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Java anonymous classes this

Содержание
  1. Anonymous Classes
  2. Declaring Anonymous Classes
  3. Syntax of Anonymous Classes
  4. Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class
  5. Examples of Anonymous Classes
  6. Java anonymous classes this
  7. Learn Latest Tutorials
  8. Preparation
  9. Trending Technologies
  10. B.Tech / MCA
  11. Javatpoint Services
  12. Training For College Campus
  13. Anonymous Inner Class in Java
  14. Java anonymous classes this

Anonymous Classes

Anonymous classes enable you to make your code more concise. They enable you to declare and instantiate a class at the same time. They are like local classes except that they do not have a name. Use them if you need to use a local class only once.

This section covers the following topics:

Declaring Anonymous Classes

While local classes are class declarations, anonymous classes are expressions, which means that you define the class in another expression. The following example, HelloWorldAnonymousClasses , uses anonymous classes in the initialization statements of the local variables frenchGreeting and spanishGreeting , but uses a local class for the initialization of the variable englishGreeting :

public class HelloWorldAnonymousClasses < interface HelloWorld < public void greet(); public void greetSomeone(String someone); >public void sayHello() < class EnglishGreeting implements HelloWorld < String name = "world"; public void greet() < greetSomeone("world"); >public void greetSomeone(String someone) < name = someone; System.out.println("Hello " + name); >> HelloWorld englishGreeting = new EnglishGreeting(); HelloWorld frenchGreeting = new HelloWorld() < String name = "tout le monde"; public void greet() < greetSomeone("tout le monde"); >public void greetSomeone(String someone) < name = someone; System.out.println("Salut " + name); >>; HelloWorld spanishGreeting = new HelloWorld() < String name = "mundo"; public void greet() < greetSomeone("mundo"); >public void greetSomeone(String someone) < name = someone; System.out.println("Hola, " + name); >>; englishGreeting.greet(); frenchGreeting.greetSomeone("Fred"); spanishGreeting.greet(); > public static void main(String. args) < HelloWorldAnonymousClasses myApp = new HelloWorldAnonymousClasses(); myApp.sayHello(); >>

Syntax of Anonymous Classes

As mentioned previously, an anonymous class is an expression. The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code.

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Consider the instantiation of the frenchGreeting object:

HelloWorld frenchGreeting = new HelloWorld() < String name = "tout le monde"; public void greet() < greetSomeone("tout le monde"); >public void greetSomeone(String someone) < name = someone; System.out.println("Salut " + name); >>;

The anonymous class expression consists of the following:

  • The new operator
  • The name of an interface to implement or a class to extend. In this example, the anonymous class is implementing the interface HelloWorld .
  • Parentheses that contain the arguments to a constructor, just like a normal class instance creation expression. Note: When you implement an interface, there is no constructor, so you use an empty pair of parentheses, as in this example.
  • A body, which is a class declaration body. More specifically, in the body, method declarations are allowed but statements are not.

Because an anonymous class definition is an expression, it must be part of a statement. In this example, the anonymous class expression is part of the statement that instantiates the frenchGreeting object. (This explains why there is a semicolon after the closing brace.)

Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class

Like local classes, anonymous classes can capture variables; they have the same access to local variables of the enclosing scope:

  • An anonymous class has access to the members of its enclosing class.
  • An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.
  • Like a nested class, a declaration of a type (such as a variable) in an anonymous class shadows any other declarations in the enclosing scope that have the same name. See Shadowing for more information.
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Anonymous classes also have the same restrictions as local classes with respect to their members:

  • You cannot declare static initializers or member interfaces in an anonymous class.
  • An anonymous class can have static members provided that they are constant variables.

Note that you can declare the following in anonymous classes:

  • Fields
  • Extra methods (even if they do not implement any methods of the supertype)
  • Instance initializers
  • Local classes

However, you cannot declare constructors in an anonymous class.

Examples of Anonymous Classes

Anonymous classes are often used in graphical user interface (GUI) applications.

Consider the JavaFX example HelloWorld.java (from the section Hello World, JavaFX Style from Getting Started with JavaFX). This sample creates a frame that contains a Say ‘Hello World’ button. The anonymous class expression is highlighted:

import javafx.event.ActionEvent; import javafx.event.EventHandler; import javafx.scene.Scene; import javafx.scene.control.Button; import javafx.scene.layout.StackPane; import javafx.stage.Stage; public class HelloWorld extends Application < public static void main(String[] args) < launch(args); >@Override public void start(Stage primaryStage) < primaryStage.setTitle("Hello World!"); Button btn = new Button(); btn.setText("Say 'Hello World'"); btn.setOnAction(new EventHandler() @Override public void handle(ActionEvent event) System.out.println("Hello World!"); > >); StackPane root = new StackPane(); root.getChildren().add(btn); primaryStage.setScene(new Scene(root, 300, 250)); primaryStage.show(); > >

In this example, the method invocation btn.setOnAction specifies what happens when you select the Say ‘Hello World’ button. This method requires an object of type EventHandler . The EventHandler interface contains only one method, handle. Instead of implementing this method with a new class, the example uses an anonymous class expression. Notice that this expression is the argument passed to the btn.setOnAction method.

Because the EventHandler interface contains only one method, you can use a lambda expression instead of an anonymous class expression. See the section Lambda Expressions for more information.

Anonymous classes are ideal for implementing an interface that contains two or more methods. The following JavaFX example is from the section Customization of UI Controls. The highlighted code creates a text field that only accepts numeric values. It redefines the default implementation of the TextField class with an anonymous class by overriding the replaceText and replaceSelection methods inherited from the TextInputControl class.

import javafx.application.Application; import javafx.event.ActionEvent; import javafx.event.EventHandler; import javafx.geometry.Insets; import javafx.scene.Group; import javafx.scene.Scene; import javafx.scene.control.*; import javafx.scene.layout.GridPane; import javafx.scene.layout.HBox; import javafx.stage.Stage; public class CustomTextFieldSample extends Application < final static Label label = new Label(); @Override public void start(Stage stage) < Group root = new Group(); Scene scene = new Scene(root, 300, 150); stage.setScene(scene); stage.setTitle("Text Field Sample"); GridPane grid = new GridPane(); grid.setPadding(new Insets(10, 10, 10, 10)); grid.setVgap(5); grid.setHgap(5); scene.setRoot(grid); final Label dollar = new Label("$"); GridPane.setConstraints(dollar, 0, 0); grid.getChildren().add(dollar); final TextField sum = new TextField() @Override public void replaceText(int start, int end, String text) if (!text.matches("[a-z, A-Z]")) super.replaceText(start, end, text); > label.setText("Enter a numeric value"); > @Override public void replaceSelection(String text) if (!text.matches("[a-z, A-Z]")) super.replaceSelection(text); > > >; sum.setPromptText("Enter the total"); sum.setPrefColumnCount(10); GridPane.setConstraints(sum, 1, 0); grid.getChildren().add(sum); Button submit = new Button("Submit"); GridPane.setConstraints(submit, 2, 0); grid.getChildren().add(submit); submit.setOnAction(new EventHandler() < @Override public void handle(ActionEvent e) < label.setText(null); >>); GridPane.setConstraints(label, 0, 1); GridPane.setColumnSpan(label, 3); grid.getChildren().add(label); scene.setRoot(grid); stage.show(); > public static void main(String[] args) < launch(args); >>

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Anonymous Inner Class in Java

Nested Classes in Java is prerequisite required before adhering forward to grasp about anonymous Inner class. It is an inner class without a name and for which only a single object is created. An anonymous inner class can be useful when making an instance of an object with certain “extras” such as overriding methods of a class or interface, without having to actually subclass a class.

Tip: Anonymous inner classes are useful in writing implementation classes for listener interfaces in graphics programming.

The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code.

// Test can be interface,abstract/concrete class Test t = new Test() < // data members and methods public void test_method() < . . >>;

Now let us do discuss the difference between regular class(normal classes) and Anonymous Inner class

  • A normal class can implement any number of interfaces but the anonymous inner class can implement only one interface at a time.
  • A regular class can extend a class and implement any number of interfaces simultaneously. But anonymous Inner class can extend a class or can implement an interface but not both at a time.
  • For regular/normal class, we can write any number of constructors but we can’t write any constructor for anonymous Inner class because the anonymous class does not have any name and while defining constructor class name and constructor name must be same.

Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class

Like local classes, anonymous classes can capture variables; they have the same access to local variables of the enclosing scope:

  • An anonymous class has access to the members of its enclosing class.
  • An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.
  • Like a nested class, a declaration of a type (such as a variable) in anonymous class shadows any other declarations in the enclosing scope that have the same name.

Anonymous classes also have the same restrictions as local classes with respect to their members:

  • We cannot declare static initializers or member interfaces in an anonymous class.
  • An anonymous class can have static members provided that they are constant variables.
  • Fields
  • Extra methods (even if they do not implement any methods of the supertype)
  • Instance initializers
  • Local classes

Anonymous inner classes are generic created via below listed two ways as follows:

Now let us take an example with which we will understand anonymous inner class, let us take a simple program

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Java anonymous classes this

У меня в идее все декларирует(компиляторо не ругается) я декларировал и переменные статические и создавал статические методы, другой вопрос что статику из анонимного класса не достать. Почему автор так написал? Может я что то не так понял? Есть ответы: Внутренний анонимный класс может содержать статические переменные и методы c версии Java 16! Статические методы или переменные достать можно, просто нужно обратиться к ним в main методе внутри воложеного класса: System.out.println(staticInt); Иначе как сделать извлечение статик элементов из анонимного класса я не придумал.

Предыдущая статья про нестатические вложенные классы (1/3 часть): https://javarush.com/groups/posts/2181-vlozhennihe-vnutrennie-klassih Продолжение про статические вложенные классы (3/3 часть): https://javarush.com/groups/posts/2183-staticheskie-vlozhennihe-klassih

вот кое что не понятно. Тут написано: «Если каждому из наших анонимных классов-модулей понадобится какое-то отличающееся поведение, свои специфические методы, которых нет у других, мы легко можем дописать их:»

 MonitoringSystem generalModule = new MonitoringSystem() < @Override public void startMonitoring() < System.out.println("Мониторинг общих показателей стартовал!"); >public void someSpecificMethod() < System.out.println("Специфический метод только для первого модуля"); >>;

Вопрос. А имеет смысл такие методы вообще писать (даже если пропустит компилятор)? будет ли вообще хоть какая-нибудь возможность достучаться до специфического метода? Ведь данный метод существует лишь в анонимном классе, имя которого нам не известно. Объект анонимного класса хранится в переменной типа интерфейса. Данному интерфейсу ничего не известно о данном методе (который есть только у наследника — в анонимном классе). И компилятор по этой причине не позволит к нему обратиться. Более того, мы не сможем создать ссылочную переменную типа анонимного класса и перекинуть туда объект, чтобы вызвать данный метод — по причине того, что нам банально не известно имя анонимного класса, чтобы создать ссылочную переменную данного типа. Я прав?

Супер лекция. Еще бы компоновали ссылки в конце по смежным темам(здесь например по всем типам вложенных классов). А то искать через поисковик неудобно. Или я чего-то не догоняю.

 import java.util.Scanner; interface Eatable < public void eat(); >public class Test < public static final Scanner menu = new Scanner(System.in); public static final String whatWouldULikeToEat = menu.nextLine(); public static void main(String[] args) < Eatable eatable = new Eatable() < @Override public void eat() < System.out.printf("Well, I would like to eat %s, please.", whatWouldULikeToEat); >public String receipt() < return whatWouldULikeToEat; >>; eatable.eat(); System.out.println("\nOkay, sir."); > >

Важный момент в том, что дополнительные методы, помимо тех, что мы переопределяем, нельзя использовать извне этого анонимного класса, а только внутри него. Т.е. в примере:

 MonitoringSystem generalModule = new MonitoringSystem() < @Override public void startMonitoring() < System.out.println("Мониторинг общих показателей стартовал!"); >public void someSpecificMethod() < System.out.println("Специфический метод только для первого модуля"); >>;

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