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Dart is the programming language for Flutter, Google's UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase.

This codelab introduces you to Dart, with a focus on features that Java developers might not expect. You can be writing Dart functions in 1 minute, scripts in 5 minutes, and apps in 10 minutes!

What you'll learn

  • How to create constructors
  • Different ways to specify parameters
  • When and how to create getters and setters
  • How Dart handles privacy
  • How to create factories
  • How functional programming works in Dart
  • Other core Dart concepts

What you'll need

To complete this codelab, all you need is a browser!

You'll write and run all the examples in DartPad, an interactive, browser-based tool that lets you play with Dart language features and core libraries. If you prefer, you can use an IDE instead, such as WebStorm, IntelliJ with the Dart plugin, or Visual Studio Code with the Dart Code extension.

What's your biggest reason for visiting this codelab?

I want to write mobile apps (Flutter). I want to write web apps. I want to write some other kind of Dart code. I'm curious about the Dart language.

You'll start by building a simple Dart class with the same functionality as the Bicycle class from the Java Tutorial. The Bicycle class contains some private instance variables with getters and setters. A main() method instantiates a Bicycle and prints it to the console.

Launch DartPad

This codelab provides a new DartPad instance for every set of exercises. The link below opens a fresh instance, which contains a default ‘Hello' example. You can continue to use the same DartPad throughout the codelab, but if you click the Reset button, DartPad takes you back to the default example, losing your work.

For your convenience, at the top of each codelab page is a DartPad instance that reflects the state at the beginning of each exercise.

Open DartPad

Note that DartPad immediately runs its code.

Define a Bicycle class

Above the main() function, add a Bicycle class with three instance variables. Also remove the contents from main(), as shown below:

class Bicycle {
  int cadence;
  int speed;
  int gear;
}

void main() {
}

Observations

  • Dart's main method is named main() or (if you need access to command line arguments) main(List<String> args).
  • The main() method lives at the top level. In Dart, you can define code outside of classes. Variables, functions, getters, and setters can all live outside of classes.
  • The original Java example declares private instance variables using the private tag, which Dart doesn't use. You'll learn more about privacy in "Step 3: Add a read-only variable".
  • Neither main() nor Bicycle is declared as public, because all identifiers are public by default. Dart doesn't have keywords for public, private, or protected.
  • Dart uses 2-character indentation, by convention, instead of 4. You don't need to worry about Dart's whitespace conventions, thanks to a handy tool called dartfmt. As the Dart code conventions (Effective Dart) say, "The official whitespace-handling rules for Dart are whatever dartfmt produces."

Define a Bicycle constructor

Add the following constructor to the Bicycle class:

Bicycle(this.cadence, this.speed, this.gear);

Observations

  • This constructor has no body, which is valid in Dart.
  • If you forget the semicolon (;) at the end of a no-body constructor, DartPad displays an error: "A function body must be provided."
  • Using this in a constructor's parameter list is a handy shortcut for assigning values to instance variables.
  • The code above is equivalent to the following:
Bicycle(int cadence, int speed, int gear) {
  this.cadence = cadence;
  this.speed = speed;
  this.gear = gear;
}

Format the code

Reformat the Dart code at any time by clicking Format at the top of the DartPad UI. Reformatting is particularly useful when you've pasted code into DartPad and the justification is off.

Click Format.

Instantiate and print a bicycle instance

Add the following code to the main() function:

void main() {
  var bike = new Bicycle(2, 0, 1);
  print(bike);
}

Remove the optional new keyword:

var bike = Bicycle(2, 0, 1);

Observation

  • The new keyword became optional in Dart 2.
  • If you know that a variable's value won't change, you can use final instead of var.

Run the example

Execute the example by clicking the enabled Run button at the top of the DartPad window. If Run is not enabled, see the Problems section below.

You should see the following output:

Instance of 'Bicycle'

Observation

  • No errors or warnings should appear, indicating that type inference is working, and that the analyzer infers that var bike = ... defines a Bicycle instance.

Improve the output

While the output "Instance of ‘Bicycle'" is correct, it's not very informative. All Dart classes have a toString() method that you can override to provide more useful output.

Add the following toString() method anywhere in the Bicycle class:

@override
String toString() => 'Bicycle: $speed mph';

Observations

  • The @override annotation tells the analyzer that you are intentionally overriding a member. The analyzer raises an error if you've failed to perform the override properly.
  • Dart supports single or double quotes when specifying strings.
  • Use string interpolation to put the value of an expression inside a string literal: ${expression}. If the expression is an identifier, you can skip the braces: $variableName.
  • Shorten one-line functions or methods using fat arrow (=>) notation.

Run the example

Click Run.

You should now see the following output:

Bicycle: 0 mph

Problems?
Check your code

Add a read-only variable

The original Java example defines speed as a read-only variable—it declares it as private and provides only a getter. Next, you'll provide the same functionality in Dart.

Open bicycle.dart in DartPad (or continue using your copy)

To mark a Dart identifier as private to its library, start its name with an underscore (_). You can convert speed to read-only by changing its name and adding a getter.

Make speed a private, read-only instance variable

In the Bicycle constructor, remove the speed parameter:

Bicycle(this.cadence, this.gear);

In main(), remove the second (speed) parameter from the call to the Bicycle constructor:

var bike = Bicycle(2, 1);

Change the remaining occurrences of speed to _speed. (2 places)

Initialize _speed to 0:

int _speed = 0;

Add the following getter to the Bicycle class:

int get speed => _speed;

Observations

  • Uninitialized variables (even numbers) have the value null.
  • The Dart compiler enforces privacy for any identifier prefixed with an underscore.
  • By default, Dart provides implicit getters and setters for all public instance variables. You don't need to define your own getters/setters unless you want to enforce read-only or write-only variables, compute or verify a value, or update a value elsewhere.
  • Since getters and setters were provided for cadence and gear in the original Java example, they aren't, by definition, considered private in the Dartiverse. Those instance variables can be accessed using bike.gear or bike.cadence.
  • You might start with a simple field, like bike.cadence, and later refactor it to use getters and setters. The API stays the same. In other words, going from a field to a getter/setter is not a breaking change in Dart.

Finish implementing speed as a read only instance variable

Add the following methods to the Bicycle class:

void applyBrake(int decrement) {
  _speed -= decrement;
}

void speedUp(int increment) {
  _speed += increment;
}

The final Dart example looks similar to the original Java but is more compact at 23 lines instead of 40:

class Bicycle {
  int cadence;
  int _speed = 0;
  int get speed => _speed;
  int gear;

  Bicycle(this.cadence, this.gear);

  void applyBrake(int decrement) {
    _speed -= decrement;
  }

  void speedUp(int increment) {
    _speed += increment;
  }

  @override
  String toString() => 'Bicycle: $_speed mph';
}

void main() {
  var bike = Bicycle(2, 1);
  print(bike);
}

Problems?
Check your code

The next exercise defines a Rectangle class, another example from the Java Tutorial.

The Java code shows overloading constructors, a common practice in Java where constructors have the same name, but differ in the number or type of parameters. Dart doesn't support overloading constructors and handles this situation differently, as you will see in this section.

Open the Rectangle example in DartPad

Add a Rectangle constructor

Add a single, empty constructor that replaces all four constructors in the Java example:

Rectangle({this.origin = const Point(0, 0), this.width = 0, this.height = 0});

This constructor uses optional named parameters.

Observations

  • this.origin, this.width, and this.height use the shorthand trick for assigning instance variables inside a constructor's declaration.
  • this.origin, this.width, and this.height are optional named parameters. Named parameters are enclosed in curly braces ({}).
  • The this.origin = const Point(0, 0) syntax specifies a default value of Point(0,0) for the origin instance variable. The specified default must be a compile-time constant. This constructor supplies default values for all three instance variables.

Improve the output

Add the following toString() function to the Rectangle class:

@override
String toString() =>
      'Origin: (${origin.x}, ${origin.y}), width: $width, height: $height';

Use the constructor

Replace main() with the following code to verify that you can instantiate Rectangle using only the parameters you need.

main() {
  print(Rectangle(origin: const Point(10, 20), width: 100, height: 200));
  print(Rectangle(origin: const Point(10, 10)));
  print(Rectangle(width: 200));
  print(Rectangle());
}

Observation

  • The Dart constructor for Rectangle is one line of code, compared to 16 lines of code for equivalent constructors in the Java version.

Run the example

You should see the following output:

Origin: (10, 20), width: 100, height: 200
Origin: (10, 10), width: 0, height: 0
Origin: (0, 0), width: 200, height: 0
Origin: (0, 0), width: 0, height: 0

Problems?
Check your code

Factories, a commonly used design pattern in Java, have several advantages over direct object instantiation, such as hiding the details of instantiation, providing the ability to return a subtype of the factory's return type, and optionally returning an existing object rather than a new object.

This step demonstrates two ways to implement a shape-creation factory:

  • Option 1: Create a top-level function
  • Option 2: Create a factory constructor

For this exercise, you'll use the Shapes example, which instantiates shapes and prints their computed area:

import 'dart:math';

abstract class Shape {
  num get area;
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);
  num get area => pi * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);
  num get area => pow(side, 2);
}

main() {
  final circle = Circle(2);
  final square = Square(2);
  print(circle.area);
  print(square.area);
}

Open the Shapes example in DartPad

In the console area, you should see the computed areas of a circle and a square:

12.566370614359172
4

Observations

  • Dart supports abstract classes.
  • You can define multiple classes in one file.
  • dart.math is one of Dart's core libraries. Other core libraries include dart:core, dart:async, dart:convert, and dart:collection.
  • In Dart 1.x, core library constants were uppercase (PI); in Dart 2, they're lowercase (pi).
  • This code includes two getters that compute a value:
    num get area => pi * pow(radius, 2); // Circle
    num get area => pow(side, 2); // Square

Option 1: Create a top-level function

Implement a factory as a top-level function by adding the following function at the highest level (outside of any class):

Shape shapeFactory(String type) {
  if (type == 'circle') return Circle(2);
  if (type == 'square') return Square(2);
  throw 'Can\'t create $type.';
}

Invoke the factory function by replacing the first two lines in the main() method:

  final circle = shapeFactory('circle');
  final square = shapeFactory('square');

Run the example

The output should look the same as before.

Observations

  • If the function is called with any string other than 'circle' or 'square', it throws an exception.
  • The Dart SDK defines classes for many common exceptions, or you can extend the Exception class to create your own more specific exceptions or (as in this example) you can throw a string that describes the problem encountered.
  • When an exception is encountered, DartPad reports Uncaught. To see information that's more helpful, wrap the code in a try-catch statement, and print the exception. As an optional exercise, check out this DartPad example.
  • To use a single quote inside a string, either escape the embedded quote using slash ('Can\'t create $type.') or specify the string with double quotes ("Can't create $type.").

Problems?
Check your code

Option 2: Create a factory constructor

Use Dart's factory keyword to create a factory constructor.

Add a factory constructor to the abstract Shape class

abstract class Shape {
  factory Shape(String type) {
    if (type == 'circle') return Circle(2);
    if (type == 'square') return Square(2);
    throw 'Can\'t create $type.';
  }
  num get area;
}

Replace the first two lines of main() with the following code for instantiating the shapes:

  final circle = Shape('circle');
  final square = Shape('square');

Delete the shapeFactory() function that you added previously.

Observation

  • The code in the factory constructor is identical to the code used in the shapeFactory() function.

Problems?
Check your code

The Dart language doesn't include an interface keyword because every class defines an interface.

Open the Shapes example in DartPad (or continue using your copy)

Add a CircleMock class that extends the Circle class:

class CircleMock implements Circle {}

You should see a "Missing concrete implementations" error. Fix this error by defining the area and radius instance variables:

class CircleMock implements Circle {
  num area;
  num radius;
}

Observation

  • Even though the CircleMock class doesn't define any behaviors, it's valid Dart—the analyzer raises no errors.

Problems?
Check your code

In functional programming you can do things like:

  • Pass functions as arguments.
  • Assign a function to a variable.
  • Deconstruct a function that takes multiple arguments into a sequence of functions that each take a single argument (also called currying).
  • Create a nameless function that can be used as a constant value (also called a lambda expression; lambda expressions were added to Java in the JDK 8 release).

Dart supports all of these features. In Dart, even functions are objects and have a type, Function. This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function, as in this example.

The following example uses imperative (not functional-style) code:

String scream(int length) => "A${'a' * length}h!";

main() {
  final values = [1, 2, 3, 5, 10, 50];
  for (var length in values) {
    print(scream(length));
  }
}

Open the Scream example in DartPad

The output should look like the following:

Aah!
Aaah!
Aaaah!
Aaaaaah!
Aaaaaaaaaaah!
Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaah!

Observation

  • When using string interpolation, the string ${'a' * length} evaluates to "the character 'a' repeated length times."

Convert imperative code to functional

Remove the imperative for() {...} loop in main() and replace it with a single line of code that uses method chaining:

  values.map(scream).forEach(print);

Run the example

The functional approach prints the same six "screams" as the imperative example.

Problems?
Check your code

Use more Iterable features

Lists and Iterables, from dart:collection, support fold, where, join, skip, and more. Dart also has Maps and Sets.

Replace the values.map() line in main() with the following:

  values.skip(1).take(3).map(scream).forEach(print);

Run the example

The output should look like the following:

Aaah!
Aaaah!
Aaaaaah!

Observations

  • skip(1)skips the first value, 1, in the values list literal.
  • take(3)gets the next 3 values—2, 3, and 5—in the values list literal.
  • The remaining values are skipped.

Problems?
Check your code

In completing this codelab, you've gained knowledge on some differences between Java and Dart. Dart is easy to learn and, in addition, its core libraries and rich set of available packages increase your productivity. Dart scales well to large applications. Hundreds of Google engineers use Dart to write mission critical apps that bring in much of Google's revenue.

Next steps

A 20-minute codelab isn't long enough to show you all of the differences between Java and Dart. For example, this codelab hasn't covered:

If you'd like to see Dart technologies in action, try the Flutter codelabs.

Learn more

You can learn much more about Dart with the following articles, resources, and websites.

Articles

Resources

Websites