docs: adding subject and main

subjects/java/checkpoints/spiral-matrix/ExerciseRunner.java

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+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        SpiralMatrix spiralMatrix = new SpiralMatrix();
+
+        // Test case 1
+        int n1 = 3;
+        int[][] matrix1 = spiralMatrix.generateMatrix(n1);
+        System.out.println("Spiral matrix for n = " + n1 + ":");
+        for (int[] row : matrix1) {
+            System.out.println(Arrays.toString(row));
+        }
+
+        // Test case 2
+        int n2 = 4;
+        int[][] matrix2 = spiralMatrix.generateMatrix(n2);
+        System.out.println("Spiral matrix for n = " + n2 + ":");
+        for (int[] row : matrix2) {
+            System.out.println(Arrays.toString(row));
+        }
+    }
+}

subjects/java/checkpoints/spiral-matrix/README.md

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+## Spiral Matrix
+
+### Instructions
+
+Create a class `SpiralMatrix` that provides a method to generate a matrix filled with elements from 1 to n\*n in spiral order, given an integer n.
+
+### Expected Class
+
+```java
+public class SpiralMatrix {
+    public int[][] generateMatrix(int n) {
+        // Implementation to generate a spiral matrix
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        SpiralMatrix spiralMatrix = new SpiralMatrix();
+
+        // Test case 1
+        int n1 = 3;
+        int[][] matrix1 = spiralMatrix.generateMatrix(n1);
+        System.out.println("Spiral matrix for n = " + n1 + ":");
+        for (int[] row : matrix1) {
+            System.out.println(Arrays.toString(row));
+        }
+
+        // Test case 2
+        int n2 = 4;
+        int[][] matrix2 = spiralMatrix.generateMatrix(n2);
+        System.out.println("Spiral matrix for n = " + n2 + ":");
+        for (int[] row : matrix2) {
+            System.out.println(Arrays.toString(row));
+        }
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Spiral matrix for n = 3:
+[1, 2, 3]
+[8, 9, 4]
+[7, 6, 5]
+
+Spiral matrix for n = 4:
+[1, 2, 3, 4]
+[12, 13, 14, 5]
+[11, 16, 15, 6]
+[10, 9, 8, 7]
+$
+```