void main() { 
   int n = "hello"; 
   print(n); 
} 

Run the script by entering −

dart Test.dart

Though there is a type-mismatch the script executes successfully as the checked mode is turned off. The script will result in the following output −

hello

Now try executing the script with the “- – checked” or the “-c” option −

dart -c Test.dart 

Or,

dart - - checked Test.dart

The Dart VM will throw an error stating that there is a type mismatch.

Unhandled exception: 
type ''String'' is not a subtype of type ''int'' of ''n'' where 
   String is from dart:core 
   int is from dart:core 
#0  main (file:///C:/Users/Administrator/Desktop/test.dart:3:9) 
#1  _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart :261) 
#2  _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:148)

Identifiers in Dart

Identifiers are names given to elements in a program like variables, functions etc. The rules for identifiers are −

Identifiers can include both, characters and digits. However, the identifier cannot begin with a digit.

• Identifiers cannot include special symbols except for underscore (_) or a dollar sign ($).

• Identifiers cannot be keywords.

• They must be unique.

• Identifiers are case-sensitive.

• Identifiers cannot contain spaces.

The following tables lists a few examples of valid and invalid identifiers −

Keywords in Dart

Keywords have a special meaning in the context of a language. The following table lists some keywords in Dart.

Whitespace and Line Breaks

Dart ignores spaces, tabs, and newlines that appear in programs. You can use spaces, tabs, and newlines freely in your program and you are free to format and indent your programs in a neat and consistent way that makes the code easy to read and understand.

Dart is Case-sensitive

Dart is case-sensitive. This means that Dart differentiates between uppercase and lowercase characters.

Statements end with a Semicolon

Each line of instruction is called a statement. Each dart statement must end with a semicolon (;). A single line can contain multiple statements. However, these statements must be separated by a semicolon.

Comments in Dart

Comments are a way to improve the readability of a program. Comments can be used to include additional information about a program like author of the code, hints about a function/ construct etc. Comments are ignored by the compiler.

Dart supports the following types of comments −

• Single-line comments ( // ) − Any text between a “//” and the end of a line is treated as a comment

• Multi-line comments (/* */) − These comments may span multiple lines.

Example

// this is single line comment  
  
/* This is a   
   Multi-line comment  
*/ 

Object-Oriented Programming in Dart

Dart is an Object-Oriented language. Object Orientation is a software development paradigm that follows real-world modelling. Object Orientation considers a program as a collection of objects that communicate with each other via mechanism called methods.

• Object − An object is a real-time representation of any entity. As per Grady Brooch, every object must have three features −

  • State − described by the attributes of an object.

  • Behavior − describes how the object will act.

  • Identity − a unique value that distinguishes an object from a set of similar such objects.

• Class − A class in terms of OOP is a blueprint for creating objects. A class encapsulates data for the object.

• Method − Methods facilitate communication between objects.

Example: Dart and Object Orientation

class TestClass {   
   void disp() {     
      print("Hello World"); 
   } 
}  
void main() {   
   TestClass c = new TestClass();   
   c.disp();  
}

Hello World