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| Chapter 30 The java.util Package |

## 30.16 java.util.Observer (JDK 1.0)

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This interface defines the `update()` method required for an object to "observe" subclasses of `Observable`. An `Observer` registers interest in an `Observable()` by calling the `Observable.addObserver()` method. `Observer` objects that have been registered in this way will have their `update()` method invoked by the `Observable` when that object has changed.

    public abstract interface Observer {
        // Public Instance Methods
                public abstract void update(Observable o, Object arg);
    }

### Passed To:

Observable.addObserver(), Observable.deleteObserver()

java.util.Observable (JDK 1.0) |  | java.util.Properties (JDK 1.0)