Anonymous classes¶

Basics¶

Anonymous classes work the same as local classes. The only difference is that anonymous classes:

have no name
should be declared if we only need them once

Anonymous class syntax¶

Creating an anonymous class object is almost identical to creating a regular object. The difference is that when creating an object, all the required methods are implemented. The expression consists of:

the new operator or the use of lambda in the case of [interface] (md-interface) for a function
the name of the interface we implement or the abstract class we inherit
constructor parameters (in the case of an interface we use an empty constructor)
class/interface body

Within the anonymous class, we can declare:

fields (including static fields)
methods (including static methods)
constants

However, you cannot declare:

constructors
interfaces
static initialization blocks

The following example shows how to implement an anonymous class using the new keyword and then using a lambda:

public interface ClickListener {
  void onClick();
}

public class UIComponents {
    void showComponents() {
        // an anonymous class implementation using the new keyword
        ClickListener buttonClick = new ClickListener() {
            @Override
            public void onClick() {
                System.out.println("On Button click!");
            }
        };
        // end of anonymous class implementation

        buttonClick.onClick();

        // an anonymous class implementation using lambda
        // this is possible because ClickListener is a functional interface, ie it has one method to implement
        ClickListener checkboxClick  = () -> System.out.println("On Checkbox click!");
        checkboxClick.onClick();
    }
}

The next example shows the additional capabilities of an anonymous class implemented using the new keyword:

public interface ClickListener {
  void onClick();
}

void showComponentsV2() {
    // anonymous class implementation
    ClickListener buttonClick = new ClickListener() {

      private String name; // field in anonymous class
      private static final String BUTTON_CLICK_MESSAGE ="On Button click!"; // static field in anonymous class

      public void sayHello() { // method implementation in anonymous class
        System.out.println("I am new method in anonymous class");
      }

      @Override
      public void onClick() {
        sayHello();
        System.out.println(BUTTON_CLICK_MESSAGE);
      }
    };

  }

Access to parent classes¶

Anonymous classes can reference parent class fields and local variables as long as they are final. As with nested classes, variables with the same names as the parent class fields override their properties.