java中如何優化大量if...else...

策略模式（Strategy Pattern）

將每個條件分支的實作作為一個獨立的策略類，然後使用一個上下文物件來選擇要執行的策略。這種方法可以將大量的if else語句轉換為物件之間的交互，從而提高程式碼的可維護性和可擴展性。

範例：

 首先，我們定義一個介面來實作所有策略的行為：

public interface PaymentStrategy {
    void pay(double amount);
}

接下來，我們定義特定的策略類別來實作不同的付款方式： 

public class CreditCardPaymentStrategy implements PaymentStrategy {
    private String name;
    private String cardNumber;
    private String cvv;
    private String dateOfExpiry;
 
    public CreditCardPaymentStrategy(String name, String cardNumber, String cvv, String dateOfExpiry) {
        this.name = name;
        this.cardNumber = cardNumber;
        this.cvv = cvv;
        this.dateOfExpiry = dateOfExpiry;
    }
 
    public void pay(double amount) {
        System.out.println(amount + " paid with credit card");
    }
}
 
public class PayPalPaymentStrategy implements PaymentStrategy {
    private String emailId;
    private String password;
 
    public PayPalPaymentStrategy(String emailId, String password) {
        this.emailId = emailId;
        this.password = password;
    }
 
    public void pay(double amount) {
        System.out.println(amount + " paid using PayPal");
    }
}
 
public class CashPaymentStrategy implements PaymentStrategy {
    public void pay(double amount) {
        System.out.println(amount + " paid in cash");
    }
}

現在，我們可以在客戶端程式碼中建立不同的策略對象，並將它們傳遞給一個統一的支付類別中，這個支付類別會根據傳入的策略物件來呼叫對應的付款方法： 

public class ShoppingCart {
    private List<Item> items;
 
    public ShoppingCart() {
        this.items = new ArrayList<>();
    }
 
    public void addItem(Item item) {
        this.items.add(item);
    }
 
    public void removeItem(Item item) {
        this.items.remove(item);
    }
 
    public double calculateTotal() {
        double sum = 0;
        for (Item item : items) {
            sum += item.getPrice();
        }
        return sum;
    }
 
    public void pay(PaymentStrategy paymentStrategy) {
        double amount = calculateTotal();
        paymentStrategy.pay(amount);
    }
}

現在我們可以使用上述程式碼來建立一個購物車，在其中加入一些商品，然後使用不同的策略來付款： 

public class Main {
    public static void main(String[] args) {
        ShoppingCart cart = new ShoppingCart();
 
        Item item1 = new Item("1234", 10);
        Item item2 = new Item("5678", 40);
 
        cart.addItem(item1);
        cart.addItem(item2);
 
        // pay by credit card
        cart.pay(new CreditCardPaymentStrategy("John Doe", "1234567890123456", "786", "12/22"));
 
        // pay by PayPal
        cart.pay(new PayPalPaymentStrategy("myemail@example.com", "mypassword"));
 
        // pay in cash
        cart.pay(new CashPaymentStrategy());
 
        //--------------------------或者提前将不同的策略对象放入map当中,如下
 
        Map<String, PaymentStrategy> paymentStrategies = new HashMap<>();
 
        paymentStrategies.put("creditcard", new CreditCardPaymentStrategy("John Doe", "1234567890123456", "786", "12/22"));
        paymentStrategies.put("paypal", new PayPalPaymentStrategy("myemail@example.com", "mypassword"));
        paymentStrategies.put("cash", new CashPaymentStrategy());
 
        String paymentMethod = "creditcard"; // 用户选择的支付方式
        PaymentStrategy paymentStrategy = paymentStrategies.get(paymentMethod);
 
        cart.pay(paymentStrategy);
 
    }
}

工廠模式（Factory Pattern）

將每個條件分支的實作作為一個獨立的產品類，然後使用一個工廠類別來建立特定的產品物件。這種方法可以將大量的if else語句轉換為物件的建立過程，從而提高程式碼的可讀性和可維護性。

範例： 

// 定义一个接口
public interface StringProcessor {
    public void processString(String str);
}
 
// 实现接口的具体类
public class LowercaseStringProcessor implements StringProcessor {
    public void processString(String str) {
        System.out.println(str.toLowerCase());
    }
}
 
public class UppercaseStringProcessor implements StringProcessor {
    public void processString(String str) {
        System.out.println(str.toUpperCase());
    }
}
 
public class ReverseStringProcessor implements StringProcessor {
    public void processString(String str) {
        StringBuilder sb = new StringBuilder(str);
        System.out.println(sb.reverse().toString());
    }
}
 
// 工厂类
public class StringProcessorFactory {
    public static StringProcessor createStringProcessor(String type) {
        if (type.equals("lowercase")) {
            return new LowercaseStringProcessor();
        } else if (type.equals("uppercase")) {
            return new UppercaseStringProcessor();
        } else if (type.equals("reverse")) {
            return new ReverseStringProcessor();
        }
        throw new IllegalArgumentException("Invalid type: " + type);
    }
}
 
// 测试代码
public class Main {
    public static void main(String[] args) {
        StringProcessor sp1 = StringProcessorFactory.createStringProcessor("lowercase");
        sp1.processString("Hello World");
        
        StringProcessor sp2 = StringProcessorFactory.createStringProcessor("uppercase");
        sp2.processString("Hello World");
        
        StringProcessor sp3 = StringProcessorFactory.createStringProcessor("reverse");
        sp3.processString("Hello World");
    }
}

 看起來還是有if...else,但這樣的程式碼更簡潔易懂,後期也便於維護....

映射表（Map）

使用一個映射表來將條件分支的實作映射到對應的函數或方法上。這種方法可以減少程式碼中的if else語句，並且可以動態地更新映射表，從而提高程式碼的靈活性和可維護性。

範例：

import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;
 
public class MappingTableExample {
    private Map<String, Function<Integer, Integer>> functionMap;
 
    public MappingTableExample() {
        functionMap = new HashMap<>();
        functionMap.put("add", x -> x + 1);
        functionMap.put("sub", x -> x - 1);
        functionMap.put("mul", x -> x * 2);
        functionMap.put("div", x -> x / 2);
    }
 
    public int calculate(String operation, int input) {
        if (functionMap.containsKey(operation)) {
            return functionMap.get(operation).apply(input);
        } else {
            throw new IllegalArgumentException("Invalid operation: " + operation);
        }
    }
 
    public static void main(String[] args) {
        MappingTableExample example = new MappingTableExample();
        System.out.println(example.calculate("add", 10));
        System.out.println(example.calculate("sub", 10));
        System.out.println(example.calculate("mul", 10));
        System.out.println(example.calculate("div", 10));
        System.out.println(example.calculate("mod", 10)); // 抛出异常
    }
}

資料驅動設計（Data-Driven Design） 

將條件分支的實作和輸入資料一起儲存在一個資料結構中，然後使用一個通用的函數或方法來處理這個資料結構。這種方法可以將大量的if else語句轉換為資料結構的處理過程，從而提高程式碼的可擴展性和可維護性。 

範例：

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
 
public class DataDrivenDesignExample {
    private List<Function<Integer, Integer>> functionList;
 
    public DataDrivenDesignExample() {
        functionList = new ArrayList<>();
        functionList.add(x -> x + 1);
        functionList.add(x -> x - 1);
        functionList.add(x -> x * 2);
        functionList.add(x -> x / 2);
    }
 
    public int calculate(int operationIndex, int input) {
        if (operationIndex < 0 || operationIndex >= functionList.size()) {
            throw new IllegalArgumentException("Invalid operation index: " + operationIndex);
        }
        return functionList.get(operationIndex).apply(input);
    }
 
    public static void main(String[] args) {
        DataDrivenDesignExample example = new DataDrivenDesignExample();
        System.out.println(example.calculate(0, 10));
        System.out.println(example.calculate(1, 10));
        System.out.println(example.calculate(2, 10));
        System.out.println(example.calculate(3, 10));
        System.out.println(example.calculate(4, 10)); // 抛出异常
    }
}

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