設計模式：深入探討常見設計模式

什麼是設計模式？

設計模式是複雜問題的解決方案。設計模式就是以解決特定設計問題的方式建構類別和介面。通常，在設計系統時，我們會遇到一些問題，針對這些問題，我們有一套設計模式。設計模式通常是涉及類別、介面以及這些類別之間的關係的範本。

設計模式的類型：

創意設計模式：

這些類型的模式以與給定情況相容的方式處理物件的建立。
在創建層面，我們可以確定係統的特定部分如何獨立創建或組合在一起，確保靈活性和相容性。
屬於此類別的設計模式清單是：

• 單例：在此設計模式中，我們只有一個實例，並且該實例將在整個應用程式中使用。

單例設計模式重點：

1. 私有建構子：將建構子標記為私有非常重要，因為我們要確保只建立該類別的實例。
2. 私人靜態實例：我們使用私人存取修飾符，因為我們要確保類別記憶體中只有該物件的實例。
3. 公用靜態方法（存取器）：這是單一實例的全域存取點。如果實例不存在，此方法基本上會建立實例，如果實例已存在，則傳回相同的實例。

單例設計模式範例

public class Singleton {
    // Private static instance of the class
    private static Singleton instance;
    private int count;

    // Private constructor to prevent instantiation
    private Singleton() {
        // initialization code
    }

    // Public static method to provide access to the instance
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    // Example method
    public void getCount() {
        System.out.println("The value of count is: " + count);
    }

    public void increaseCount() {
        count++;
    }

    public void decreaseCount() {
        count--;
    }
}

public class Main {
    public static void main(String[] args) {
        // Get the single instance of Singleton
        Singleton singleton = Singleton.getInstance();
        singleton.increaseCount();
        singleton.getCount();  // Output: The value of count is: 1

        // Get the same instance of Singleton
        Singleton anotherSingleton = Singleton.getInstance();
        anotherSingleton.decreaseCount();
        anotherSingleton.getCount();  // Output: The value of count is: 0

        // Both singleton and anotherSingleton refer to the same instance
    }
}

• Builder： 在Builder模式中，我們定義了一種逐步建立物件的方法。此模式還提供了靈活性，允許使用相同的建構過程建立同一物件的不同版本。

建構器模式的關鍵要素：

1. 產品：它是正在建構的複雜物件。
2. 建構器介面： 定義建立產品不同部分的方法。這些方法通常會傳回建構器物件本身以允許方法連結。
3. 具體建構器：實作建構器介面並提供用於建立產品部分的特定實作。

建構器模式範例：
此範例展示如何使用建構器設計模式透過逐步添加成分來製作巧克力醬麵包。

// Product Class
class Bread {
    private String bread;
    private String spread;
    private String chiaSeeds;
    private String pumpkinSeeds;

    public void setBread(String bread) {
        this.bread = bread;
    }

    public void setSpread(String spread) {
        this.spread = spread;
    }

    public void setChiaSeeds(String chiaSeeds) {
        this.chiaSeeds = chiaSeeds;
    }

    public void setPumpkinSeeds(String pumpkinSeeds) {
        this.pumpkinSeeds = pumpkinSeeds;
    }

    @Override
    public String toString() {
        return "Bread with " + spread + ", topped with " + chiaSeeds + " and " + pumpkinSeeds;
    }
}

// Builder Interface
interface BreadBuilder {
    BreadBuilder addBread();
    BreadBuilder addChocolateSpread();
    BreadBuilder addChiaSeeds();
    BreadBuilder addPumpkinSeeds();
    Bread build();
}

// Concrete Builder
class ChocolateBreadBuilder implements BreadBuilder {
    private Bread bread = new Bread();

    @Override
    public BreadBuilder addBread() {
        bread.setBread("Whole grain bread");
        return this;
    }

    @Override
    public BreadBuilder addChocolateSpread() {
        bread.setSpread("Chocolate spread");
        return this;
    }

    @Override
    public BreadBuilder addChiaSeeds() {
        bread.setChiaSeeds("Chia seeds");
        return this;
    }

    @Override
    public BreadBuilder addPumpkinSeeds() {
        bread.setPumpkinSeeds("Pumpkin seeds");
        return this;
    }

    @Override
    public Bread build() {
        return bread;
    }
}

// Client Code
public class Main {
    public static void main(String[] args) {
        // Create a builder and build the chocolate spread bread
        BreadBuilder builder = new ChocolateBreadBuilder();
        Bread myBread = builder.addBread()
                               .addChocolateSpread()
                               .addChiaSeeds()
                               .addPumpkinSeeds()
                               .build();

        // Output the result
        System.out.println(myBread);
    }
}

• 工廠方法：在工廠方法模式中，我們定義了一種建立物件的方法，但我們允許子類別決定將建立的物件的具體類型。

工廠模式的關鍵要點：

1. 產品介面：定義所有產品的通用介面。
2. 具體產品：實作產品介面。
3. 創建者： 聲明工廠方法。
4. 具體創建者： 實作工廠方法以傳回不同的特定產品。

// Product Interface
interface Juice {
    void serve();
}

// Concrete Product 1
class OrangeJuice implements Juice {
    @Override
    public void serve() {
        System.out.println("Serving Orange Juice.");
    }
}

// Concrete Product 2
class MangoJuice implements Juice {
    @Override
    public void serve() {
        System.out.println("Serving Mango Juice.");
    }
}

// Creator Abstract Class
abstract class JuiceFactory {
    // Factory method
    public abstract Juice createJuice();
}

// Concrete Creator 1
class OrangeJuiceFactory extends JuiceFactory {
    @Override
    public Juice createJuice() {
        return new OrangeJuice();
    }
}

// Concrete Creator 2
class MangoJuiceFactory extends JuiceFactory {
    @Override
    public Juice createJuice() {
        return new MangoJuice();
    }
}

// Client Code
public class Main {
    public static void main(String[] args) {
        // Create an Orange Juice using its factory
        JuiceFactory orangeJuiceFactory = new OrangeJuiceFactory();
        Juice orangeJuice = orangeJuiceFactory.createJuice();
        orangeJuice.serve();  // Output: Serving Orange Juice.

        // Create a Mango Juice using its factory
        JuiceFactory mangoJuiceFactory = new MangoJuiceFactory();
        Juice mangoJuice = mangoJuiceFactory.createJuice();
        mangoJuice.serve();  // Output: Serving Mango Juice.
    }
}

結構設計模式

這種設計模式主要關注類別和物件如何組合以形成更大的結構。他們專注於物件和類別之間的組織和關係，簡化結構，增強靈活性，提高可維護性。

• 適配器模式： 在這種模式中，我們允許介面不相容的物件一起工作。它充當兩個不相容介面之間的橋樑，使它們能夠在不更改現有程式碼的情況下進行通訊。

適配器模式的關鍵要素：

1. Target Interface: It is an interface that will solve the problem (bridging the gap between the incompatible interfaces).
2. Client: The class or code that interacts with the target interface.
3. Adaptee: This is the interface which is not compatible with the current client requirements.
4. Adapter: Implements the target interface and contains an instance of the adaptee. It translates requests from the target interface to the adaptee’s interface, making them compatible.

// Target Interface (Menu)
interface Menu {
    void orderDish(String dish);
}

// Adaptee (Chef)
class Chef {
    public void prepareDish(String dishName) {
        System.out.println("Chef is preparing " + dishName + ".");
    }
}

// Adapter (Waiter)
class Waiter implements Menu {
    private Chef chef;

    public Waiter(Chef chef) {
        this.chef = chef;
    }

    @Override
    public void orderDish(String dish) {
        chef.prepareDish(dish);
    }
}

// Client Code
public class Restaurant {
    public static void main(String[] args) {
        Chef chef = new Chef();
        Menu waiter = new Waiter(chef);

        // Customer places an order via the waiter
        waiter.orderDish("Spaghetti Carbonara");  // Output: Chef is preparing Spaghetti Carbonara.
    }
}

• Facade pattern: Simplifies the interaction with a complex system by providing a unified interface (facade). Instead of directly calling several different methods across various objects, the client interacts with the facade, which internally manages those operations.

Key essentials of the facade design pattern:

1. Facade: It is an interface that wraps all the complex subsystem interfaces and delegates the complex tasks to the subsystems that actually perform the work.
2. Subsystem Classes: These are the classes that acutally perform the work.

An example of facade design pattern:
The example illustrates the Facade Pattern which simplifies the process of washing, drying, and pressing clothes. It hides the complexity of interacting with multiple subsystems behind a single, unified interface.

// Subsystem Classes
class WashingMachine {
    public void wash() {
        System.out.println("Washing clothes.");
    }
}

class Dryer {
    public void dry() {
        System.out.println("Drying clothes.");
    }
}

class Iron {
    public void press() {
        System.out.println("Pressing clothes.");
    }
}

// Facade Class
class LaundryFacade {
    private WashingMachine washingMachine;
    private Dryer dryer;
    private Iron iron;

    public LaundryFacade(WashingMachine washingMachine, Dryer dryer, Iron iron) {
        this.washingMachine = washingMachine;
        this.dryer = dryer;
        this.iron = iron;
    }

    public void doLaundry() {
        System.out.println("Starting the laundry process...");
        washingMachine.wash();
        dryer.dry();
        iron.press();
        System.out.println("Laundry process complete.");
    }
}

// Client Code
public class Main {
    public static void main(String[] args) {
        WashingMachine washingMachine = new WashingMachine();
        Dryer dryer = new Dryer();
        Iron iron = new Iron();

        LaundryFacade laundryFacade = new LaundryFacade(washingMachine, dryer, iron);

        // Use the facade to do the laundry
        laundryFacade.doLaundry();
    }
}

Behavioral design patterns

The patterns that fall under this category mainly deals with communication between objects and how they interact with each other.

• Iterator pattern: In the Iterator Pattern, we define a way to sequentially access elements of a collection without needing to use conventional methods, such as for loops or direct indexing. Instead, the pattern provides a standard interface (usually methods like next() and hasNext()) to traverse the collection. This approach abstracts the iteration process, allowing the client to navigate through the collection without needing to understand its internal structure or use traditional iteration methods.

Key essentials of this pattern are:

1. Iterator Interface: We define all the methods such as next(), hasNext(), and currentItem().These are used to traverse the collection.
2. Concrete Iterator: This is the concrete implementation of the iterator interface.
3. Aggregate Interface: In this interface,we define methods to create iterators.All the methods returns an instance of the Iterator.
4. Concrete Aggregate: It's just a concrete implementation of the aggregate interface.

Example of iterator pattern:
This example demostrates a simple usecase of iterators a employees object using iterator pattern.

// Iterator Interface
interface Iterator {
    boolean hasNext();
    Object next();
}

// Aggregate Interface 
interface Aggregate {
    Iterator createIterator();
}

// Employee Class
class Employee {
    public String Name;
    public int Age;
    public String Department;
    public int EmployeeId;

    public Employee(String name, int age, String department, int employeeId) {
        this.Name = name;
        this.Age = age;
        this.Department = department;
        this.EmployeeId = employeeId;
    }
}

// Concrete Aggregate
class EmployeeCollection implements Aggregate {
    private Employee[] employees;

    public EmployeeCollection(Employee[] employees) {
        this.employees = employees;
    }

    @Override
    public Iterator createIterator() {
        return new EmployeeIterator(this.employees);
    }
}

// Concrete Iterator
class EmployeeIterator implements Iterator {
    private Employee[] employees;
    private int position = 0;

    public EmployeeIterator(Employee[] employees) {
        this.employees = employees;
    }

    @Override
    public boolean hasNext() {
        return position < employees.length;
    }

    @Override
    public Object next() {
        return hasNext() ? employees[position++].Name : null;
    }
}

// Client Code
public class Main {
    public static void main(String[] args) {
        // Creating employee array
        Employee[] employees = {
            new Employee("John", 28, "Engineering", 101),
            new Employee("Jane", 32, "Marketing", 102),
            new Employee("Tom", 25, "Sales", 103)
        };

        // Creating employee collection and iterator
        EmployeeCollection employeeCollection = new EmployeeCollection(employees);
        Iterator iterator = employeeCollection.createIterator();

        // Iterating through employees
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

• Strategy pattern: In this pattern we define a family of algorithms, and at the runtime we choose the algorithm.Instead of implementing a single algorithm directly, the code receives runtime instructions on which algorithm to use from a family of algorithms. This pattern allows the algorithm to vary independently from the clients that use it.

Key essentials of this pattern are:

1.Strategy Interface: Defines the common interface for all supported algorithms.
2.Concrete Strategies: Implement the Strategy interface with specific algorithms.
3.Context: Uses a Strategy to execute the algorithm.

Example of strategy pattern:
Imagine we are building an encoding system where we may need to use different encoding algorithms depending on the situation. We will demonstrate this system using the Strategy Pattern.

// Strategy Interface
interface EncoderStrategy {
    void encode(String string);
}

// Concrete Strategy for Base64 Encoding
class Base64Encoder implements EncoderStrategy {
    @Override
    public void encode(String string) {
        // Implement Base64 encoding logic here
        System.out.println("This method uses Base64 encoding algorithm for: " + string);
    }
}

// Concrete Strategy for MD5 Encoding
class MD5Encoder implements EncoderStrategy {
    @Override
    public void encode(String string) {
        // Implement MD5 encoding logic here
        System.out.println("This method uses MD5 encoding algorithm for: " + string);
    }
}

// Context Class
class EncoderContext {
    private EncoderStrategy strategy;

    public void setEncoderMethod(EncoderStrategy strategy) {
        this.strategy = strategy;
    }

    public void encode(String string) {
        strategy.encode(string);
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        EncoderContext context = new EncoderContext();

        // Use Base64 encoding method
        context.setEncoderMethod(new Base64Encoder());
        context.encode("A34937ifdsuhfweiur");

        // Use MD5 encoding method
        context.setEncoderMethod(new MD5Encoder());
        context.encode("89743297dfhksdhWOJO");
    }
}

Explanation:

1. Firstly, we define the interface for the
2. Next, we create concrete implementations of the interfaces that we have defined.
3. Finally, we use these implementations to observe how a change in the subject also updates its dependents.

• Observer pattern: behavioral design pattern that establishes a one-to-many dependency between objects. This means that when one object (the subject) changes its state, all its dependent objects (observers) are notified and updated automatically. This pattern is particularly useful for implementing distributed event-handling systems in event-driven software.

Key essentials of this pattern are:

1. Subject: It is an object which holds the state and informs the observers when it updates it's state.
2. Observer: An interface or abstract class that defines the update method, which is called when the subject’s state changes.
3. Concrete Subject: A class that implements the Subject interface and maintains the state of interest to observers.
4. Concrete Observer: A class that implements the Observer interface and updates its state to match the subject’s state.

Example of observer pattern:
In a stock trading application, the stock ticker acts as the subject. Whenever the price of a stock is updated, various observers—such as investors and regulatory bodies—are notified of the change. This allows them to respond to price fluctuations in real-time.

import java.util.ArrayList;
import java.util.List;

// Observer interface
interface Observer {
    void update(String stockSymbol, double stockPrice);
}

// Subject interface
interface Subject {
    void register(Observer o);
    void remove(Observer o);
    void notify();
}

// Concrete Subject
class Stock implements Subject {
    private List<Observer> observers;
    private String stockSymbol;
    private double stockPrice;

    public Stock() {
        observers = new ArrayList<>();
    }

    public void setStock(String stockSymbol, double stockPrice) {
        this.stockSymbol = stockSymbol;
        this.stockPrice = stockPrice;
        notify();
    }

    @Override
    public void register(Observer o) {
        observers.add(o);
    }

    @Override
    public void remove(Observer o) {
        observers.remove(o);
    }

    @Override
    public void notify() {
        for (Observer observer : observers) {
            observer.update(stockSymbol, stockPrice);
        }
    }
}

// Concrete Observer
class StockTrader implements Observer {
    private String traderName;

    public StockTrader(String traderName) {
        this.traderName = traderName;
    }

    @Override
    public void update(String stockSymbol, double stockPrice) {
        System.out.println("Trader " + traderName + " notified. Stock: " + stockSymbol + " is now $" + stockPrice);
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        Stock stock = new Stock();

        StockTrader trader1 = new StockTrader("Niharika");
        StockTrader trader2 = new StockTrader("Goulikar");

        stock.register(trader1);
        stock.register(trader2);

        stock.setStock("Niha", 9500.00);
        stock.setStock("Rika", 2800.00);
    }
}

Explanation:

• Firstly, we define the interface for the subject which is responsible for sending updates. Similarly, we also define an interface for the observer, which is responsible for receiving updates.
• Next, we create concrete implementations of the interfaces that we have defined.
• Finally, we use these implementations to observe how a change in the subject also updates its dependents.

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