[英]How do I create a event that fires up every time a new element is added to ArrayList
我想創建一個方法,每次將新消息添加到groupchat arraylist時都會啟動。 偽代碼:
public void listenForChange(){
while(true){
if(new message is added to the groupchat){
System.out.println(print the last added message);
}
}
}
我嘗試過但無法使用的內容:
public class Groupe{
ArrayList<String> groupechat;
int liveChange;
public void listenForChange() {
while(true){
if (groupchat.size() > liveChange){
liveChange= gruppenchat.size();
System.out.println(gruppenchat.get(liveChange-1));
}
}
}
測試類別:
public class testGruppen extends Thread {
Gruppe gruppe;
public TestGroup(){
gruppe= new Gruppe("Monday");
}
public void run() {
System.out.println("listen");
gruppe.listenForChange();
}
public static void main(String[] args) {
testGruppen test = new testGruppen();
test.start();
test.gruppe.write("1"); // write just adds a new String to groupchat
test.gruppe.write("2");
test.gruppe.write("3");
test.gruppe.write("4");
}
}
輸出: 4
而不是1\\n 2\\n 3\\n 4\\n
怎樣使用裝飾器:
public static void main(String... args) {
List<Integer> group = new FireEventListDecorator<>(new ArrayList<>());
for (int i = 0; i < 3; i++)
group.add(i);
}
public static class FireEventListDecorator<E> extends AbstractList<E> {
private final List<E> delegate;
public FireEventListDecorator(List<E> delegate) {
this.delegate = delegate;
}
@Override
public void add(int index, E element) {
delegate.add(index, element);
fireEvent(element);
}
@Override
public E get(int index) {
return delegate.get(index);
}
@Override
public int size() {
return delegate.size();
}
private void fireEvent(E element) {
System.out.println("add: " + element);
}
}
為了避免CPU while (true)
輪詢時浪費while (true)
循環,請通過觀察者/偵聽器模式使用回調方法。 一種方法是給持有ArrayList的類一個PropertyChangeSupport
實例,允許它接受偵聽器,然后在更改ArrayList的方法中通知偵聽器。
例如,
public class Group {
// property listened to: ADD_TEXT
public static final String ADD_TEXT = "add text";
// the support object
private PropertyChangeSupport support = new PropertyChangeSupport(this);
private List<String> chatText = new ArrayList<>();
public void addPropertyChangeListener(PropertyChangeListener listener) {
support.addPropertyChangeListener(ADD_TEXT, listener);
}
public void addText(String text) {
String oldValue = "";
String newValue = text;
chatText.add(text + "\n");
// notify listeners
support.firePropertyChange(ADD_TEXT, oldValue, newValue);
}
}
然后可以像這樣使用它:
public class TestGroupChat {
public static void main(String[] args) {
final Group group = new Group();
group.addPropertyChangeListener(new GroupListener());
final String[] texts = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday"};
new Thread(() -> {
for (String text : texts) {
group.addText(text);
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {}
}
}) .start();
}
private static class GroupListener implements PropertyChangeListener {
@Override
public void propertyChange(PropertyChangeEvent evt) {
// call back method that is called any time the listened-to
// property has been changed
System.out.println("Notification: "+ evt.getNewValue());
}
}
}
您應該看看LinkedBlockingQueue類 。 當您想要在將新元素添加到隊列時喚醒線程時,此類非常有用。 在下面的示例中,每次將新消息添加到隊列時,線程都會打印該消息並等待下一條消息。
public class Foo extends Thread {
LinkedBlockingQueue<String> messagesQueue;
public Foo(LinkedBlockingQueue<String> messagesQueue) {
this.messagesQueue = messagesQueue;
}
@Override
public voir run() {
while(true) {
String message = messagesQueue.take();
//The thread will sleep until there is a new message in the queue.
System.out.println(message);
}
}
}
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