Added support for nullable types

.gitignore

@@ -0,0 +1,2 @@
+.idea
+.vscode

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.
+	*/
+
 }

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);

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");

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!}");
 }

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");

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
 	}

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");

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
 

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;

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 !");

23_method_overriding.dart

@@ -21,7 +21,7 @@ class Animal {
 
 class Dog extends Animal {
 
-	String breed;
+	String? breed;
 
 	String color = "Black";     // Property Overriding
 

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;

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
 

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");