diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..706fd07
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.idea
+.vscode
diff --git a/02_built_in_data_types.dart b/02_built_in_data_types.dart
index 5f1e485..5b76da2 100644
--- a/02_built_in_data_types.dart
+++ b/02_built_in_data_types.dart
@@ -19,9 +19,19 @@ void main(List<String> arguments) {
 	bool isValid = true;
 	var isAlive = false;
 
+	// Nullable types: dataType? varName = null;
+	int? number = null;
+
 	print(score);
 	print(exponents);
 
-	// NOTE: All data types in Dart are Objects.
-	// Therefore, their initial value is by default 'null'
+	/*
+	NOTE: by default, the initial value of a data type
+	cannot be null. unless explicitly specified with the ? symbol.
+	example:
+		int number = null; // Throws the error bellow
+		// Error: The value 'null' can't be assigned to a variable of type
+					'int' because 'int' is not nullable.
+	*/
+
 }
diff --git a/06_conditional_expressions.dart b/06_conditional_expressions.dart
index c0f3751..8c9a885 100644
--- a/06_conditional_expressions.dart
+++ b/06_conditional_expressions.dart
@@ -19,7 +19,7 @@ void main() {
 	// If expr1 is non-null, returns its value; otherwise, evaluates and
 	// returns the value of expr2.
 
-	String name = null;
+	String? name = null;
 
 	String nameToPrint = name ?? "Guest User";
 	print(nameToPrint);
diff --git a/15_optional_positional_params.dart b/15_optional_positional_params.dart
index 74d81b8..7c320f3 100644
--- a/15_optional_positional_params.dart
+++ b/15_optional_positional_params.dart
@@ -20,7 +20,7 @@ void printCities(String name1, String name2, String name3) {
 }
 
 // Optional Positional Parameters
-void printCountries(String name1, [String name2, String name3]) {
+void printCountries(String name1, [String? name2, String? name3]) {
 
 	print("Name 1 is $name1");
 	print("Name 2 is $name2");
diff --git a/16_named_parameters.dart b/16_named_parameters.dart
index 24dd43e..f97681e 100644
--- a/16_named_parameters.dart
+++ b/16_named_parameters.dart
@@ -6,14 +6,18 @@ void main() {
 	print("");
 
 	findVolume(10, height: 20, breadth: 5);     // Sequence doesn't matter in Named Parameter
-}
 
+	print("");
+
+	findVolume(10);
+}
 
-int findVolume(int length, {int breadth, int height}) {
 
+void findVolume(int length, {int? breadth, int? height}) {
 	print("Length is $length");
 	print("Breadth is $breadth");
 	print("Height is $height");
 
-	print("Volume is ${length * breadth * height}");
+	// the ! symbol prevents app crashes when a variable is null
+	print("Volume is ${length * breadth! * height!}");
 }
diff --git a/17_default_parameters.dart b/17_default_parameters.dart
index 122f30c..cead4fc 100644
--- a/17_default_parameters.dart
+++ b/17_default_parameters.dart
@@ -13,7 +13,7 @@ void main() {
 }
 
 
-int findVolume(int length, {int breadth = 2, int height = 20}) {
+void findVolume(int length, {int breadth = 2, int height = 20}) {
 
 	print("Lenght is $length");
 	print("Breadth is $breadth");
diff --git a/18_exception_handling.dart b/18_exception_handling.dart
index 6f3b486..c6fae2c 100644
--- a/18_exception_handling.dart
+++ b/18_exception_handling.dart
@@ -13,7 +13,7 @@ void main() {
 	try {
 		int result = 12 ~/ 0;
 		print("The result is $result");
-	} on IntegerDivisionByZeroException {
+	} on UnsupportedError {
 		print("Cannot divide by Zero");
 	}
 
@@ -52,7 +52,7 @@ void main() {
 	try {
 		depositMoney(-200);
 	} catch (e) {
-		print(e.errorMessage());
+		print(e);
 	} finally {
 		// Code
 	}
diff --git a/19_class_and_objects.dart b/19_class_and_objects.dart
index 086ccab..f4ab833 100644
--- a/19_class_and_objects.dart
+++ b/19_class_and_objects.dart
@@ -19,7 +19,7 @@ void main() {
 // Define states (properties) and behavior of a Student
 class Student {
 	int id = -1; 			// Instance or Field Variable, default value is -1
-	String name;  		// Instance or Field Variable, default value is null
+	String ?name;  		// Instance or Field Variable, default value is null
 
 	void study() {
 		print("${this.name} is now studying");
diff --git a/20_constructors.dart b/20_constructors.dart
index d39ac4a..69a14d6 100644
--- a/20_constructors.dart
+++ b/20_constructors.dart
@@ -33,7 +33,7 @@ void main() {
 // Define states (properties) and behavior of a Student
 class Student {
 	int id = -1;
-	String name;
+	String? name;
 
 	Student(this.id, this.name);    // Parameterised Constructor
 
diff --git a/21_getters_setters.dart b/21_getters_setters.dart
index 1d28de5..4e3f7f0 100644
--- a/21_getters_setters.dart
+++ b/21_getters_setters.dart
@@ -16,9 +16,9 @@ void main() {
 
 class Student {
 
-  String name;        // Instance Variable with default Getter and Setter
+  String? name;        // Instance Variable with default Getter and Setter
 
-  double _percent;    // Private Instance Variable for its own library
+  double _percent = 0;    // Private Instance Variable for its own library
 
   // Instance variable with Custom Setter
   void set percentage(double marksSecured) => _percent = (marksSecured / 500) * 100;
diff --git a/22_inheritance.dart b/22_inheritance.dart
index d1ed8f0..3ad2891 100644
--- a/22_inheritance.dart
+++ b/22_inheritance.dart
@@ -23,7 +23,7 @@ void main() {
 
 class Animal {
 
-	String color;
+	String? color;
 
 	void eat() {
 		print("Eat !");
@@ -32,7 +32,7 @@ class Animal {
 
 class Dog extends Animal {      // Dog is Child class or sub class, Animal is super or parent class
 
-	String breed;
+	String? breed;
 
 	void bark() {
 		print("Bark !");
@@ -41,7 +41,7 @@ class Dog extends Animal {      // Dog is Child class or sub class, Animal is su
 
 class Cat extends Animal {      // Cat is Child class or sub class, Animal is super or parent class
 
-	int age;
+	int? age;
 
 	void meow() {
 		print("Meow !");
diff --git a/23_method_overriding.dart b/23_method_overriding.dart
index 99011d6..b335793 100644
--- a/23_method_overriding.dart
+++ b/23_method_overriding.dart
@@ -21,7 +21,7 @@ class Animal {
 
 class Dog extends Animal {
 
-	String breed;
+	String? breed;
 
 	String color = "Black";     // Property Overriding
 
diff --git a/24_inheritance_with_constructors.dart b/24_inheritance_with_constructors.dart
index deae98a..543d1ae 100644
--- a/24_inheritance_with_constructors.dart
+++ b/24_inheritance_with_constructors.dart
@@ -18,7 +18,7 @@ void main() {
 
 class Animal {
 
-	String color;
+	String? color;
 
 	Animal(String color) {
 		this.color = color;
@@ -32,7 +32,7 @@ class Animal {
 
 class Dog extends Animal {
 
-	String breed;
+	String? breed;
 
 	Dog(String breed, String color) : super(color) {
 		this.breed = breed;
diff --git a/25_abstract_class_method.dart b/25_abstract_class_method.dart
index 181a2f3..a335795 100644
--- a/25_abstract_class_method.dart
+++ b/25_abstract_class_method.dart
@@ -17,8 +17,8 @@ void main() {
 abstract class Shape {
 
 	// Define your Instance variable if needed
-	int x;
-	int y;
+	int? x;
+	int? y;
 
 	void draw();        // Abstract Method
 
diff --git a/27_static_method_variable.dart b/27_static_method_variable.dart
index 0d3f2ce..e52bf46 100644
--- a/27_static_method_variable.dart
+++ b/27_static_method_variable.dart
@@ -28,7 +28,7 @@ class Circle {
 	static const double pi = 3.14;
 	static int maxRadius = 5;
 
-	String color;
+	String? color;
 
 	static void calculateArea() {
 		print("Some code to calculate area of Circle");