docs: adding subject and main

subjects/java/checkpoints/maximal-square/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        MaximalSquare finder = new MaximalSquare();
+
+        // Test case 1
+        char[][] matrix1 = {
+            {'1', '0', '1', '0', '0'},
+            {'1', '0', '1', '1', '1'},
+            {'1', '1', '1', '1', '1'},
+            {'1', '0', '0', '1', '0'}
+        };
+        System.out.println("Maximal square area: " + finder.maximalSquare(matrix1)); // Expected output: 4
+
+        // Test case 2
+        char[][] matrix2 = {
+            {'0', '1'},
+            {'1', '0'}
+        };
+        System.out.println("Maximal square area: " + finder.maximalSquare(matrix2)); // Expected output: 1
+
+        // Test case 3
+        char[][] matrix3 = {
+            {'0'}
+        };
+        System.out.println("Maximal square area: " + finder.maximalSquare(matrix3)); // Expected output: 0
+    }
+}

subjects/java/checkpoints/maximal-square/README.md

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+## Maximal Square
+
+### Instructions
+
+Create a class `MaximalSquare` that provides a method to find the area of the largest square containing only 1's in a given 2D binary matrix.
+
+### Expected Class
+
+```java
+public class MaximalSquare {
+    public int maximalSquare(char[][] matrix) {
+        // Implementation to find the area of the largest square containing only 1's
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        MaximalSquare finder = new MaximalSquare();
+
+        // Test case 1
+        char[][] matrix1 = {
+            {'1', '0', '1', '0', '0'},
+            {'1', '0', '1', '1', '1'},
+            {'1', '1', '1', '1', '1'},
+            {'1', '0', '0', '1', '0'}
+        };
+        System.out.println("Maximal square area: " + finder.maximalSquare(matrix1)); // Expected output: 4
+
+        // Test case 2
+        char[][] matrix2 = {
+            {'0', '1'},
+            {'1', '0'}
+        };
+        System.out.println("Maximal square area: " + finder.maximalSquare(matrix2)); // Expected output: 1
+
+        // Test case 3
+        char[][] matrix3 = {
+            {'0'}
+        };
+        System.out.println("Maximal square area: " + finder.maximalSquare(matrix3)); // Expected output: 0
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Maximal square area: 4
+Maximal square area: 1
+Maximal square area: 0
+$
+```