# Java

**Usage examples:** The Observer pattern is pretty common in Java code, especially in the GUI components. It provides a way to react to events happening in other objects without coupling to their classes.

Here are some examples of the pattern in core Java libraries:

* [`java.util.Observer`](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)/[`java.util.Observable`](http://docs.oracle.com/javase/8/docs/api/java/util/Observable.html) (rarely used in real world)
* All implementations of [`java.util.EventListener`](http://docs.oracle.com/javase/8/docs/api/java/util/EventListener.html) (practically all over Swing components)
* [`javax.servlet.http.HttpSessionBindingListener`](http://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionBindingListener.html)
* [`javax.servlet.http.HttpSessionAttributeListener`](http://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionAttributeListener.html)
* [`javax.faces.event.PhaseListener`](http://docs.oracle.com/javaee/7/api/javax/faces/event/PhaseListener.html)

**Identification:** The pattern can be recognized if you see a subscription method that stores incoming objects in a list. You can confirm the identification, if you see some sort of notification method that iterates over objects in that list and calls their “update” method.

### Event subscription <a href="#example-0-title" id="example-0-title"></a>

In this example, the Observer pattern establishes indirect collaboration between objects of a text editor. Each time the `Editor` object changes, it notifies its subscribers. `EmailNotificationListener` and `LogOpenListener` react to these notifications by executing their primary behaviors.

Subscriber classes aren’t coupled to the editor class and can be reused in other apps if needed. The `Editor` class depends only on the abstract subscriber interface. This allows adding new subscriber types without changing the editor’s code.

#### &#x20;**publisher**

&#x20;**publisher/EventManager.java: Basic publisher**

```java
package refactoring_guru.observer.example.publisher;

import refactoring_guru.observer.example.listeners.EventListener;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class EventManager {
    Map<String, List<EventListener>> listeners = new HashMap<>();

    public EventManager(String... operations) {
        for (String operation : operations) {
            this.listeners.put(operation, new ArrayList<>());
        }
    }

    public void subscribe(String eventType, EventListener listener) {
        List<EventListener> users = listeners.get(eventType);
        users.add(listener);
    }

    public void unsubscribe(String eventType, EventListener listener) {
        List<EventListener> users = listeners.get(eventType);
        users.remove(listener);
    }

    public void notify(String eventType, File file) {
        List<EventListener> users = listeners.get(eventType);
        for (EventListener listener : users) {
            listener.update(eventType, file);
        }
    }
}
```

#### &#x20;**editor**

&#x20;**editor/Editor.java: Concrete publisher, tracked by other objects**

```java
package refactoring_guru.observer.example.editor;

import refactoring_guru.observer.example.publisher.EventManager;

import java.io.File;

public class Editor {
    public EventManager events;
    private File file;

    public Editor() {
        this.events = new EventManager("open", "save");
    }

    public void openFile(String filePath) {
        this.file = new File(filePath);
        events.notify("open", file);
    }

    public void saveFile() throws Exception {
        if (this.file != null) {
            events.notify("save", file);
        } else {
            throw new Exception("Please open a file first.");
        }
    }
}
```

#### &#x20;**listeners**

&#x20;**listeners/EventListener.java: Common observer interface**

```java
package refactoring_guru.observer.example.listeners;

import java.io.File;

public interface EventListener {
    void update(String eventType, File file);
}
```

&#x20;**listeners/EmailNotificationListener.java: Sends emails upon receiving notification**

```java
package refactoring_guru.observer.example.listeners;

import java.io.File;

public class EmailNotificationListener implements EventListener {
    private String email;

    public EmailNotificationListener(String email) {
        this.email = email;
    }

    @Override
    public void update(String eventType, File file) {
        System.out.println("Email to " + email + ": Someone has performed " + eventType + " operation with the following file: " + file.getName());
    }
}
```

&#x20;**listeners/LogOpenListener.java: Writes a message to a log upon receiving notification**

```java
package refactoring_guru.observer.example.listeners;
ja
import java.io.File;

public class LogOpenListener implements EventListener {
    private File log;

    public LogOpenListener(String fileName) {
        this.log = new File(fileName);
    }

    @Override
    public void update(String eventType, File file) {
        System.out.println("Save to log " + log + ": Someone has performed " + eventType + " operation with the following file: " + file.getName());
    }
}
```

&#x20;**Demo.java: Initialization code**

```java
package refactoring_guru.observer.example;

import refactoring_guru.observer.example.editor.Editor;
import refactoring_guru.observer.example.listeners.EmailNotificationListener;
import refactoring_guru.observer.example.listeners.LogOpenListener;

public class Demo {
    public static void main(String[] args) {
        Editor editor = new Editor();
        editor.events.subscribe("open", new LogOpenListener("/path/to/log/file.txt"));
        editor.events.subscribe("save", new EmailNotificationListener("admin@example.com"));

        try {
            editor.openFile("test.txt");
            editor.saveFile();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
```

&#x20;**OutputDemo.txt: Execution result**

```
Save to log \path\to\log\file.txt: Someone has performed open operation with the following file: test.txt
Email to admin@example.com: Someone has performed save operation with the following file: test.txt
```