docs: adding subject and main

subjects/java/checkpoints/day-of-week/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        DayOfWeekFinder finder = new DayOfWeek();
+
+        // Test case 1
+        String startDate1 = "2023-06-22";
+        String dayOfWeek1 = "Monday";
+        System.out.println("Next " + dayOfWeek1 + " after " + startDate1 + ": " + finder.findNextDayOfWeek(startDate1, dayOfWeek1)); // Expected output: 2023-06-26
+
+        // Test case 2
+        String startDate2 = "2023-06-22";
+        String dayOfWeek2 = "Friday";
+        System.out.println("Next " + dayOfWeek2 + " after " + startDate2 + ": " + finder.findNextDayOfWeek(startDate2, dayOfWeek2)); // Expected output: 2023-06-23
+
+        // Test case 3
+        String startDate3 = "2023-06-22";
+        String dayOfWeek3 = "Sunday";
+        System.out.println("Next " + dayOfWeek3 + " after " + startDate3 + ": " + finder.findNextDayOfWeek(startDate3, dayOfWeek3)); // Expected output: 2023-06-25
+
+        // Error case: invalid date format
+        String startDate4 = "invalid-date";
+        String dayOfWeek4 = "Monday";
+        System.out.println("Next " + dayOfWeek4 + " after " + startDate4 + ": " + finder.findNextDayOfWeek(startDate4, dayOfWeek4)); // Expected output: Error
+
+        // Error case: invalid day of the week
+        String startDate5 = "2023-06-22";
+        String dayOfWeek5 = "Funday";
+        System.out.println("Next " + dayOfWeek5 + " after " + startDate5 + ": " + finder.findNextDayOfWeek(startDate5, dayOfWeek5)); // Expected output: Error
+    }
+}

subjects/java/checkpoints/day-of-week/README.md

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+## Day Of Week
+
+### Instructions
+
+Create a class `DayOfWeekFinder` that provides a method to find the date of the next specified day of the week from a given date. The date will be provided in the format `yyyy-MM-dd`, and the day of the week will be provided as a string (e.g., "Monday", "Tuesday").
+
+### Expected Class
+
+```java
+import java.time.DayOfWeek;
+import java.time.LocalDate;
+import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeParseException;
+
+public class DayOfWeekFinder {
+    public String findNextDayOfWeek(String startDate, String dayOfWeek) {
+        ...
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        DayOfWeekFinder finder = new DayOfWeekFinder();
+
+        // Test case 1
+        String startDate1 = "2023-06-22";
+        String dayOfWeek1 = "Monday";
+        System.out.println("Next " + dayOfWeek1 + " after " + startDate1 + ": " + finder.findNextDayOfWeek(startDate1, dayOfWeek1));
+
+        // Test case 2
+        String startDate2 = "2023-06-22";
+        String dayOfWeek2 = "Friday";
+        System.out.println("Next " + dayOfWeek2 + " after " + startDate2 + ": " + finder.findNextDayOfWeek(startDate2, dayOfWeek2));
+
+        // Test case 3
+        String startDate3 = "2023-06-22";
+        String dayOfWeek3 = "Sunday";
+        System.out.println("Next " + dayOfWeek3 + " after " + startDate3 + ": " + finder.findNextDayOfWeek(startDate3, dayOfWeek3));
+
+        // Error case: invalid date format
+        String startDate4 = "invalid-date";
+        String dayOfWeek4 = "Monday";
+        System.out.println("Next " + dayOfWeek4 + " after " + startDate4 + ": " + finder.findNextDayOfWeek(startDate4, dayOfWeek4));
+
+        // Error case: invalid day of the week
+        String startDate5 = "2023-06-22";
+        String dayOfWeek5 = "Funday";
+        System.out.println("Next " + dayOfWeek5 + " after " + startDate5 + ": " + finder.findNextDayOfWeek(startDate5, dayOfWeek5));
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Next Monday after 2023-06-22: 2023-06-26
+Next Friday after 2023-06-22: 2023-06-23
+Next Sunday after 2023-06-22: 2023-06-25
+Next Monday after invalid-date: Error
+Next Funday after 2023-06-22: Error
+$
+```