Passing and setting booleans - Java/Android

Question

I have a class like so:

public class Achievements(){

    boolean score100_Earned_Offline;
    boolean score1000_Earned_Offline;

    final String score100 = "xxxxxxxxxx" //where xxxxxxxxxx would be replaced with the achievement's Google Play Games ID
    final  String score1000 = "xxxxxxxxxx" //where xxxxxxxxxx would be replaced with the achievement's Google Play Games ID
}

In my Game class, I check the state of the achievements every tick and act on them as necessary like so (assume all methods to be valid and defined - this is cut down to provide the code necessary to the question).......

public class Game(){

    public void checkAchievements(Achievements achievements){

        if (score>=100){
            unlockAchievement(achievements.score100, achievements.score100_Earned_Offline);     
        }

        if (score>1000){
            unlockAchievement(achievements.score100, achievements.score1000_Earned_Offline);  
        }
    }

    public void unlockAchievement(String achievementToUnlock, boolean thisAchievementOfflineFlag){

        //If user is signed in, then we are ready to go, so go ahead and unlock the relevant achievement....
        if (checkSignedIn()){
            Games.Achievements.unlock(getApiClient(), achievementToUnlock);
        //Otherwise, I want to do is set the relevant flag to true so it can be checked when the user does eventually log in
        else{
            thisAchievementOfflineFlag=true;
        }
    }
}

Pass by value

In the 'unlockAchievement' method, the boolean 'thisAchievementOfflineFlag' does get set to true if the user is not logged in, however, it doesn't effect the actual boolean that was originally sent into the method (which as you can see is defined in my 'Achievements' class). I'm guessing this is because Java is Pass by Value and is therefore, creating a local copy of the variable which is valid inside the method only. I did try using Boolean too (wrapper class) but got the same results.

Other ways to achieve this?

I've currently got it set up so I can define each achievement as an enum so each one will have it's own copy of the boolean flag. However, I'm aware that it's not recommended to use enums in Android so if there is a better way that I am missing, I would rather avoid them.

Please note that I don't want to use if checks or switch statements as this is taking place in a game-loop.

Any suggestions appreciated

Answer 1

This is all because Java's implementation of Boolean (also, for example String) is immutable for safety reasons. You can see it here: http://www.explain-java.com/is-string-mutable-in-java-why-wrapper-classes-immutable-in-java/

You can solve your problem by introducing an object wrapper for that boolean:

public class BooleanWrapper {

    private boolean value;

    public void set(boolean value) {
        this.value = value;
    }

    public boolean get() {
        return value;
    }
}

Now, this object reference will be passed by value but will still point to the same BooleanWrapper object on the heap . You can simply use getters and setters to change the inner boolean value.

Then your code would become:

public void unlockAchievement(String achievementToUnlock, BooleanWrapper thisAchievementOfflineFlag){
    if (checkSignedIn()){
        Games.Achievements.unlock(getApiClient(), achievementToUnlock);
    else {
        thisAchievementOfflineFlag.set(true);
    }
}

---

Java is pass-by-value:

When you pass boolean then you for sure passed it by value, while it is a primitive type. When you pass Boolean , you would think it's an object and that you can change it's state, but actually you cannot because Boolean is implemented as an immutable object (as already said). You can confirm this just by reading the code of java.lang.Boolean .

But if you create your own wrapper class, and in a sense, you control whether you implement it in immutable or mutable way. BooleanWrapper I wrote lets you change the state of that object. And when you pass an object such as this one to the method, it's passed by value. That means that another reference is created, but it points to the same object on heap (see image below).

Answer 2

您可以使用AtomicBoolean ，它具有按引用传递的语义。