Hi

Some of the code on the page https://www.dukascopy.com/wiki/#Threading is unhelpful.

As written, this code doesn't work if you pass in variable values. In the example you pass in hard-coded values, but this is hardly useful in real-life.

My understanding is that to work with parameters passed to the constructor you need to declare the class variables as final:

private final Instrument instrument;

You could have saved at least one Java newbie an hour of head-scratching if the example actually worked in real-life. Java threading is hard enough to get your head around without misleading examples!

Hope you find this useful.

private class BuyTask implements Callable<Object>{
        private Instrument instrument;
        private double stopLossPips;            
        
        public BuyTask(Instrument instrument, double stopLossPips){
                this.instrument = instrument;
                this.stopLossPips = stopLossPips;
}
                        
        public Object call() throws Exception {
                double stopLossPrice = history.getLastTick(this.instrument).getBid() - this.stopLossPips * this.instrument.getPipValue();
                return engine.submitOrder("Buy_order", this.instrument, OrderCommand.BUY, 0.001, 0, 20, stopLossPrice, 0);
        }
    }

Do you mean configurable parameters? The example does not contain any configurable parameters. If you modify the strategy code of course it may not work as described or as you expect.
The examples are not written to be universal, rather the opposite - they are there to show the idea - the simpler the implementation the better.
This is the case with anonymous classes and when the variable is defined in the respective method. The particular example has its Callable implemented in a separate class, i.e., BuyTask. Thus, this is not necessary in this case.

For further inquiries please provide full example strategy that you have come up to so far, describe what you expect the strategy to do and what it does instead (i.e., what you find being wrong). Also please provide specific launch conditions (if any).

A note: this is an instance variable, not a class variable.

With the class as written in the docs:

private class BuyTask implements Callable<Object>{
        private Instrument instrument;
        private double stopLossPips;            
         
        public BuyTask(Instrument instrument, double stopLossPips){
                this.instrument = instrument;
                this.stopLossPips = stopLossPips;
}
                         
        public Object call() throws Exception {
                double stopLossPrice = history.getLastTick(this.instrument).getBid() - this.stopLossPips * this.instrument.getPipValue();
                return engine.submitOrder("Buy_order", this.instrument, OrderCommand.BUY, 0.001, 0, 20, stopLossPrice, 0);
        }
    }

This works:

BuyTask task = new BuyTask(Instrument.EURUSD, 40);
context.executeTask(task);  

But this doesn't:

Double pips = 40;
BuyTask task = new BuyTask(Instrument.EURUSD, pips);
context.executeTask(task);  

In other words, it only works if the parameter is hard coded, as I said.

To make the second call work, you need to rewrite the class as:

private class BuyTask implements Callable<Object>{
        private Instrument instrument;
        private final double stopLossPips;            
         
        public BuyTask(Instrument instrument, double stopLossPips){
                this.instrument = instrument;
                this.stopLossPips = stopLossPips;
}
                         
        public Object call() throws Exception {
                double stopLossPrice = history.getLastTick(this.instrument).getBid() - this.stopLossPips * this.instrument.getPipValue();
                return engine.submitOrder("Buy_order", this.instrument, OrderCommand.BUY, 0.001, 0, 20, stopLossPrice, 0);
        }
    }

I agree that you should be keeping your examples simple, but when it's too simple to work in any real-life situation, and is difficult to convert to real-life use without quite deep knowledge of Java idioms, I would still be suggesting that you have got the balance wrong here.

This does not even compile - it yields: "Type mismatch: cannot convert from int to Double". You can't assign a java.lang.Double a numeric value. Probably you wanted to use double instead of Double. For automatic assistance on compile-time errors, consider using some IDE, e.g., Eclipse or NetBeans.

The variable being final or not does not make any difference in the behaviour of the program, see our attached program.

P.S. For further inquiries please always provide full example strategies. Otherwise we need to speculate if there are any parts of the strategy that cause the unexpected behaviour that have not been included in the snippets.

Well, I'm a total beginner with Java so I could very well be misunderstanding something.

All I can say is that whenever I passed in a variable to the Callable constructor and placed it into an instance variable, call() couldn't see it unless the variable was final.

I fixed the problem with the advice on this thread in StackOverflow about how to pass values into a Callable class. Please note how all the replies use final.

https://stackoverflow.com/questions/9992992/is-there-a-way-to-take-an-argument-in-a-callable-method

Then how would you explain that our example given in the previous post does work without the field variable being final? Until you provide a full example strategy for the case that you report, we can't check where the actual problem was.