docs: adding subject and main

subjects/java/checkpoints/factory-blueprint/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        Factory factory = new Factory();
+
+        // Handle invalid product type
+        Object invalidProduct = factory.createProduct("C");
+        if (invalidProduct != null) {
+            invalidProduct.showDetails();
+        } else {
+            System.out.println("Invalid product type");
+        }
+    }
+}

subjects/java/checkpoints/factory-blueprint/README.md

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+## Factory Blueprint
+
+### Instructions
+
+You are given an incomplete Factory design pattern implementation with some incorrect parts. Complete and fix the class to demonstrate your understanding of how the Factory design pattern works. 
+
+### Expected Classes
+
+```java
+// Product interface
+public interface Product {
+    void showDetails();
+}
+
+// ConcreteProductA class
+public class ConcreteProductA {
+    ...
+}
+
+// ConcreteProductB class
+public class ConcreteProductB {
+    ...
+}
+
+// Factory class
+public class Factory {
+    public ConcreteProductB createProduct(String type) { // the type parametre accept two values `A` and `B`
+
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your classes:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        Factory factory = new Factory();
+
+        // Handle invalid product type
+        Object invalidProduct = factory.createProduct("C");
+        if (invalidProduct != null) {
+            invalidProduct.showDetails();
+        } else {
+            System.out.println("Invalid product type");
+        }
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Invalid product type
+$
+```