docs: adding subject and main

subjects/java/checkpoints/flexisort/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        int[] array = {64, 34, 25, 12, 22, 11, 90};
+
+        // Test BubbleSort
+        Sorter bubbleSorter = new BubbleSort();
+        bubbleSorter.setArray(array.clone());
+        bubbleSorter.sort();
+        System.out.println("BubbleSorted array: " + Arrays.toString(bubbleSorter.getArray()));
+
+        // Test InsertionSort
+        Sorter insertionSorter = new InsertionSort();
+        insertionSorter.setArray(array.clone());
+        insertionSorter.sort();
+        System.out.println("InsertionSorted array: " + Arrays.toString(insertionSorter.getArray()));
+    }
+}

subjects/java/checkpoints/flexisort/README.md

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+## Flexisort
+
+### Instructions
+
+Create an abstract class `Sorter` that will be used as a base class for sorting algorithms. This class should have an abstract method `sort` that will be implemented by its child classes. Additionally, it should have methods to set and get the array to sort.
+
+Implement two child classes:
+
+- `BubbleSort` which implements the bubble sort algorithm.
+- `InsertionSort` which implements the insertion sort algorithm.
+
+### Expected Classes
+
+```java
+public abstract class Sorter {
+    private int[] array;
+
+    public int[] getArray() {
+        // getter method
+    }
+
+    public void setArray(int[] array) {
+        // setter method
+    }
+
+    public abstract void sort();
+}
+
+public class BubbleSort extends Sorter {
+    @Override
+    public void sort() {
+        // bubble sort algorithm implementation
+    }
+}
+
+public class InsertionSort extends Sorter {
+    @Override
+    public void sort() {
+        // insertion sort algorithm implementation
+    }
+}
+```
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your classes:
+
+```java
+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        int[] array = {64, 34, 25, 12, 22, 11, 90};
+
+        // Test BubbleSort
+        Sorter bubbleSorter = new BubbleSort();
+        bubbleSorter.setArray(array.clone());
+        bubbleSorter.sort();
+        System.out.println("BubbleSorted array: " + Arrays.toString(bubbleSorter.getArray()));
+
+        // Test InsertionSort
+        Sorter insertionSorter = new InsertionSort();
+        insertionSorter.setArray(array.clone());
+        insertionSorter.sort();
+        System.out.println("InsertionSorted array: " + Arrays.toString(insertionSorter.getArray()));
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+BubbleSorted array: [11, 12, 22, 25, 34, 64, 90]
+InsertionSorted array: [11, 12, 22, 25, 34, 64, 90]
+$
+```