docs: adding subject and main

subjects/java/checkpoints/factorial-master/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        Factorial iterativeFactorial = new IterativeFactorial();
+        Factorial recursiveFactorial = new RecursiveFactorial();
+
+        // Test iterative factorial
+        int number = 5;
+        long iterativeResult = iterativeFactorial.calculate(number);
+        System.out.println("Iterative Factorial of " + number + " is: " + iterativeResult); // Expected output: 120
+
+        // Test recursive factorial
+        long recursiveResult = recursiveFactorial.calculate(number);
+        System.out.println("Recursive Factorial of " + number + " is: " + recursiveResult); // Expected output: 120
+    }
+}

subjects/java/checkpoints/factorial-master/README.md

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+## Factorial Master
+
+### Instructions
+
+Create an abstract class `Factorial` that will serve as the base class for calculating the factorial of a number. This class should have an abstract method `calculate` that will be implemented by its child classes.
+
+Implement two child classes:
+
+- `IterativeFactorial` which implements the factorial calculation using an iterative approach.
+- `RecursiveFactorial` which implements the factorial calculation using a recursive approach.
+
+> Note: The factorial of 0 is 1, according to the [convention](https://www.chilimath.com/lessons/intermediate-algebra/zero-factorial/)
+
+#### Formula for factorial calculation:
+
+n! = n × (n−1) × (n−2) × … × 1
+
+And an example:
+5! = 5 × 4 × 3 × 2 × 1 = 120
+
+### Expected Abstract Class
+
+```java
+public abstract class Factorial {
+    public abstract long calculate(int n);
+}
+
+public class IterativeFactorial extends Factorial {
+    @Override
+    public long calculate(int n) {
+        // iterative factorial calculation
+    }
+}
+
+public class RecursiveFactorial extends Factorial {
+    @Override
+    public long calculate(int n) {
+        // recursive factorial calculation
+    }
+}
+```
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your classes:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        Factorial iterativeFactorial = new IterativeFactorial();
+        Factorial recursiveFactorial = new RecursiveFactorial();
+
+        // Test iterative factorial
+        int number = 5;
+        long iterativeResult = iterativeFactorial.calculate(number);
+        System.out.println("Iterative Factorial of " + number + " is: " + iterativeResult); // Expected output: 120
+
+        // Test recursive factorial
+        long recursiveResult = recursiveFactorial.calculate(number);
+        System.out.println("Recursive Factorial of " + number + " is: " + recursiveResult); // Expected output: 120
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Iterative Factorial of 5 is: 120
+Recursive Factorial of 5 is: 120
+$
+```