What are anonymous classes in Java and where can they be applied?

Anonymous classes in Java are local classes without a name. They are declared and instantiated simultaneously. They are typically used when a one-time implementation of an interface or extension of an abstract or regular class is needed.

Uses of anonymous classes:

• Event handlers in GUI: Often used to create handlers for button clicks or other user actions.

  // Example with Swing ActionListener
  JButton button = new JButton("Click me");
  button.addActionListener(new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent e) {
          System.out.println("Button clicked!");
      }
  });
  

• Implementation of interfaces with a single abstract method (SAM interfaces): Before lambda expressions, this was the main way to implement functionality for such interfaces.

  // Example with Runnable
  new Thread(new Runnable() {
      @Override
      public void run() {
          System.out.println("Thread started");
      }
  }).start();
  

• Creating objects "on the fly": When you need to quickly create an object with modified behavior from an existing class without creating a separate named class.

  // Example of extending an abstract class
  AbstractList<String> cityList = new AbstractList<String>() {
      private String[] cities = {"Moscow", "Saint Petersburg"};
  
      @Override
      public String get(int index) {
          return cities[index];
      }
  
      @Override
      public int size() {
          return cities.length;
      }
  };
  System.out.println(cityList.get(0)); // Outputs: Moscow
  

Anonymous classes have access to final or effectively final local variables in the scope where they are declared. They cannot have static members and cannot be abstract or final.