[SalesForce] Best way to implement a singleton class in Apex

I am trying to create a singleton class for one of my apex classes and i am running into challenges. I created some static variables to control the instances but still i am running into multiple instances as static works within the execution context.

See the static boolean variable i use to control the instance. But when i call getInstance, i always end up creating a new instance each time. Can anybody tell me what i am doing wrong and how to create a guaranteed singleton class?

public with sharing class Helper_PK_ScreenDriver {
    private static  Helper_PK_ScreenDriver instance ;
    private static  boolean isHelperInstantiated = false;
}

public static Helper_PK_ScreenDriver getInstance(String module,String entityId)
{
    System.debug('Helper pk screen driver instance'+isHelperInstantiated + 'instance' +instance);
    if(isHelperInstantiated)
    {
        return instance;
    }
    else
    {
        //This part gets executed everytime i call this from outside..
        instance = new Helper_PK_ScreenDriver(module,entityId);
        isHelperInstantiated = true;
        return instance;
    }   
}

Best Answer

If you need a static, stateful singleton, there is no such thing in Apex. That's because static in Apex does not mean the same thing as static in Java. In Apex, none of your Apex can remain in memory after a request, and none of your Apex can share memory. Static is request-scoped.

If you can provide some more detail on what you're trying to achieve, we can try to help you solve your problem; in my experience, genuinely stateful singletons are a rare requirement that almost always have some alternative.

The general solution I'd recommend that most closely resembles a stateful singleton, and allows you to pass information between requests, is some combination of an abstract utility class with static methods and persisting state via either a custom object instance or a custom setting. Locking could be an issue depending on how fancy your state needs to be.

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[SalesForce] Difficulties with Appleman’s Trigger Classes

This recent blog post by Hari Krishnan might be helpful. He's taken the trigger architecture and used it as a foundation in his framework. It's downloadable and might help to more cleanly separate concerns for you.

But as pbattisson mentioned, the volume of code you posted makes determining the issue difficult. I'd suggest starting fresh with new bare-bones triggers/classes that only solve the specific issue you're having with extra code execution (DML etc.) Or install Hari's unmanaged app in your org and see if it solves the problem.

Also I noticed you're doing an after insert / after update. If you plan on changing Opportunity records you might consider doing this on before update/insert and avoid the extra DML altogether.

Good luck!

[SalesForce] Simple Apex class to return a list of strings

You can't instantiate a List like that using an integer. You don't need to explicitly specify how much items are going into the list when it is created. Instead, just remove the integer:

//Instantiate the list
String[] myArray = new List<String>();

You're logic in your loop seems to be incorrect as well. Specifically, myArray's size will always be 0 when you instantiate it. Change your loop logic to:

for(Integer i=0;i<length;i++) {
   // code
}

You want to generate a List of strings based on the i variable. You will want to change your internal logic to:

//Populate the array
myArray.add('Test ' + i);

// Write value to the debug log
System.debug(myArray[i]);

Finally, you want to return that Array. You need to change your method so the return type is no longer void by changing its signature:

public static String[] generateStringArray(Integer length)

then it is just a matter of returning that array at the end of your method:

return myArray;

This would bring it all together as:

public class StringArrayTest {
    //Public Method
    public static String[] generateStringArray(Integer length) {
        //Instantiate the list
        String[] myArray = new List<String>();

        //Iterate throught the list
        for(Integer i=0;i<length;i++) {
            //Populate the array
            myArray.add('Test ' + i);

            // Write value to the debug log
            System.debug(myArray[i]);
        } //end loop

        return myArray;
    }//end method       
}// end class

Some more info on Arrays and Lists, Loops, and Class methods for future reference.

Best Answer

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[SalesForce] Simple Apex class to return a list of strings

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