuse
- composition — black-box reuse, forwarding requests with a reference to the original object
  - delegation — State, Strategy, Visitor patterns and some more conditionally
object aggregation and acquaintance
- aggregation — one object owns or is responsible for another object
- acquaintance — an object merely knows of another object

# Creational

Creational design patterns abstract the instantiation process, They help make a system independent of how its objects are created, composed, and represented.

Singleton

other implementation — eager, lazy without thread safe, method synchronized lazy

volatile and double null check

public class Singleton {
    private volatile static Singleton uniqueInstance;
    private Singleton() { }
    public static Singleton getUniqueInstance() {
        if (uniqueInstance == null) {
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}

static inner class

public class Singleton {
    private Singleton() { }
    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getUniqueInstance() {
        return SingletonHolder.INSTANCE;
    }
}

as an enum — prevent reflection attack; no duplicates after serialization and de-serialization
examples — beans, loggers, Runtime::getRuntime, System::getSecurityManager

factory

Abstract Factory — provide an interface for creating families of related or dependent objects without specifying their concrete classes
```
public abstract class AbstractFactory {
    abstract AbstractProductA createProductA();
    abstract AbstractProductB createProductB();
}
```
- Factory Method for each

Factory Method — define an interface for creating an object, but let subclasses decide which class to instantiate; lets a class defer instantiation to subclasses

public abstract class Factory {
    abstract public Product factoryMethod();
    public void doSomething() {
        Product product = factoryMethod();
        // do something with the product
    }
}

examples — java.util.concurrent.ThreadFactory, org.springframework.beans.factory.BeanFactory, java.net.SocketImplFactory
```
public interface ThreadFactory {
    Thread newThread(Runnable r);
}
```

Simple Factory — 把实例化的操作单独放到一个类中，这个类就成为简单工厂类，让简单工厂类来决定应该用哪个具体子类来实例化; in some literature there is no Simple Factory, but the definition of Factory Method is expanded

public class SimpleFactory {
    public Product createProduct(int type) {
        if (type == 1) {
            return new ConcreteProduct1();
        } else if (type == 2) {
            return new ConcreteProduct2();
        }
        return new ConcreteProduct();
    }
}

examples — java.util.EnumSet, java.util.ResourceBundle, java.text.NumberFormat, java.nio.charset.Charset, org.springframework.aop.framework.ProxyFactory, React.createElement

// EnumSet::noneOf
public static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> elementType) {
    Enum<?>[] universe = getUniverse(elementType);
    if (universe == null)
        throw new ClassCastException(elementType + " not an enum");
    if (universe.length <= 64)
        return new RegularEnumSet<>(elementType, universe);
    else
        return new JumboEnumSet<>(elementType, universe);
}

Builder — separate the construction of a complex object from its representation so that the same construction process can create different representations
- examples — Spring security, StringBuilder, java.nio.ByteBuffer, Appendable, java.util.stream.Stream.Builder
Prototype — specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype
- examples — Object::clone, JavaScript prototype

# Structural

Structural patterns are concerned with how classes and objects are composed to form larger structures.

Adapter — convert the interface of a class into another interface clients expect
- examples — java.util.Collections::asLifoQueue, java.io.InputStreamReader, java.io.OutputStreamWriter, java.util.Arrays::asList

Bridge — decouple an abstraction from its implementation so that the two can vary independently

public abstract class Abstraction {
    protected AbstractionImpl impl;
    public Abstraction(AbstractionImpl impl) {
        this.impl = impl;
    }
    public abstract void operation(); // do something with impl
}

examples — JDBC, java.net.Socket and java.net.SocketImpl

Composite — Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

public class Composite extends Component {
    private List<Component> child;
    public Composite(String name) {
        super(name);
        child = new ArrayList<>();
    }
    @Override
    public void operation() { /* ... */ }
    @Override
    public void add(Component component) {
        child.add(component);
    }
    @Override
    public void remove(Component component) {
        child.remove(component);
    }
}

examples — React.Fragment

Decorator — Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
- examples — @ in Python and TypeScript, java.io.FilterInputStream, java.io.FilterOutputStream, java.io.FilterReader, java.io.FilterWriter, some views in java.util.Collections
Facade (Façade) — Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
- example — Compiler: Scanner, Parser, ProgramNode, BytecodeStream, and ProgramNodeBuilder
Flyweight — Use sharing to support large numbers of fine-grained objects efficiently, where intrinsic states are immutable but extrinsic states can vary
- examples — Integer::valueOf, also on Boolean, Byte, Character, Short, Long, BigDecimal and BigInteger
Proxy — Provide a surrogate or placeholder for another object to control access to it.
- taxonomy
  - remote proxy — provides a local representative for an object in a different address space
  - virtual proxy — creates expensive objects on demand, like @org.springframework.context.annotation.Lazy
  - protection proxy — controls access to the original object
  - smart reference — a replacement for a bare pointer that performs additional actions when an object is accessed; for example, counting the number of references to the real object so that it can be freed automatically when there are no more references (also called smart pointers)
- examples — java.lang.reflect.Proxy, Spring AOP, java.rmi.*

# Behavioral

Behavioral patterns are concerned with algorithms and the assignment of responsibilities between objects. Behavioral patterns describe not just patterns of objects or classes but also the patterns of communication between them. These patterns characterize complex control flow that's difficult to follow at run-time. They shift your focus away from flow of control to let you concentrate just on the way objects are interconnected.

Chain of Responsibility — Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
- examples — javax.servlet.Filter::doFilter, java.util.logging.Logger::log
Command — Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
```
public interface Command {
    void execute();
}
```
- use
  - callback — Commands are an object-oriented replacement for callbacks
  - specify, queue, and execute requests at different times
  - support undo, redo — store states and add an undo operation to the Command interface; for unlimited-level undo and redo, store executed commands in a history list
  - support logging — for durability, similar to undo and redo
- examples — Runnable, Netflix Hystrix, lambda
Interpreter — Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
- examples — java.util.Pattern, java.text.Format subclasses (Context is not String), SpEL
Iterator — Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
- examples — java.util.Iterator (also java.util.Scanner), java.util.Enumeration
Mediator — Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
```
public class Client {
    public static void main(String[] args) {
        Alarm alarm = new Alarm();
        CoffeePot coffeePot = new CoffeePot();
        Calender calender = new Calender();
        Sprinkler sprinkler = new Sprinkler();
        Mediator mediator = new ConcreteMediator(alarm, coffeePot, calender, sprinkler);
        alarm.onEvent(mediator);
    }
}
```
- examples
  - java.util.Timer (all scheduleXXX() methods)
  - java.util.concurrent.Executor::execute
  - java.util.concurrent.ExecutorService (the invokeXXX() and submit() methods)
  - java.util.concurrent.ScheduledExecutorService (all scheduleXXX() methods)
  - java.lang.reflect.Method::invoke

Memento — Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

public interface Originator {
  Memento createMemento();
  void setMemento(Memento memo);
}
public interface Memento {
  State getState();
  void setState();
}

examples — java.io.Serializable

Observer — Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
- examples — java.util.EventListener, java.util.concurrent.Flow, RxJava (opens new window)
  - java.util.Observer, java.util.Observable — deprecated since JDK 9, see Event Handling

State — Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

public interface State {
  void handle();
}

example

public class TCPConnection {
  private TCPState state;
  // ...
}
public class TCPEstablished implements TCPState {}
public class TCPListen implements TCPState {}
public class TCPClosed implements TCPState {}

Strategy — Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
```
public class Context<T extends Strategy> {
  private T strategy;
  public void operation() { strategy.doAlgorithm(); }
}
```
- examples — javax.servlet.http.HttpServlet, java.security.MessageDigest::digest
Template Method — Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.
- examples
  - all non-abstract methods of java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer
  - default methods in collection interfaces and all non-abstract methods in collection companion classes
    - LinkedHashMap::afterNodeRemoval, LinkedHashMap::afterNodeInsertion, LinkedHashMap::afterNodeAccess overrides those in HashMap
  - React lifecycle

Visitor — Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

examples

javax.lang.model.element.AnnotationValue and javax.lang.model.element.AnnotationValueVisitor
javax.lang.model.element.Element and javax.lang.model.element.ElementVisitor
javax.lang.model.type.TypeMirror and javax.lang.model.type.TypeVisitor

java.nio.file.FileVisitor used in java.nio.file.Files::walkFileTree

// example from javadoc of FileVisitor
Path start = ...
Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
        throws IOException
    {
        Files.delete(file);
        return FileVisitResult.CONTINUE;
    }
    @Override
    public FileVisitResult postVisitDirectory(Path dir, IOException e)
        throws IOException
    {
        if (e == null) {
            Files.delete(dir);
            return FileVisitResult.CONTINUE;
        } else {
            // directory iteration failed
            throw e;
        }
    }
});

org.springframework.asm.ClassVisitor, org.springframework.asm.FieldVisitor, org.springframework.asm.MethodVisitor, org.springframework.asm.ModuleVisitor, org.springframework.asm.AnnotationVisitor

空对象 — 使用什么都不做的空对象来代替 null。