Merge remote-tracking branch 'upstream/master' into master

1 files changed, 345 insertions, 154 deletions

diff --git a/dart.html.markdown b/dart.html.markdown
index 07f755f7..b215474a 100644
--- a/dart.html.markdown
+++ b/dart.html.markdown
@@ -2,127 +2,228 @@
 language: dart
 filename: learndart.dart
 contributors:
-    - ["Joao Pedrosa", "https://github.com/jpedrosa/"]
+  - ["Joao Pedrosa", "https://github.com/jpedrosa/"]
+  - ["Vince Ramces Oliveros", "https://github.com/ram231"]
 ---
 
-Dart is a newcomer into the realm of programming languages.
-It borrows a lot from other mainstream languages, having as a goal not to deviate too much from
-its JavaScript sibling. Like JavaScript, Dart aims for great browser integration.
+**Dart** is a single threaded, general purpose programming language.
+It borrows a lot from other mainstream languages.
+It supports Streams, Futures(known as Promises in JavaScript), Generics, First-class functions(closures) and static type checking.
+Dart can run in any platform such as Web, CLI, Desktop, Mobile and IoT devices.
 
-Dart's most controversial feature must be its Optional Typing.
+Dart's most controversial feature is its ~~Optional Typing~~ Static Type safety and [Sound Type checks](https://dart.dev/guides/language/sound-dart).
 
 ```dart
 import "dart:collection";
-import "dart:math" as DM;
-
-// Welcome to Learn Dart in 15 minutes. http://www.dartlang.org/
-// This is an executable tutorial. You can run it with Dart or on
-// the Try Dart! site if you copy/paste it there. http://try.dartlang.org/
-
-// Function declaration and method declaration look the same. Function
-// declarations can be nested. The declaration takes the form of
-// name() {} or name() => singleLineExpression;
-// The fat arrow function declaration has an implicit return for the result of
-// the expression.
+import "dart:math" as math;
+
+/// Welcome to Learn Dart in 15 minutes. http://dart.dev/
+/// This is an executable tutorial. You can run it with Dart or on
+/// the Try Dart! site if you copy/paste it there. http://dartpad.dev/
+/// You can also run Flutter in DartPad by click the `< > New Pad ` and choose Flutter
+
+
+/// In Dart, Everything is an Object.
+/// Every declaration of an object is an instance of Null and
+/// Null is also an object.
+
+
+/// 3 Types of comments in dart
+// Single line comment
+/**
+* Multi-line comment
+* Can comment more than 2 lines
+*/
+/// Code doc comment
+/// It uses markdown syntax to generate code docs when making an API.
+/// Code doc comment is the recommended choice when documenting your APIs, classes and methods.
+
+/// 4 types of variable declaration.
+/// Constants are variables that are immutable cannot be change or altered.
+/// `const` in dart should practice SCREAMING_SNAKE_CASE name declaration.
+const CONSTANT_VALUE = "I CANNOT CHANGE";
+CONSTANT_VALUE = "DID I?"; //Error
+/// Final is another variable declaration that cannot be change once it has been instantiated. Commonly used in classes and functions
+/// `final` can be declared in pascalCase.
+final finalValue = "value cannot be change once instantiated";
+finalValue = "Seems not"; //Error
+
+/// `var` is another variable declaration that is mutable and can change its value. Dart will infer types and will not change its data type
+var mutableValue = "Variable string";
+mutableValue = "this is valid";
+mutableValue = false; // Error.
+
+/// `dynamic` is another variable declaration in which the type is not evaluated by the dart static type checking.
+/// It can change its value and data type.
+/// Some dartisans uses dynamic cautiously as it cannot keep track of its data type. so use it at your own risk
+dynamic dynamicValue = "I'm a string";
+dynamicValue = false; // false
+
+
+/// Functions can be declared in a global space
+/// Function declaration and method declaration look the same. Function
+/// declarations can be nested. The declaration takes the form of
+/// name() {} or name() => singleLineExpression;
+/// The fat arrow function declaration can be an implicit or
+/// explicit return for the result of the expression.
+/// Dart will execute a function called `main()` anywhere in the dart project.
+///
 example1() {
   nested1() {
     nested2() => print("Example1 nested 1 nested 2");
     nested2();
   }
+
   nested1();
 }
 
-// Anonymous functions don't include a name.
+/// Anonymous functions don't include a name but can take number of arguments
 example2() {
-  nested1(fn) {
+  //// Explicit return type.
+  nested1(Function<void> fn) {
     fn();
   }
+
   nested1(() => print("Example2 nested 1"));
 }
 
-// When a function parameter is declared, the declaration can include the
-// number of parameters the function takes by specifying the names of the
-// parameters it takes.
+/// When a function parameter is declared, the declaration can include the
+/// number of parameters the function takes by explicitly specifying the names of the
+/// parameters it takes.
 example3() {
-  planA(fn(informSomething)) {
+  planA(fn(String informSomething)) {
     fn("Example3 plan A");
   }
-  planB(fn) { // Or don't declare number of parameters.
+  planB(fn) {
+    // Or don't declare number of parameters.
     fn("Example3 plan B");
   }
+
   planA((s) => print(s));
   planB((s) => print(s));
 }
 
-// Functions have closure access to outer variables.
+/// Functions have closure access to outer variables.
+/// Dart will infer types when the variable has a value of something.
+/// In this example dart knows that this variable is a String.
 var example4Something = "Example4 nested 1";
 example4() {
   nested1(fn(informSomething)) {
     fn(example4Something);
   }
+
   nested1((s) => print(s));
 }
 
-// Class declaration with a sayIt method, which also has closure access
-// to the outer variable as though it were a function as seen before.
+/// Class declaration with a sayIt method, which also has closure access
+/// to the outer variable as though it were a function as seen before.
 var example5method = "Example5 sayIt";
+
 class Example5Class {
   sayIt() {
     print(example5method);
   }
 }
+
 example5() {
-  // Create an anonymous instance of the Example5Class and call the sayIt
-  // method on it.
+  /// Create an anonymous instance of the Example5Class and call the sayIt
+  /// method on it.
+  /// the `new` keyword is optional in Dart.
   new Example5Class().sayIt();
 }
 
-// Class declaration takes the form of class name { [classBody] }.
-// Where classBody can include instance methods and variables, but also
-// class methods and variables.
+/// Class declaration takes the form of class name { [classBody] }.
+/// Where classBody can include instance methods and variables, but also
+/// class methods and variables.
 class Example6Class {
   var instanceVariable = "Example6 instance variable";
   sayIt() {
     print(instanceVariable);
   }
 }
+
 example6() {
-  new Example6Class().sayIt();
+   Example6Class().sayIt();
 }
 
-// Class methods and variables are declared with "static" terms.
+/// Class methods and variables are declared with "static" terms.
 class Example7Class {
   static var classVariable = "Example7 class variable";
   static sayItFromClass() {
     print(classVariable);
   }
+
   sayItFromInstance() {
     print(classVariable);
   }
 }
+
 example7() {
   Example7Class.sayItFromClass();
   new Example7Class().sayItFromInstance();
 }
 
-// Literals are great, but there's a restriction for what literals can be
-// outside of function/method bodies. Literals on the outer scope of class
-// or outside of class have to be constant. Strings and numbers are constant
-// by default. But arrays and maps are not. They can be made constant by
-// declaring them "const".
-var example8Array = const ["Example8 const array"],
-  example8Map = const {"someKey": "Example8 const map"};
+/// Dart supports Generics.
+/// Generics refers to the technique of writing the code for a class
+/// without specifying the data type(s) that the class works on.
+/// Source: https://stackoverflow.com/questions/4560890/what-are-generics-in-c
+
+/// Type `T` refers to any type that has been instantiated
+/// you can call whatever you want
+/// Programmers uses the convention in the following
+/// T - Type(used for class and primitype types)
+/// E - Element(used for List, Set, or Iterable)
+/// K,V - Key Value(used for Map)
+class GenericExample<T>{
+  void printType(){
+    print("$T")
+  }
+  // methods can also have generics
+  genericMethod<M>(){
+    print("class:$T, method: $M");
+  }
+}
+
+
+/// List are similar to arrays but list is a child of Iterable<E>
+/// Therefore Maps, List, LinkedList are all child of Iterable<E> to be able to loop using the keyword `for`
+/// Important things to remember:
+/// () - Iterable<E>
+/// [] - List<E>
+/// {} - Map<K,V>
+
+
+/// List are great, but there's a restriction for what List can be
+/// outside of function/method bodies. List on the outer scope of class
+/// or outside of class have to be constant. Strings and numbers are constant
+/// by default. But arrays and maps are not. They can be made constant by
+/// declaring them "const". Kind of similar to Javascript's Object.freeze()
+const example8List = ["Example8 const array"];
+const  example8Map = {"someKey": "Example8 const map"};
+/// Declare List or Maps as Objects.
+ List<String> explicitList = new List<String>();
+ Map<String,dynamic> explicitMaps = new Map<String,dynamic>();
+
+ explicitList.add("SomeArray");
 example8() {
-  print(example8Array[0]);
   print(example8Map["someKey"]);
+  print(explicitList[0]);
 }
 
-// Loops in Dart take the form of standard for () {} or while () {} loops,
-// slightly more modern for (.. in ..) {}, or functional callbacks with many
-// supported features, starting with forEach.
+/// Assigning a list from one variable to another will not be the same result.
+/// Because dart is pass-reference-by-value.
+/// So when you assign an existing list to a new variable.
+/// Instead of List, it becomes an Iterable
+var iterableExplicitList = explicitList;
+print(iterableExplicitList) // ("SomeArray"); "[]" becomes "()"
+var newExplicitLists = explicitList.toList() // Converts Iterable<E> to List<E>
+
+/// Loops in Dart take the form of standard for () {} or while () {} loops,
+/// slightly more modern for (.. in ..) {}, or functional callbacks with many
+/// supported features, starting with forEach,map and where.
 var example9Array = const ["a", "b"];
 example9() {
-  for (var i = 0; i < example9Array.length; i++) {
+  for (int i = 0; i < example9Array.length; i++) {
     print("Example9 for loop '${example9Array[i]}'");
   }
   var i = 0;
@@ -130,13 +231,15 @@ example9() {
     print("Example9 while loop '${example9Array[i]}'");
     i++;
   }
-  for (var e in example9Array) {
+  for (final e in example9Array) {
     print("Example9 for-in loop '${e}'");
   }
+
   example9Array.forEach((e) => print("Example9 forEach loop '${e}'"));
+
 }
 
-// To loop over the characters of a string or to extract a substring.
+/// To loop over the characters of a string or to extract a substring.
 var example10String = "ab";
 example10() {
   for (var i = 0; i < example10String.length; i++) {
@@ -147,14 +250,37 @@ example10() {
   }
 }
 
-// Int and double are the two supported number formats.
+/// `int`, `double`  and `num` are the three supported number formats.
+/// `num` can be either `int` or `double`.
+/// `int` and `double` are children of type `num`
 example11() {
   var i = 1 + 320, d = 3.2 + 0.01;
+  num myNumDouble = 2.2;
+  num myNumInt = 2;
+  int myInt = 1;
+  double myDouble = 0; // Dart will add decimal prefix, becomes 0.0;
+  myNumDouble = myInt; // valid
+  myNumDouble = myDouble; //valid
+  myNumDouble = myNumInt; //valid
+
+  myNumInt = myInt; // valid
+  myNumInt = myDouble; // valid
+  myNumInt = myNumDouble; // valid
+
+  myInt = myNumDouble; //Error
+  myInt = myDouble; //Error
+  myInt = myNumInt; //valid
+
+  myDouble = myInt; //error
+  myDouble = myNumInt; //valid
+  myDouble = myNumDouble; //valid
+
   print("Example11 int ${i}");
   print("Example11 double ${d}");
+
 }
 
-// DateTime provides date/time arithmetic.
+/// DateTime provides date/time arithmetic.
 example12() {
   var now = new DateTime.now();
   print("Example12 now '${now}'");
@@ -162,7 +288,7 @@ example12() {
   print("Example12 tomorrow '${now}'");
 }
 
-// Regular expressions are supported.
+/// Regular expressions are supported.
 example13() {
   var s1 = "some string", s2 = "some", re = new RegExp("^s.+?g\$");
   match(s) {
@@ -172,11 +298,12 @@ example13() {
       print("Example13 regexp doesn't match '${s}'");
     }
   }
+
   match(s1);
   match(s2);
 }
 
-// Boolean expressions support implicit conversions and dynamic type
+/// Boolean expressions support implicit conversions and dynamic type
 example14() {
   var a = true;
   if (a) {
@@ -186,11 +313,11 @@ example14() {
   if (a) {
     print("true, a is $a");
   } else {
-    print("false, a is $a"); // runs here
+    print("false, a is $a"); /// runs here
   }
 
-  // dynamic typed null can be convert to bool
-  var b;// b is dynamic type
+  /// dynamic typed null can be convert to bool
+  var b;/// b is dynamic type
   b = "abc";
   try {
     if (b) {
@@ -199,35 +326,35 @@ example14() {
       print("false, b is $b");
     }
   } catch (e) {
-    print("error, b is $b"); // this could be run but got error
+    print("error, b is $b"); /// this could be run but got error
   }
   b = null;
   if (b) {
     print("true, b is $b");
   } else {
-    print("false, b is $b"); // runs here
+    print("false, b is $b"); /// runs here
   }
 
-  // statically typed null can not be convert to bool
+  /// statically typed null can not be convert to bool
   var c = "abc";
   c = null;
-  // complie failed
-  // if (c) {
-  //   print("true, c is $c");
-  // } else {
-  //   print("false, c is $c");
-  // }
+  /// complie failed
+  /// if (c) {
+  ///   print("true, c is $c");
+  /// } else {
+  ///   print("false, c is $c");
+  /// }
 }
 
-// try/catch/finally and throw are used for exception handling.
-// throw takes any object as parameter;
+/// try/catch/finally and throw are used for exception handling.
+/// throw takes any object as parameter;
 example15() {
   try {
     try {
       throw "Some unexpected error.";
     } catch (e) {
       print("Example15 an exception: '${e}'");
-      throw e; // Re-throw
+      throw e; /// Re-throw
     }
   } catch (e) {
     print("Example15 catch exception being re-thrown: '${e}'");
@@ -236,18 +363,21 @@ example15() {
   }
 }
 
-// To be efficient when creating a long string dynamically, use
-// StringBuffer. Or you could join a string array.
+/// To be efficient when creating a long string dynamically, use
+/// StringBuffer. Or you could join a string array.
 example16() {
   var sb = new StringBuffer(), a = ["a", "b", "c", "d"], e;
-  for (e in a) { sb.write(e); }
+  for (e in a) {
+    sb.write(e);
+  }
   print("Example16 dynamic string created with "
-    "StringBuffer '${sb.toString()}'");
+      "StringBuffer '${sb.toString()}'");
   print("Example16 join string array '${a.join()}'");
 }
 
-// Strings can be concatenated by just having string literals next to
-// one another with no further operator needed.
+/// Strings can be concatenated by just having string List next to
+/// one another with no further operator needed.
+
 example17() {
   print("Example17 "
       "concatenate "
@@ -255,44 +385,44 @@ example17() {
       "just like that");
 }
 
-// Strings have single-quote or double-quote for delimiters with no
-// actual difference between the two. The given flexibility can be good
-// to avoid the need to escape content that matches the delimiter being
-// used. For example, double-quotes of HTML attributes if the string
-// contains HTML content.
+/// Strings have single-quote or double-quote for delimiters with no
+/// actual difference between the two. The given flexibility can be good
+/// to avoid the need to escape content that matches the delimiter being
+/// used. For example, double-quotes of HTML attributes if the string
+/// contains HTML content.
 example18() {
   print('Example18 <a href="etc">'
       "Don't can't I'm Etc"
       '</a>');
 }
 
-// Strings with triple single-quotes or triple double-quotes span
-// multiple lines and include line delimiters.
+/// Strings with triple single-quotes or triple double-quotes span
+/// multiple lines and include line delimiters.
 example19() {
   print('''Example19 <a href="etc">
 Example19 Don't can't I'm Etc
 Example19 </a>''');
 }
 
-// Strings have the nice interpolation feature with the $ character.
-// With $ { [expression] }, the return of the expression is interpolated.
-// $ followed by a variable name interpolates the content of that variable.
-// $ can be escaped like so \$ to just add it to the string instead.
+/// Strings have the nice interpolation feature with the $ character.
+/// With $ { [expression] }, the return of the expression is interpolated.
+/// $ followed by a variable name interpolates the content of that variable.
+/// $ can be escaped like so \$ to just add it to the string instead.
 example20() {
   var s1 = "'\${s}'", s2 = "'\$s'";
   print("Example20 \$ interpolation ${s1} or $s2 works.");
 }
 
-// Optional types allow for the annotation of APIs and come to the aid of
-// IDEs so the IDEs can better refactor, auto-complete and check for
-// errors. So far we haven't declared any types and the programs have
-// worked just fine. In fact, types are disregarded during runtime.
-// Types can even be wrong and the program will still be given the
-// benefit of the doubt and be run as though the types didn't matter.
-// There's a runtime parameter that checks for type errors which is
-// the checked mode, which is said to be useful during development time,
-// but which is also slower because of the extra checking and is thus
-// avoided during deployment runtime.
+/// Optional types allow for the annotation of APIs and come to the aid of
+/// IDEs so the IDEs can better refactor, auto-complete and check for
+/// errors. So far we haven't declared any types and the programs have
+/// worked just fine. In fact, types are disregarded during runtime.
+/// Types can even be wrong and the program will still be given the
+/// benefit of the doubt and be run as though the types didn't matter.
+/// There's a runtime parameter that checks for type errors which is
+/// the checked mode, which is said to be useful during development time,
+/// but which is also slower because of the extra checking and is thus
+/// avoided during deployment runtime.
 class Example21 {
   List<String> _names;
   Example21() {
@@ -302,11 +432,13 @@ class Example21 {
   set names(List<String> list) {
     _names = list;
   }
+
   int get length => _names.length;
   void add(String name) {
     _names.add(name);
   }
 }
+
 void example21() {
   Example21 o = new Example21();
   o.add("c");
@@ -315,46 +447,50 @@ void example21() {
   print("Example21 names '${o.names}' and length '${o.length}'");
 }
 
-// Class inheritance takes the form of class name extends AnotherClassName {}.
+/// Class inheritance takes the form of class name extends AnotherClassName {}.
 class Example22A {
   var _name = "Some Name!";
   get name => _name;
 }
+
 class Example22B extends Example22A {}
+
 example22() {
   var o = new Example22B();
   print("Example22 class inheritance '${o.name}'");
 }
 
-// Class mixin is also available, and takes the form of
-// class name extends SomeClass with AnotherClassName {}.
-// It's necessary to extend some class to be able to mixin another one.
-// The template class of mixin cannot at the moment have a constructor.
-// Mixin is mostly used to share methods with distant classes, so the
-// single inheritance doesn't get in the way of reusable code.
-// Mixins follow the "with" statement during the class declaration.
+/// Class mixin is also available, and takes the form of
+/// class name extends SomeClass with AnotherClassName {}.
+/// It's necessary to extend some class to be able to mixin another one.
+/// The template class of mixin cannot at the moment have a constructor.
+/// Mixin is mostly used to share methods with distant classes, so the
+/// single inheritance doesn't get in the way of reusable code.
+/// Mixins follow the "with" statement during the class declaration.
 class Example23A {}
+
 class Example23Utils {
   addTwo(n1, n2) {
     return n1 + n2;
   }
 }
+
 class Example23B extends Example23A with Example23Utils {
   addThree(n1, n2, n3) {
     return addTwo(n1, n2) + n3;
   }
 }
+
 example23() {
-  var o = new Example23B(), r1 = o.addThree(1, 2, 3),
-    r2 = o.addTwo(1, 2);
+  var o = new Example23B(), r1 = o.addThree(1, 2, 3), r2 = o.addTwo(1, 2);
   print("Example23 addThree(1, 2, 3) results in '${r1}'");
   print("Example23 addTwo(1, 2) results in '${r2}'");
 }
 
-// The Class constructor method uses the same name of the class and
-// takes the form of SomeClass() : super() {}, where the ": super()"
-// part is optional and it's used to delegate constant parameters to the
-// super-parent's constructor.
+/// The Class constructor method uses the same name of the class and
+/// takes the form of SomeClass() : super() {}, where the ": super()"
+/// part is optional and it's used to delegate constant parameters to the
+/// super-parent's constructor.
 class Example24A {
   var _value;
   Example24A({value: "someValue"}) {
@@ -362,71 +498,76 @@ class Example24A {
   }
   get value => _value;
 }
+
 class Example24B extends Example24A {
   Example24B({value: "someOtherValue"}) : super(value: value);
 }
+
 example24() {
-  var o1 = new Example24B(),
-    o2 = new Example24B(value: "evenMore");
+  var o1 = new Example24B(), o2 = new Example24B(value: "evenMore");
   print("Example24 calling super during constructor '${o1.value}'");
   print("Example24 calling super during constructor '${o2.value}'");
 }
 
-// There's a shortcut to set constructor parameters in case of simpler classes.
-// Just use the this.parameterName prefix and it will set the parameter on
-// an instance variable of same name.
+/// There's a shortcut to set constructor parameters in case of simpler classes.
+/// Just use the this.parameterName prefix and it will set the parameter on
+/// an instance variable of same name.
 class Example25 {
   var value, anotherValue;
   Example25({this.value, this.anotherValue});
 }
+
 example25() {
   var o = new Example25(value: "a", anotherValue: "b");
   print("Example25 shortcut for constructor '${o.value}' and "
-    "'${o.anotherValue}'");
+      "'${o.anotherValue}'");
 }
 
-// Named parameters are available when declared between {}.
-// Parameter order can be optional when declared between {}.
-// Parameters can be made optional when declared between [].
+/// Named parameters are available when declared between {}.
+/// Parameter order can be optional when declared between {}.
+/// Parameters can be made optional when declared between [].
 example26() {
   var _name, _surname, _email;
   setConfig1({name, surname}) {
     _name = name;
     _surname = surname;
   }
+
   setConfig2(name, [surname, email]) {
     _name = name;
     _surname = surname;
     _email = email;
   }
+
   setConfig1(surname: "Doe", name: "John");
   print("Example26 name '${_name}', surname '${_surname}', "
-    "email '${_email}'");
+      "email '${_email}'");
   setConfig2("Mary", "Jane");
   print("Example26 name '${_name}', surname '${_surname}', "
-  "email '${_email}'");
+      "email '${_email}'");
 }
 
-// Variables declared with final can only be set once.
-// In case of classes, final instance variables can be set via constant
-// constructor parameter.
+/// Variables declared with final can only be set once.
+/// In case of classes, final instance variables can be set via constant
+/// constructor parameter.
 class Example27 {
   final color1, color2;
-  // A little flexibility to set final instance variables with syntax
-  // that follows the :
+  /// A little flexibility to set final instance variables with syntax
+  /// that follows the :
   Example27({this.color1, color2}) : color2 = color2;
 }
+
 example27() {
   final color = "orange", o = new Example27(color1: "lilac", color2: "white");
   print("Example27 color is '${color}'");
   print("Example27 color is '${o.color1}' and '${o.color2}'");
 }
 
-// To import a library, use import "libraryPath" or if it's a core library,
-// import "dart:libraryName". There's also the "pub" package management with
-// its own convention of import "package:packageName".
-// See import "dart:collection"; at the top. Imports must come before
-// other code declarations. IterableBase comes from dart:collection.
+/// To import a library, use import "libraryPath" or if it's a core library,
+/// import "dart:libraryName". There's also the "pub" package management with
+/// its own convention of import "package:packageName".
+/// See import "dart:collection"; at the top. Imports must come before
+/// other code declarations. IterableBase comes from dart:collection.
 class Example28 extends IterableBase {
   var names;
   Example28() {
@@ -434,16 +575,17 @@ class Example28 extends IterableBase {
   }
   get iterator => names.iterator;
 }
+
 example28() {
   var o = new Example28();
   o.forEach((name) => print("Example28 '${name}'"));
 }
 
-// For control flow we have:
-// * standard switch with must break statements
-// * if-else if-else and ternary ..?..:.. operator
-// * closures and anonymous functions
-// * break, continue and return statements
+/// For control flow we have:
+/// * standard switch with must break statements
+/// * if-else if-else and ternary ..?..:.. operator
+/// * closures and anonymous functions
+/// * break, continue and return statements
 example29() {
   var v = true ? 30 : 60;
   switch (v) {
@@ -459,10 +601,12 @@ example29() {
   callItForMe(fn()) {
     return fn();
   }
+
   rand() {
     v = new DM.Random().nextInt(50);
     return v;
   }
+
   while (true) {
     print("Example29 callItForMe(rand) '${callItForMe(rand)}'");
     if (v != 30) {
@@ -470,17 +614,21 @@ example29() {
     } else {
       continue;
     }
-    // Never gets here.
+    /// Never gets here.
   }
 }
 
-// Parse int, convert double to int, or just keep int when dividing numbers
-// by using the ~/ operation. Let's play a guess game too.
+/// Parse int, convert double to int, or just keep int when dividing numbers
+/// by using the ~/ operation. Let's play a guess game too.
 example30() {
-  var gn, tooHigh = false,
-    n, n2 = (2.0).toInt(), top = int.parse("123") ~/ n2, bottom = 0;
+  var gn,
+      tooHigh = false,
+      n,
+      n2 = (2.0).toInt(),
+      top = int.parse("123") ~/ n2,
+      bottom = 0;
   top = top ~/ 6;
-  gn = new DM.Random().nextInt(top + 1); // +1 because nextInt top is exclusive
+  gn = new DM.Random().nextInt(top + 1); /// +1 because nextInt top is exclusive
   print("Example30 Guess a number between 0 and ${top}");
   guessNumber(i) {
     if (n == gn) {
@@ -488,10 +636,11 @@ example30() {
     } else {
       tooHigh = n > gn;
       print("Example30 Number ${n} is too "
-        "${tooHigh ? 'high' : 'low'}. Try again");
+          "${tooHigh ? 'high' : 'low'}. Try again");
     }
     return n == gn;
   }
+
   n = (top - bottom) ~/ 2;
   while (!guessNumber(n)) {
     if (tooHigh) {
@@ -503,19 +652,64 @@ example30() {
   }
 }
 
-// Programs have only one entry point in the main function.
-// Nothing is expected to be executed on the outer scope before a program
-// starts running with what's in its main function.
-// This helps with faster loading and even lazily loading of just what
-// the program needs to startup with.
+/// Optional Positional Parameter:
+/// parameter will be disclosed with square bracket [ ] & square bracketed parameter are optional.
+example31() {
+    findVolume31(int length, int breath, [int height]) {
+      print('length = $length, breath = $breath, height = $height');
+    }
+
+    findVolume31(10,20,30); //valid
+    findVolume31(10,20); //also valid
+}
+
+/// Optional Named Parameter:
+/// parameter will be disclosed with curly bracket { }
+/// curly bracketed parameter are optional.
+/// have to use parameter name to assign a value which separated with colan :
+/// in curly bracketed parameter order does not matter
+/// these type parameter help us to avoid confusion while passing value for a function which has many parameter.
+example32() {
+    findVolume32(int length, int breath, {int height}) {
+    print('length = $length, breath = $breath, height = $height');
+    }
+
+    findVolume32(10,20,height:30);//valid & we can see the parameter name is mentioned here.
+    findVolume32(10,20);//also valid
+}
+
+/// Optional Default Parameter:
+/// same like optional named parameter in addition we can assign default value for this parameter.
+/// which means no value is passed this default value will be taken.
+example33() {
+    findVolume33(int length, int breath, {int height=10}) {
+     print('length = $length, breath = $breath, height = $height');
+    }
+
+    findVolume33(10,20,height:30);//valid
+    findVolume33(10,20);//valid
+}
+
+/// Dart has also added feature such as Null aware operators
+var isBool = true;
+var hasString = isBool ?? "default String";
+
+/// Programs have only one entry point in the main function.
+/// Nothing is expected to be executed on the outer scope before a program
+/// starts running with what's in its main function.
+/// This helps with faster loading and even lazily loading of just what
+/// the program needs to startup with.
 main() {
   print("Learn Dart in 15 minutes!");
-  [example1, example2, example3, example4, example5, example6, example7,
-    example8, example9, example10, example11, example12, example13, example14,
-    example15, example16, example17, example18, example19, example20,
-    example21, example22, example23, example24, example25, example26,
-    example27, example28, example29, example30
-    ].forEach((ef) => ef());
+  [
+    example1, example2, example3, example4, example5,
+    example6, example7, example8, example9, example10,
+    example11, example12, example13, example14, example15,
+    example16, example17, example18, example19, example20,
+    example21, example22, example23, example24, example25,
+    example26, example27, example28, example29,
+    example30 // Adding this comment stops the dart formatter from putting all items on a new line
+  ].forEach((ef) => ef());
 }
 
 ```
@@ -526,6 +720,3 @@ Dart has a comprehensive web-site. It covers API reference, tutorials, articles
 useful Try Dart online.
 [https://www.dartlang.org](https://www.dartlang.org)
 [https://try.dartlang.org](https://try.dartlang.org)
-
-
-