2022/5/24/Dart/Intro to Dart for Java Developersの訳



Intro to Dart for Java Developers

1. Introduction

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

  • 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.

DartPad は、Dart 言語機能やコアライブラリを操作できるブラウザベースの対話型ツールで、すべてのサンプルを作成し、実行します。ご希望であれば、WebStorm、Dartプラグインを備えたIntelliJ、Dart Code拡張機能を備えたVisual Studio CodeなどのIDEを代わりに使用することができます。

2. Create a simple Dart class

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.

まず、Java チュートリアルの Bicycle クラスと同じ機能を持つ簡単な Dart クラスを作成します。Bicycleクラスには、ゲッターとセッターを持ついくつかのプライベートなインスタンス変数が含まれています。main() メソッドは Bicycle をインスタンス化し、コンソールに表示します。


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.


Note: 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

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() {

cf1e10b838bf60ee.png Observations

  • With this example, the Dart analyzer produces an error informing you that the variables must be initialized because they’re non-nullable. You’ll fix that in the next section.

この例では、Dart アナライザーは、「変数がnon-nullable型なので初期化する必要があること」を通知するエラーを生成します。これは次のセクションで修正します。

int gear;


int? gear;


  • Dart’s main method is named main(). If you need access to command-line arguments, you can add them: main(List<String> args).

Dartのmain関数はmain()です。コマンドライン引数にアクセスする必要があるときは、main(List<String> args)のようにすることができます。

  • 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 a little later, in “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.

main()メソッドもBicycleクラスもpublicとして宣言されていません。なぜなら全ての識別子はデフォルトでpublicになります。Dartにはpublic, privateprotectedのキーワードはありません。

  • Dart uses two-character indentation by convention instead of four. You don’t need to worry about Dart’s whitespace conventions, thanks to a handy tool called dart format. As the Dart code conventions ( Effective Dart) say, “The official whitespace-handling rules for Dart are whatever dart format produces.”

Dart では、慣例的に 4 文字ではなく 2 文字のインデントが使用されています。Dartの空白の規則については、dartフォーマットという便利なツールのおかげで心配する必要はありません。Dartのコード規約(Effective Dart)には、「Dartの公式な空白処理規則は、dart formatが生成するものである」と書かれています。

Define a Bicycle constructor

Add the following constructor to the Bicycle class:


Bicycle(this.cadence, this.speed, this.gear);
  • 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 the following error: “A function body must be provided.”

no-bodyのコンストラクタの最後にセミコロン(;)を付けないと、DartPadは下記のエラーを示します。 “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, which uses an initializer list:


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.

DartPad UI の上部にある[フォーマット]をクリックすると、いつでも Dart コードを再フォーマットすることができます。再フォーマットは、DartPad にコードを貼り付ける際、両端揃えがずれている場合に特に便利です。

Instantiate and print a Bicycle instance

b2f84ff91b0e1396.png Add the following code to the main() function:


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

b2f84ff91b0e1396.png Remove the optional new keyword:


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

cf1e10b838bf60ee.png Observation

  • The new keyword became optional in Dart 2.


  • If you know that a variable’s value won’t change, then you can use final instead of var.


  • The print() function accepts any object (not just strings). It converts it to a String using the object’s toString() method.


Run the example

b2f84ff91b0e1396.png 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'

cf1e10b838bf60ee.png Observation

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

全くエラーも警告も出ない、ということは、型推測(type inference)が機能していることを示しています。アナライザーは、var bike =で始まる文について、変数bikeの型はBicycle型(クラス)である、と推測しています。

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


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.

“Instance of ‘Bicycle'”という出力は正しいのですが、(型情報だけでは)あまり参考になりません。すべてのDartクラスにはtoString()メソッドがあり、これをオーバーライドすればより有用な出力を得ることができます。

b2f84ff91b0e1396.png Add the following toString() method anywhere in the Bicycle class:


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

cf1e10b838bf60ee.png Observations

  • The @override annotation tells the analyzer that you are intentionally overriding a member. The analyzer raises an error if you fail to properly perform the override.


  • 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

b2f84ff91b0e1396.png 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.



b2f84ff91b0e1396.png 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.

Dartの識別子(変数)をライブラリに対してプライベートにしたい場合、識別子名をアンダースコア( _ )で書き始めてください。



Make speed a private, read-only instance variable

b2f84ff91b0e1396.png In the Bicycle constructor, remove the speed parameter:


Bicycle(this.cadence, this.gear);

b2f84ff91b0e1396.png In main(), remove the second (speed) parameter from the call to the Bicycle constructor:


var bike = Bicycle(2, 1);

b2f84ff91b0e1396.png Change the remaining occurrences of speed to _speed. (Two places)


Tip: If you are using a JetBrains IDE, you can simultaneously rename all instances of a variable by right-clicking the name, and choosing Refactor > Rename from the popup menu.

JetBrains IDEを使用している場合、「右クリック > Refactor > Rename」で、変数のインスタンス全箇所の変数名を一度に修正できます。

b2f84ff91b0e1396.png Initialize _speed to 0:


int _speed = 0;

b2f84ff91b0e1396.png Add the following getter to the Bicycle class:


int get speed => _speed;


cf1e10b838bf60ee.png Observations

  • Each variable (even if it’s a number) must either be initialized or be declared nullable by adding ? to its type declaration.



  • The Dart compiler enforces library privacy for any identifier prefixed with an underscore. Library privacy generally means that the identifier is visible only inside the file (not just the class) that the identifier is defined in.

Dart コンパイラは、アンダースコアを先頭に持つ識別子に対して、ライブラリのプライバシーを強制します。ライブラリのプライバシーとは、一般に識別子が定義されているファイル (クラスだけではありません) の中だけで識別子が見えるようにすることを意味します。

  • By default, Dart provides implicit getters and setters for all public instance variables. You don’t need to define your own getters or setters unless you want to enforce read-only or write-only variables, compute or verify a value, or update a value elsewhere.

デフォルトでは、Dart はすべてのパブリックインスタンス変数に対して暗黙のゲッターとセッターを提供します。読み取り専用や書き込み専用の変数を強制したり、値を計算したり検証したり、 別の場所で値を更新したりするのでなければ、自分でゲッターやセッターを定義する必要はありません。

  • The original Java sample provided getters and setters for cadence and gear. The Dart sample doesn’t need explicit getters and setters for those, so it just uses instance variables.


  • You might start with a simple field, such as 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 and setter is not a breaking change in Dart.

bike.cadenceのようなシンプルなフィールドから始めて、後でゲッターとセッターを使うようにリファクタリングするかもしれません。APIは同じままです(APIにとって重大な変更にはならない)。言い換えれば、フィールドからゲッターとセッターに変更しても、Dartでは壊れるような変更(breaking change)ではありません。

b2f84ff91b0e1396.png 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;

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

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

Problems?Check your code.

3. Use optional parameters (instead of overloading)

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.

Java のコードでは、コンストラクタのオーバーロードが示されています。Java では、コンストラクタは同じ名前ですが、パラメータの数または種類が異なる「overloading constructors」を使う、という一般的な慣行があります。Dart はオーバーロード コンストラクタ(overloading constructors)をサポートしておらず、このセクションで説明するように、この状況は異なる方法で処理されます。

b2f84ff91b0e1396.png Open the Rectangle example in DartPad.

Add a Rectangle constructor

b2f84ff91b0e1396.png 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.

このコンストラクタではオプショナル名前付き引数(named parameters)が使用されています。

cf1e10b838bf60ee.png 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 ({}).

origin,width,heightはオプショナル名前付き引数(optional named parameters)です。上記のように引数を中括弧( { } )で囲むとオプショナル名前付き引数になります。

  • The this.origin = const Point(0, 0) syntax specifies a default value of Point(0,0) for the origininstance variable. The specified default must be a compile-time constant. This constructor supplies default values for all three instance variables.

this.origin = const Point(0, 0)と書くことで、originについてデフォルト値を設定することになります。設定されたデフォルト値はコンパイル時定数である必要があります。このコンストラクタは三つのインスタンス変数全てにデフォルト値を設定しています。

Improve the output

b2f84ff91b0e1396.png Add the following toString() function to the Rectangle class:


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


Use the constructor

b2f84ff91b0e1396.png 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));

cf1e10b838bf60ee.png 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:

このステップではshape-creation factoryの実装について二つの方法を示します。

  • Option 1: Create a top-level function.


  • Option 2: Create a factory constructor.

Option 2:factoryコンストラクタを定義する。

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;
  num get area => pi * pow(radius, 2);

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

main() {
  final circle = Circle(2);
  final square = Square(2);

b2f84ff91b0e1396.png Open the Shapes example in DartPad.

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



cf1e10b838bf60ee.png 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.

dart:mathはDartのコアライブラリの一つです。コアライブラリには他にdart:core, dart:async, dart:convert, and dart:collectionなどがあります。

慣例により、Dartのライブラリの定数はlowerCamelCase(例えば PI の代わりに pi )が使われます。詳しくはをPREFER using lowerCamelCase for constant names.ご覧ください。

  • The following code shows 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

b2f84ff91b0e1396.png 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.';

b2f84ff91b0e1396.png 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.


cf1e10b838bf60ee.png 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 implement the Exception class to create more specific exceptions or (as in this example) you can throw a string that describes the problem encountered.

Dart SDKでは色々な例外クラスを定義しています。あるいは自分でもっと具体的なExceptionクラスを実装することもできます。あるいはこのサンプルのように、問題の内容を示す文字列をスローすることも可能です。

  • 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.


DartPad exampleもご覧ください。

  • 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.


b2f84ff91b0e1396.png 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;

b2f84ff91b0e1396.png Replace the first two lines of main() with the following code for instantiating the shapes:


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

b2f84ff91b0e1396.png Delete the shapeFactory() function that you previously added.


cf1e10b838bf60ee.png Observation

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


Problems?Check your code.

5. Implement an interface

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



b2f84ff91b0e1396.png Open the Shapes example in DartPad (or continue using your copy).


b2f84ff91b0e1396.png Add a CircleMock class that implements the Circle interface:


class CircleMock implements Circle {}

b2f84ff91b0e1396.png You should see a “Missing concrete implementations” error because CircleMock doesn’t inherit the implementationof Circle—it only uses its interface. Fix this error by defining the area and radius instance variables:

“Missing concrete implementations”が出てくると思います。なぜならCircleMockクラスはまだ、areaとradiusというメンバーを実装していないからです。

class CircleMock implements Circle {
  num area = 0;
  num radius = 0;

cf1e10b838bf60ee.png Observation

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


  • The area instance variable of CircleMock implements the area getter of Circle.


Problems?Check your code.

6. Use Dart for functional programming

In functional programming you can do things like the following:


  • 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).

複数の引数を取る関数を、それぞれが単一の引数を取る一連の関数に分解する(currying とも呼ばれる)。

  • 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).

無名関数を定数値として使う(ラムダ式と呼ばれる、ラムダ式はJDK 8リリースで導入された)

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) {

b2f84ff91b0e1396.png Open the Scream example in DartPad.

The output should look like the following:



cf1e10b838bf60ee.png Observation

  • When using string interpolation, the string ${'a' * length} evaluates to “the character 'a' repeated lengthtimes.”

文字列補間を使用する場合、文字列 ${‘a’ * length} は “文字 ‘a’ を length 回繰り返したもの” と評価されます。

Convert imperative code to functional

b2f84ff91b0e1396.png Remove the imperative for() {...} loop in main() and replace it with a single line of code that uses method chaining:



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.


b2f84ff91b0e1396.png Replace the values.map() line in main() with the following:



Run the example

The output should look like the following:



cf1e10b838bf60ee.png 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.

7. Congratulations!

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:


  • async/await, which allows you to write asynchronous code as if it were synchronous. Check out this DartPad example, which animates the calculation of the first five decimals of π.



Null aware演算子。

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

Dartの技術を実際に見てみたい方は、Flutter codelabsをお試しください。

Learn more

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