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：
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public interface PaymentStrategy {
void pay( double amount);
}
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接下来，我们定义具体的策略类来实现不同的支付方式：
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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" );
}
}
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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：
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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);
}
}
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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测试数据" , "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测试数据" , "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);
}
}
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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" );
}
}
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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：
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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
}
}
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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：
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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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总结
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原文链接：https://blog.csdn.net/qq_38238956/article/details/129563541
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