How to use Observer Pattern in java to detect change in Java List?

Question

I want to detect change in my ArrayList myList. Everytime I add, remove or update anything from ArrayList. I want to notify user about it. I have used following code for it. It notifies user when setValues function is used to set list. Is there any other way to implement it so that every time I change list user will get notified? Thank you.

// observable class which will contain my List. 
package com.psl;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Observable;

public class MyList extends Observable{

      private List<Map<String,Object>> values;    
    public List<Map<String, Object>> getValues() {
        return values;
    }

    public void setValues(List<Map<String, Object>> values) {

        if(getValues()==null && values!=null){

            setChanged();
            notifyObservers();
        }

        else if( !this.values.equals(values)){
            setChanged();
            notifyObservers();
        }

        this.values = values;
    }

    public static void main(String[] args) {

        MyList myList = new MyList();
        List<Map<String, Object>> values = new ArrayList<Map<String, Object>>();
        Notify notify = new Notify();
        myList.addObserver(notify);
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("string_value", null);
        myList.setValues(values);                       
    }


}
// Observer which will notify user when list is updated. 

package com.psl;

import java.util.Observable;
import java.util.Observer;

public class Notify implements Observer{

    @Override
    public void update(Observable o, Object arg) {
            System.out.println("List has been changed");                
    }

}

If reinventing the wheel is not absolutely required, and assuming you have not considered it or are unaware of its existence, perhaps ObservableList is suitable? — Abra
– Abra, Commented Jun 19, 2019 at 9:50
The Observer pattern is considered obsolete, and the relevant Java classes aren't much practical use, as you have to extend Observable, which ArrayList and friends don't. Consider PropertyChangeListener/Support. — user207421
– user207421, Commented Dec 8, 2019 at 6:32

eltabo · Accepted Answer · 2019-06-19 09:49:19Z

4

JavaFX has an implementation of ObservableList, maybe you can use it:

javafx.collections.ObservableList a = javafx.collections.FXCollections.observableArrayList();
a.addListener(new javafx.collections.ListChangeListener<String>() {
    @Override
    public void onChanged(Change<? extends String> c) {
        System.out.println(c);
    }});

a.add("aa");
a.add("bb");

answered Jun 19, 2019 at 9:49

eltabo

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Comments

Marat · Accepted Answer · 2019-06-19 10:22:35Z

You can use inheritance and two own interfaces. (Because we can't extend 2 classes)

import java.util.ArrayList;
import java.util.List;

public class ObserverExample {
    public static void main(String[] args) {
        MyList<Integer> myList = new MyList<>();
        Notify notify = new Notify();
        myList.addObserver(notify);
        myList.add(1);
    }
}


interface Observable {
    void notifyObservers();
    void addObserver(Observer o);
}

interface Observer {
    void update(Observable o, Object arg);
}


class MyList<E> extends ArrayList<E> implements Observable{
    List<Observer> observers = new ArrayList<>();

    @Override
    public boolean add(E e) {
        notifyObservers();
        return super.add(e);
    }

    @Override
    public void notifyObservers() {
        for (Observer o :observers) {
            o.update(this, null);
        }
    }

    @Override
    public void addObserver(Observer o) {
        observers.add(o);
    }
}

class Notify implements Observer{
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("List has been changed");
    }
}

You also can override other list methods and add notifyObservers(); into each of them.

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