docs: adding subject and main

subjects/java/checkpoints/age-finder/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        AgeFinder AgeFinder = new AgeFinder();
+
+        // Test case 1
+        String date1 = "2000-01-01";
+        System.out.println("Age: " + AgeFinder.calculateAge(date1));
+
+        // Test case 2
+        String date2 = "1990-06-15";
+        System.out.println("Age: " + AgeFinder.calculateAge(date2));
+
+        // Test case 3
+        String date3 = "2010-12-25";
+        System.out.println("Age: " + AgeFinder.calculateAge(date3));
+    }
+}

subjects/java/checkpoints/age-finder/README.md

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+## Age Finder
+
+### Instructions
+
+Create a class `AgeFinder` that provides a method to calculate the age from a given date. The date will be provided in the format `yyyy-MM-dd`.
+
+In case of any error the method `calculateAge` should return `-1`
+
+### Expected Class
+
+```java
+import java.time.LocalDate;
+import java.time.Period;
+import java.time.format.DateTimeFormatter;
+
+public class AgeFinder {
+    public int calculateAge(String date) {
+        // Implementation to calculate the age from the given date
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        AgeFinder AgeFinder = new AgeFinder();
+
+        // Test case 1
+        String date1 = "2000-01-01";
+        System.out.println("Age: " + AgeFinder.calculateAge(date1));
+
+        // Test case 2
+        String date2 = "1990-06-15";
+        System.out.println("Age: " + AgeFinder.calculateAge(date2));
+
+        // Test case 3
+        String date3 = "2010-12-25";
+        System.out.println("Age: " + AgeFinder.calculateAge(date3));
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Age: 24
+Age: 34
+Age: 13
+$
+```