src/java.base/share/classes/java/util/Observer.java

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package java.util;

/**
 * A class can implement the {@code Observer} interface when it
 * wants to be informed of changes in observable objects.
 *
 * @author  Chris Warth
 * @see     java.util.Observable
 * @since   1.0
 *
 * @deprecated
 * This interface has been deprecated. See the {@link Observable}
 * class for further information.
 */
@Deprecated(since="9")
public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an {@code Observable} object's
     * {@code notifyObservers} method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the {@code notifyObservers}
     *                 method.
     */
    void update(Observable o, Object arg);
}