docs: adding subject and main

subjects/java/checkpoints/date-formatter/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        DateFormatter df = new DateFormatter(1656374400, "DD/MM/YYYY");
+        System.out.println(df.getFormattedDate());
+
+        df.setFormat("dd.MM.yyyy");
+        System.out.println(df.getFormattedDate());
+
+        df.setDate(1672531200);
+        System.out.println(df.getFormattedDate());
+
+        df.setFormat("DD month yyyy");
+        System.out.println(df.getFormattedDate());
+    }
+}

subjects/java/checkpoints/date-formatter/README.md

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+## Date Formatter
+
+### Instructions
+
+Write a class `DateFormatter` that has three private attributes: 
+* `date`: the date to be formatted (as a date).
+* `format`: the format to convert the date to.
+* `formattedDate`: contains the date converted into the given format.
+
+Create getters for all the attributes and setters for `date` and `format`. The conversion should happen when you set the `format` or the `date`. If the `format` is not correct, the conversion shouldn't happen. Additionally, create methods to output the date in the specified `format`, and constructors with only the `date`, with `date` and `format`, and a default constructor.
+
+#### The accepted date formats are:
+- `DD/MM/YYYY`
+- `DD Month YYYY`
+- `DD.MM.YYYY`
+
+
+#### The accepted date formats are:
+- `DD/MM/YYYY`
+- `DD Month YYYY`
+- `DD.MM.YYYY`
+
+
+
+### Expected Functions
+
+```java
+import java.text.SimpleDateFormat;
+import java.util.Date;
+
+public class DateFormatter {
+    private long timestamp;
+    private String formattedDate;
+    private String format;
+
+    ...
+}
+```
+
+### Usage
+Here is a possible ExerciseRunner.java to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        DateFormatter df = new DateFormatter(1656374400, "DD/MM/YYYY");
+        System.out.println(df.getFormattedDate());
+
+        df.setFormat("dd.MM.yyyy");
+        System.out.println(df.getFormattedDate());
+
+        df.setDate(1672531200);
+        System.out.println(df.getFormattedDate());
+
+        df.setFormat("DD month yyyy");
+        System.out.println(df.getFormattedDate());
+    }
+}
+```
+
+### Expected Output
+```shell
+Copy code
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+28/06/2022
+28.06.2022
+31.12.2022
+31 December 2022
+$
+```