java中如何优化大量if...else...

策略模式（Strategy Pattern）

将每个条件分支的实现作为一个独立的策略类，然后使用一个上下文对象来选择要执行的策略。这种方法可以将大量的if else语句转换为对象之间的交互，从而提高代码的可维护性和可扩展性。

示例：

 首先，我们定义一个接口来实现所有策略的行为：

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

接下来，我们定义具体的策略类来实现不同的支付方式： 

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");
    }
}

现在，我们可以在客户端代码中创建不同的策略对象，并将它们传递给一个统一的支付类中，这个支付类会根据传入的策略对象来调用相应的支付方法： 

public class ShoppingCart {
    private List 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);
    }
}

现在我们可以使用上述代码来创建一个购物车，向其中添加一些商品，然后使用不同的策略来支付： 

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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 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);
 
    }
}

工厂模式（Factory Pattern）

将每个条件分支的实现作为一个独立的产品类，然后使用一个工厂类来创建具体的产品对象。这种方法可以将大量的if else语句转换为对象的创建过程，从而提高代码的可读性和可维护性。

示例： 

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// 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");
    }
}

 看起来还是有if...else,但这样的代码更加简洁易懂,后期也便于维护....

映射表（Map）

使用一个映射表来将条件分支的实现映射到对应的函数或方法上。这种方法可以减少代码中的if else语句，并且可以动态地更新映射表，从而提高代码的灵活性和可维护性。 

示例：

import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;
 
public class MappingTableExample {
    private Map> 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
    }
}

数据驱动设计（Data-Driven Design） 

将条件分支的实现和输入数据一起存储在一个数据结构中，然后使用一个通用的函数或方法来处理这个数据结构。这种方法可以将大量的if else语句转换为数据结构的处理过程，从而提高代码的可扩展性和可维护性。 

示例：

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
 
public class DataDrivenDesignExample {
    private List> 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
    }
}