Re: Workaround to overcome the generics limitation causes by Erasure

Daniel Pitts <>
2 May 2007 07:50:29 -0700
On May 2, 6:23 am, wrote:

Initially, I thought the introduce of generics in Java would solve me
some problem in pattern design. Here, I make use of generic to design
Observer/Subject pattern.

public interface Observer<S, A> {
    public void update(S subject, A arg);


public class Subject<S, A> {

    public void attach(Observer<S, A> observer) {

    void notify(S subject, A arg) {
        for (Observer<S, A> obs : observers) {
            obs.update(subject, arg);

    private List<Observer<S, A>> observers = new
CopyOnWriteArrayList<Observer<S, A>>();


However, due to the limitation of generics causes by erasure, I am
unable to have two different instantiations with same generics

Compiler will complain if I tend to make an Object act as more than
one type of Observer.

public class MainFrame extends javax.swing.JFrame implements

Currently, my workaround for this is using

public class MainFrame extends javax.swing.JFrame implements
org.yccheok.jstock.engine.Observer<Object, Object>

and performing instanceof checking (to check whether it is
RealTimeStockMonitor or StockHistoryMonitor) during runtime.

I had gone through severals articles :

Till now, I still do not have any better workaround, in order to have
a typed-safe Observer/ Subject pattern. Can anyone of you share your
experience, how do you overcome this situation?

Thank you very much!

public interface Observer<S, A> {
     void update(S subject, A arg);

Where does the compiler complain about here:

class MyObserver {
   public void update(Foo subject, Integer arg) {

   public void update(Bar subject, String arg) {

   public Observer<Foo, Integer> getFooObserver() {
      return new Observer<Foo, Integer>() {
        public void update(Foo subject, Integer arg) {
          MyObserver.this.update(subject, arg);
   public Observer<Bar, Integer> getBarObserver() {
      return new Observer<Bar, Integer>() {
        public void update(Bar subject, Integer arg) {
          MyObserver.this.update(subject, arg);

Hope this helps,

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