How to add listener on ArrayList in java

问题:

I want to create my own implementation of ArrayList in java, that can listen when the list is changing and to do action when this happens. From what I have read, I understand that I can't extend ArrayList and then add listener.

I want to use MyList in class as a variable with public modifier, so users can change it directly and to be done action when he changes it.

class MyList extends ArrayList<object>.... {  ... }
 class UseOfMyList {
 public MyList places = new MyList<Object>();
 places.add("Buenos Aires");
 //and to be able to do that
 List cities = new ArrayList<Object>();
 cities.add("Belmopan");
 places = cities;

So how to create and when do add,remove or pass another list to MyList an action to be performed?

回答1:

You're not going to be able to do this by extending ArrayList, as it has no built-in notification mechanism (and, further, because it is has been declared final and thus cannot be extended). However, you can achieve your desired result by creating your own List implementation and adding your "listener" functionality vis a vis the add() and remove() methods:

class MyList<T>{
    private ArrayList<T> list;

    public MyList(){
        list = new ArrayList<>();
        ...
    }
    public void add(T t){
        list.add(t) 
        //do other things you want to do when items are added 
    }
    public T remove(T t){
        list.remove(t);
        //do other things you want to do when items are removed
    }
}

回答2:

the resp. ;)

private class MyList extends ArrayList<Objects> {

      @Override
      public void sort(Comparator c) {
        super.sort(c); 
        resetLancamentos(); // call some metod ;)
      }
    //...
     @Override
     public boolean removeAll(Collection c) {
        //To change body of generated methods, choose Tools | Templates.
        boolean ret = super.removeAll(c);
        resetLancamentos(); // some metod like fireObjChanged() will do the job too
         return ret;
     }

}