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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package java.util;
/**
* Observable is used to notify a group of Observer objects when a change
* occurs. On creation, the set of observers is empty. After a change occurred,
* the application can call the {@link #notifyObservers()} method. This will
* cause the invocation of the {@code update()} method of all registered
* Observers. The order of invocation is not specified. This implementation will
* call the Observers in the order they registered. Subclasses are completely
* free in what order they call the update methods.
*
* @see Observer
*/
public class Observable {
List<Observer> observers = new ArrayList<Observer>();
boolean changed = false;
/**
* Constructs a new {@code Observable} object.
*/
public Observable() {
}
/**
* Adds the specified observer to the list of observers. If it is already
* registered, it is not added a second time.
*
* @param observer
* the Observer to add.
*/
public void addObserver(Observer observer) {
if (observer == null) {
throw new NullPointerException();
}
synchronized (this) {
if (!observers.contains(observer))
observers.add(observer);
}
}
/**
* Clears the changed flag for this {@code Observable}. After calling
* {@code clearChanged()}, {@code hasChanged()} will return {@code false}.
*/
protected void clearChanged() {
changed = false;
}
/**
* Returns the number of observers registered to this {@code Observable}.
*
* @return the number of observers.
*/
public int countObservers() {
return observers.size();
}
/**
* Removes the specified observer from the list of observers. Passing null
* won't do anything.
*
* @param observer
* the observer to remove.
*/
public synchronized void deleteObserver(Observer observer) {
observers.remove(observer);
}
/**
* Removes all observers from the list of observers.
*/
public synchronized void deleteObservers() {
observers.clear();
}
/**
* Returns the changed flag for this {@code Observable}.
*
* @return {@code true} when the changed flag for this {@code Observable} is
* set, {@code false} otherwise.
*/
public boolean hasChanged() {
return changed;
}
/**
* If {@code hasChanged()} returns {@code true}, calls the {@code update()}
* method for every observer in the list of observers using null as the
* argument. Afterwards, calls {@code clearChanged()}.
* <p>
* Equivalent to calling {@code notifyObservers(null)}.
*/
public void notifyObservers() {
notifyObservers(null);
}
/**
* If {@code hasChanged()} returns {@code true}, calls the {@code update()}
* method for every Observer in the list of observers using the specified
* argument. Afterwards calls {@code clearChanged()}.
*
* @param data
* the argument passed to {@code update()}.
*/
@SuppressWarnings("unchecked")
public void notifyObservers(Object data) {
int size = 0;
Observer[] arrays = null;
synchronized (this) {
if (hasChanged()) {
clearChanged();
size = observers.size();
arrays = new Observer[size];
observers.toArray(arrays);
}
}
if (arrays != null) {
for (Observer observer : arrays) {
observer.update(this, data);
}
}
}
/**
* Sets the changed flag for this {@code Observable}. After calling
* {@code setChanged()}, {@code hasChanged()} will return {@code true}.
*/
protected void setChanged() {
changed = true;
}
}
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