docs: adding subject and main

subjects/java/checkpoints/area-calculator/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        AreaCalculator calculator = new AreaCalculator();
+
+        // Test calculate area of a circle
+        double circleArea = calculator.calculate(5.0);
+        System.out.println("Area of circle: " + circleArea);
+
+        // Test calculate area of a rectangle
+        double rectangleArea = calculator.calculate(4.0, 6.0);
+        System.out.println("Area of rectangle: " + rectangleArea);
+
+        // Test calculate area of a triangle
+        double triangleArea = calculator.calculate(4.0, 6.0, true);
+        System.out.println("Area of triangle: " + triangleArea);
+
+        // Test calculate area of a square
+        double squareArea = calculator.calculate(4.0, true);
+        System.out.println("Area of square: " + squareArea);
+    }
+}

subjects/java/checkpoints/area-calculator/README.md

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+## Area Calculator
+
+### Instructions
+
+Create a class `AreaCalculator` that has an overloaded method `calculate` to calculate the area of different shapes. The method should be able to calculate the area for:
+
+- A circle (given the radius)
+- A rectangle (given the width and height)
+- A triangle (given the base and height)
+- A square (given the side length)
+
+#### Formulas for area calculations:
+
+- Circle: Area = 𝜋 × radius²
+
+- Rectangle: Area = width × height
+
+- Triangle: Area = ½ × base × height
+
+- Square: Area = side²
+
+### Expected Class
+
+```java
+public class AreaCalculator {
+    public double calculate(double radius) {
+        // calculate area of circle
+    }
+
+    public double calculate(double width, double height) {
+        // calculate area of rectangle
+    }
+
+    public double calculate(double base, double height, boolean isTriangle) {
+        // calculate area of triangle
+    }
+
+    public double calculate(double side, boolean isSquare) {
+        // calculate area of square
+    }
+}
+```
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        AreaCalculator calculator = new AreaCalculator();
+
+        // Test calculate area of a circle
+        double circleArea = calculator.calculate(5.0);
+        System.out.println("Area of circle: " + circleArea);
+
+        // Test calculate area of a rectangle
+        double rectangleArea = calculator.calculate(4.0, 6.0);
+        System.out.println("Area of rectangle: " + rectangleArea);
+
+        // Test calculate area of a triangle
+        double triangleArea = calculator.calculate(4.0, 6.0, true);
+        System.out.println("Area of triangle: " + triangleArea);
+
+        // Test calculate area of a square
+        double squareArea = calculator.calculate(4.0, true);
+        System.out.println("Area of square: " + squareArea);
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Area of circle: 78.54
+Area of rectangle: 24.0
+Area of triangle: 12.0
+Area of square: 16.0
+$
+```