The Wayback Machine - https://web.archive.org/web/20200513060922/https://codelabs.developers.google.com/codelabs/from-java-to-dart/

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

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

What you'll learn

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 Reset, DartPad takes you back to the default example, losing your work.

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 in the following code snippet:

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

void main() {
}

Observations

Define a Bicycle constructor

Add the following constructor to the Bicycle class:

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

Observations

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 paste 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

Run the example

Execute the example by clicking Run at the top of the DartPad window. If Run isn't enabled, see the Problems section later in this page.

You should see the following output:

Instance of 'Bicycle'

Observation

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

Run the example

Click Run.

You should 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. (Two places)

Initialize _speed to 0:

int _speed = 0;

Add the following getter to the Bicycle class:

int get speed => _speed;

Observations

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'll 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

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

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:

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

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

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 previously added.

Observation

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 implements the Circle interface:

class CircleMock implements Circle {}

You should see a "Missing concrete implementations" error because CircleMock doesn't inherit the implementation of Circle—it only uses its interface. Fix this error by defining the area and radius instance variables:

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

Observation

Problems?
Check your code.

In functional programming you can do things like the following:

Dart supports all those 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

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

The core List and Iterable classes support fold(), where(), join(), skip(), and more. Dart also has built-in support for 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

Problems?
Check your code.

By completing this codelab, you gained knowledge of 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 apps. 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