patterns.md

title
Design Patterns

Creational Patterns

Abstract Factory

Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Builder

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Factory Method

Define an interface for creating an object, but let subclasses decide which class to instantiate.

Prototype

Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Singleton

Ensure a class only has one instance, and provide a global point of access to it.

Structural Patterns

Adapter

Convert the interface of a class into another interface clients expect.

Bridge

Decouple an abstraction from its implementation so that the two can vary independently.

Composite

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

Decorator

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Facade

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.

Flyweight

Use sharing to support large numbers of fine-grained objects efficiently.

Proxy

Provide a surrogate or placeholder for another object to control access to it.

Behavioral Patterns

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.

Command

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Interpreter

Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Iterator

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

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.

Memento

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

Observer

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

State

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

Strategy

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

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.

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.

Language Support

Java

implements an interface

public interface Comparable<T> {
  public abstract int compareTo(T that);
}

public class Point implements Comparable<Point> {
  private final double x;
  private final double y;
  
  public Point(double x, double y) {
    this.x = x;
    this.y = y;
  }
  
  @Override
  public int compareTo(Point that) {
    if (this.x < that.x) return -1;
    if (this.x > that.x) return +1;
    if (this.y < that.y) return -1;
    if (this.y > that.y) return +1;
    return 0;
  }
}

extends a class

所有用 class 定义的类都是 java.lang.Object 的“子类 (subclass)”或“派生类 (derived classs)”。

public class Object {
  public String toString();
}

public class Point extends Object {
  private final double x;
  private final double y;
  
  public Point(double x, double y) {
    this.x = x;
    this.y = y;
  }
  
  @Override
  public String toString() {
    StringBuilder sb = new StringBuilder();
    sb.append('(');
    sb.append(this.x);
    sb.append(',').append(' ');
    sb.append(this.y);
    sb.append(')');
    return sb.toString();
  }
  
  // extended methods
  // ...
}

C++

Inheritance

#include <iostream>
#include <string>

class Object {
 public:
  virtual std::string to_string() = 0;
};

class Point : public Object {
 private:
  const double _x;
  const double _y;
  
 public:
  Point(double x, double y) : _x(x), _y(y) { }
  virtual std::string to_string() override {
    return '(' + std::to_string(_x) + ',' + ' ' + std::to_string(_y) + ')';
  }
};

int main() {
  Point p = Point(1.0, 0.0);
  std::cout << p.to_string() << std::endl;
}

Inheritance + template

#include <iostream>

template<class T>
class Comparable {
  virtual int compareTo(const T& that) = 0;
};

class Point : public Comparable<Point> {
 private:
  const double _x;
  const double _y;
  
 public:
  Point(double x, double y) : _x(x), _y(y) { }
  virtual int compareTo(const Point& that) override {
    if (_x < that._x) return -1;
    if (_x > that._x) return +1;
    if (_y < that._y) return -1;
    if (_y > that._y) return +1;
    return 0;
  }
};

int main() {
  using P = Point;
  P o = P(0.0, 0.0);
  P p = P(1.0, 0.0);
  std::cout << p.compareTo(o) << std::endl;
}