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edited Apr 13, 2017 at 12:40

Community Bot

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edited Jan 8, 2015 at 21:30

Jamal

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Does javaJava need an Observable object with generics?

asked Jan 8, 2015 at 21:04

hkoosha

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This is the follow-up question from here

This is a Observable class similar to java.util.Observable. difference is, it uses generics to avoid casts. There are two questions

• Is it really worth the effort? or it's just making things more complex than needed?
• Should this class be declared final? or it's ok to be subclassed? java.util.Observable is not final.

The code:

package cc.koosha.silkroad.lang; 
import java.util.ArrayList;
import java.util.Collection;
/**
 * like java.util.Observable, But uses generics to avoid need for a cast.
 * For any un-documented variable, parameter or method, see java.util.Observable
 */
public class Observable<T> {
 public interface Observer<T> {
 public void update(Observable observable, T args);
 }
 /**
 * Holds registered observers.
 * @TODO if class is not final can we make this protected? (should we?).
 */
 private final Collection<Observer<? super T>> registry;
 private boolean changed = false;
 /**
 * @TODO should this be private and the class final?.
 */
 protected Observable(final Collection<Observer<? super T>> registry) {
 this.registry = registry;
 }
 /**
 * Static constructor.
 */
 public static <F> Observable<F> getInstance() {
 ArrayList<Observer<? super F>> container = new ArrayList<Observer<? super F>>();
 return new Observable<F>(container);
 }
 // @TODO Should o be checked for null?
 public void addObserver(final Observer<? super T> o) {
 synchronized (this.registry) {
 if (!this.registry.contains(o))
 this.registry.add(o);
 }
 }
 
 protected void update(final T args) {
 ArrayList<Observer<? super T>> toNotify;
 synchronized (this.registry) {
 if (!changed)
 return;
 toNotify = new ArrayList<Observer<? super T>>(this.registry);
 this.changed = false;
 }
 // Look at OpenJDK to see why a new Collection is used.
 for (Observer<? super T> o : toNotify)
 o.update(this, args);
 }
 public void notifyObservers(final T args) {
 this.update(args);
 }

Methods `deleteObserver`, `deleteObservers`, `setChanged`, `clearChanged`, ` hasChanged`, `countObservers`, `notifyObservers` are removed for brevity. *the rest of code at [GitHub](https://github.com/loolooyyyy/GenericObservable/blob/master/src/main/java/cc/koosha/silkroad/lang/Observable.java)*

java generics observer-pattern