add factory

study_go/design_pattern/1_simple_factory/README.md

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+# UML
+
+```mermaid
+classDiagram
+    class Product {
+        <<interface>>
+        +operation()
+    }
+    class ConcreteProductA {
+        +operation()
+    }
+    class ConcreteProductB {
+        +operation()
+    }
+    class SimpleFactory {
+        +createProduct(string) Product
+    }
+    Product <|.. ConcreteProductA
+    Product <|.. ConcreteProductB
+    SimpleFactory ..> Product
+    SimpleFactory ..> ConcreteProductA
+    SimpleFactory ..> ConcreteProductB
+````

study_go/design_pattern/1_simple_factory/main.go

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+package main
+
+import "fmt"
+
+// Product 接口
+type Product interface {
+	Operation() string
+}
+
+// ConcreteProductA 结构体
+type ConcreteProductA struct{}
+
+func (p *ConcreteProductA) Operation() string {
+	return "ConcreteProductA"
+}
+
+// ConcreteProductB 结构体
+type ConcreteProductB struct{}
+
+func (p *ConcreteProductB) Operation() string {
+	return "ConcreteProductB"
+}
+
+// SimpleFactory 结构体
+type SimpleFactory struct{}
+
+func (f *SimpleFactory) CreateProduct(productType string) Product {
+	switch productType {
+	case "A":
+		return &ConcreteProductA{}
+	case "B":
+		return &ConcreteProductB{}
+	default:
+		return nil
+	}
+}
+
+func main() {
+	factory := &SimpleFactory{}
+
+	productA := factory.CreateProduct("A")
+	fmt.Println(productA.Operation())
+
+	productB := factory.CreateProduct("B")
+	fmt.Println(productB.Operation())
+}