Using Observable/Observer with two different double variables?

Question

I have a class that monitors the stock market. It holds 2 values (doubles) the daily high and the daily low. I want to monitor these variables from another class and take action if either changes. (i.e. change a limit order)

So, I have a class LiveOrderBook extends Observable and two methods inside that update the price:

public void setDailyLow(double price){
    low = price;
    setChanged();
    notifyObservers(low);
}

public void setDailyHigh(double price){
    high = price;
    setChanged();
    notifyObservers(high);
}

I need to observe these price variables so I made a class PriceObserver implements Observer. My plan is to create PriceObserver objects inside my Bid class that changes stock market bids.

My PriceObserver class

private double low;
private double high;

public PriceObserver(){

    low = 0;
    high = 0;
}

public void update(Observable arg0, Object arg1) {
    // TODO Auto-generated method stub
}

How do I now specify which double should be updated? I can't check if arg0 == the variable name from the other class, so how is this done?

Answer 1

An easy (and useful) approach is to first create different event classes that can get dispatched:

public class LowPriceChangedEvent {
    private double price;
    // Constructor and getter up to you.
}

public class HighPriceChangedEvent {
    private double price;
    // Constructor and getter up to you.
}

Now you can dispatch these events in your LiveOrderBook class:

public void setDailyLow(double price){
    low = price;
    setChanged();
    notifyObservers(new LowPriceChangedEvent(low));
}

public void setDailyHigh(double price){
    high = price;
    setChanged();
    notifyObservers(new HighPriceChangedEvent(low));
}

Your PriceObserver now easily can distinguish the events by doing a simple instanceOf check:

public class PriceObserver implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        if (arg instanceOf LowPriceChangedEvent) {
            ...
        } else  if (arg instanceOf HighPriceChangedEvent) {
            ...
        } else {
            ...
        }
    }
}