Strategy mode_

quote ：https://blog.csdn.net/qq_51165184/article/details/123997917

The strategy pattern Stragery ： For a set of algorithms , Encapsulate each algorithm into a separate class with a common interface , Make them interchangeable

The essence ： Separation algorithm , Selective implementation
Strategic patterns can separate behavior from the environment , The environment class Context Be responsible for inquiring what to do , Various algorithms are in the specific strategy class （ConcreteStrategy） Provided in the

Characteristics of the strategic model

• The specific algorithm is independent from the specific business
• Multiple if-else Consider using a policy pattern
• Policy algorithms are different implementations of the same behavior ( polymorphic )
• Client selection . Context to implement the policy algorithm

Advantages and disadvantages

advantage ：

• Avoid involving the client with important algorithms and data
• Avoid using multiple conditional selection statements that are difficult to maintain
• Easy to expand

shortcoming

• The judgment logic is on the client side , When the demand changes , To change the client program
• The client must know all policy classes , And decide which strategy class to use , This means that the client must understand the differences between these algorithms , In order to timely select the appropriate algorithm class
• Increased the number of objects
• Only suitable for flat algorithm structure

Code :
Discount policy interface ：

public interface DiscountStrategy {
    
    BigDecimal getDiscount(BigDecimal total);
}

Two discount strategies ：

public class OverDiscountStrategy implements DiscountStrategy{
    
    @Override
    public BigDecimal getDiscount(BigDecimal total) {
    
        //  full 100 reduce 20
        return total.compareTo(BigDecimal.valueOf(100)) >= 0?BigDecimal.valueOf(20): BigDecimal.ZERO;
    }
}

public class UserDiscountStrategy implements DiscountStrategy{
    
    @Override
    public BigDecimal getDiscount(BigDecimal total) {
    
        //  10% discount for members 
        return total.multiply(new BigDecimal("0.1")).setScale(2, RoundingMode.HALF_UP);
    }
}

The default policy used by environment settings ,, And you can switch strategies ：

public class DiscountContext {
    
    //  Hold a strategy 
    private DiscountStrategy strategy = new UserDiscountStrategy();

    //  Allow clients to set new policies 
    public void setStrategy(DiscountStrategy discountStrategy){
    
        this.strategy = discountStrategy;
    }

    public BigDecimal calculatePrice(BigDecimal  total){
    
        return total.subtract(strategy.getDiscount(total));
    }
}

test ：

 public static void main(String[] args) {
    
        DiscountContext context = new DiscountContext();
        context.setStrategy(new OverDiscountStrategy());
        BigDecimal result = context.calculatePrice(BigDecimal.valueOf(200));
        System.out.println("result = " + result);
    }