test: adding subject and main

subjects/java/checkpoints/armstrong-number/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        ArmstrongNumber armstrongNumber = new ArmstrongNumber();
+
+        // Test case 1
+        int number1 = 153;
+        System.out.println("Is " + number1 + " an Armstrong number? " + armstrongNumber.isArmstrong(number1)); // Expected output: true
+
+        // Test case 2
+        int number2 = 123;
+        System.out.println("Is " + number2 + " an Armstrong number? " + armstrongNumber.isArmstrong(number2)); // Expected output: false
+
+        // Test case 3
+        int number3 = 9474;
+        System.out.println("Is " + number3 + " an Armstrong number? " + armstrongNumber.isArmstrong(number3)); // Expected output: true
+    }
+}

subjects/java/checkpoints/armstrong-number/README.md

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+## Armstrong Number
+
+### Instructions
+
+Write a program to check if a given number is an Armstrong number. An Armstrong number of n digits is an integer such that the sum of its digits raised to the power n is equal to the number itself.
+
+Example: 153 is an Armstrong number bacause `1^3 + 5^3 + 3^3 = 153`
+
+### Expected Class
+
+```java
+public class ArmstrongNumber {
+    public boolean isArmstrong(int number) {
+        // Implementation to check if the given number is an Armstrong number
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        ArmstrongNumber armstrongNumber = new ArmstrongNumber();
+
+        // Test case 1
+        int number1 = 153;
+        System.out.println("Is " + number1 + " an Armstrong number? " + armstrongNumber.isArmstrong(number1)); // Expected output: true
+
+        // Test case 2
+        int number2 = 123;
+        System.out.println("Is " + number2 + " an Armstrong number? " + armstrongNumber.isArmstrong(number2)); // Expected output: false
+
+        // Test case 3
+        int number3 = 9474;
+        System.out.println("Is " + number3 + " an Armstrong number? " + armstrongNumber.isArmstrong(number3)); // Expected output: true
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Is 153 an Armstrong number? true
+Is 123 an Armstrong number? false
+Is 9474 an Armstrong number? true
+$
+```