docs: adding subject and main

subjects/java/checkpoints/config-protector/ExerciseRunner.java

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+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        ConfigProtector protector = new ConfigProtector();
+
+        // Test case 1
+        String configFile1 = "username=admin\npassword=secret\nhost=localhost\n";
+        List<String> sensitiveKeys1 = Arrays.asList("password");
+        System.out.println("Protected Config 1:\n" + protector.hideSensitiveData(configFile1, sensitiveKeys1));
+
+        // Test case 2
+        String configFile2 = "apiKey=12345\napiSecret=abcdef\nendpoint=https://api.example.com\n";
+        List<String> sensitiveKeys2 = Arrays.asList("apiKey", "apiSecret");
+        System.out.println("Protected Config 2:\n" + protector.hideSensitiveData(configFile2, sensitiveKeys2));
+
+        // Test case 3
+        String configFile3 = "username=user\npassword=pass\n";
+        List<String> sensitiveKeys3 = Arrays.asList("username", "password");
+        System.out.println("Protected Config 3:\n" + protector.hideSensitiveData(configFile3, sensitiveKeys3));
+    }
+}

subjects/java/checkpoints/config-protector/README.md

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+## Config Protector
+
+### Instructions
+
+Create a class `ConfigProtector` that provides a method to hide sensitive data in a configuration file using regex. The method should replace sensitive values with asterisks. The configuration file will be provided as a string, and the keys for the sensitive data will be given in a list.
+
+### Expected Class
+
+```java
+import java.util.List;
+
+public class ConfigProtector {
+    public String hideSensitiveData(String configFile, List<String> sensitiveKeys) {
+        // Implementation to hide sensitive data in the configuration file using regex
+    }
+}
+```
+
+### 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) {
+        ConfigProtector protector = new ConfigProtector();
+
+        // Test case 1
+        String configFile1 = "username=admin\npassword=secret\nhost=localhost\n";
+        List<String> sensitiveKeys1 = Arrays.asList("password");
+        System.out.println("Protected Config 1:\n" + protector.hideSensitiveData(configFile1, sensitiveKeys1));
+
+        // Test case 2
+        String configFile2 = "apiKey=12345\napiSecret=abcdef\nendpoint=https://api.example.com\n";
+        List<String> sensitiveKeys2 = Arrays.asList("apiKey", "apiSecret");
+        System.out.println("Protected Config 2:\n" + protector.hideSensitiveData(configFile2, sensitiveKeys2));
+
+        // Test case 3
+        String configFile3 = "username=user\npassword=pass\n";
+        List<String> sensitiveKeys3 = Arrays.asList("username", "password");
+        System.out.println("Protected Config 3:\n" + protector.hideSensitiveData(configFile3, sensitiveKeys3));
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Protected Config 1:
+username=admin
+password=******
+host=localhost
+
+Protected Config 2:
+apiKey=*****
+apiSecret=******
+endpoint=https://api.example.com
+
+Protected Config 3:
+username=****
+password=****
+$
+```