Package java.util

## Interface Observer

-     public interface Observer

  A class can implement the `Observer` interface when it wants to be informed of changes in observable objects.

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `update​(Observable o, Object arg)` | This method is called whenever the observed object is changed.

- - ### Method Detail

    - #### update

          void update​(Observable o,
                      @Nullable
                      Object arg)

      This method is called whenever the observed object is changed. An application calls an `Observable` object's `notifyObservers` method to have all the object's observers notified of the change.
      Parameters:
        `o` - the observable object.
        `arg` - an argument passed to the `notifyObservers` method.