How to create multiple lists with common elements in java

Question

Lets say I have list of Scenarios in a Game, and lists of Players. Each player plays some scenario from the Scenario list in Game. I don't want to have copy of Scenario in Player, I want to have something like pointer to the list in Game object, so that if any scenario is edited, it is also edited in Player list.

Game
Scenarios : list

Player
Scenarios : list -> each scenario here is a reference to scenario in Game

Now It would be simple in language supporting pointers, but Java is supposedly pass-by-value. At the same time, I know there are hacks using passing values inside tables etc. Is it possible to do this or not?

Adam Arold · Accepted Answer · 2013-11-04T12:59:48.417

What have you tried? If you would have tried putting the reference into separate lists you would have known that they are the same reference. The term "pass-by-value" is tricky here because you will get a reference to the same object in your methods for example but java passes around copies of references. So basically java passes around values of references.

If you put the same reference in all your lists the changes to the original will be reflected in them.

SomeObject o = new SomeObject();

list0.add(o);
list1.add(o);
list2.add(o);

o.setSomeValue(true);

// all lists will "see" the changes

list0.get(0).equals(o); // assuming list0 only contains o -> this is true

As a rule of thumb: if you don't mess (swap, assign, etc.) with references and only call methods on the objects the references point to you'll be fine.

I'm currently planning a bigger (for me) project, and was wondering whether this is possible or if I should try to implement it differently. Also, I would try to get the someObject from list0, instead of creating one object and then sharing it, would it make a difference? — Xyzk, Nov 04 '13 at 12:55

Maxim Shoustin · Answer 2 · 2013-11-04T13:01:09.567

1st way

I would use interface in your case. Something like:

public class Scenario implements  ScenarioItf{/**/}

And the Player will store only ScenarioItf.

2nd way

To store Scenarios list into Factory class:

public class ScenariosFactory {

    private static List<Scenarios> mScenarios = null;


    public static void create(List<Scenarios> scenarios) {
        mScenarios = scenarios;
    }


    public static List<Scenarios> getFactory(){
        if ( mScenarios == null )
            throw new NullPointerException( "...null..." );
        return mScenarios;
    }
}

So both classes Player and Game can use the same source.

score 1 · Answer 3 · answered Nov 04 '13 at 12:58

In your case, if Player object has a Scenario object, not the whole scenario will be copied (or cloned), only the reference of Scenario will be copied. And you can modify the Scenario either through reference by Player or through the List; as it is one object many references.

René Jensen · Answer 4 · 2013-11-04T13:01:29.267

0

Java is pass-by-reference, so if you have a Scenario object in the list AND in the Player object, they will both be a pointer to the actual Scenario object in memory.

Edit: Apparently, it's sort of by-reference but technically not really. As long as you don't go scenario = new Scenario() at either end, it will be the same object.

edited Nov 04 '13 at 13:01

answered Nov 04 '13 at 12:55

René Jensen

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3
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http://stackoverflow.com/questions/40480/is-java-pass-by-reference appears to disagree – Xyzk Nov 04 '13 at 12:57
It's Pass-by-value, but the value is a reference (so it's rather Pass-by-value-of-reference). But in this case the effect is what you described. – Axel Nov 04 '13 at 12:59
This is false. Check out my explanation in my answer. – Adam Arold Nov 04 '13 at 13:01
@RomanVottner If you change the *state* (aka modify its members) of an object passed as a parameter, you will see the effect in the invoking routine. But if you bind the reference to another object, it will have no effect in the invoking method. A famous example for the difference between Java and languages that support pass-by-reference is that implementing a generic swap()-method isn't possible in Java (you should find lots of info on this via google). – Axel Nov 04 '13 at 13:18
I've always HATED the statement, "Java is pass by value." If it was, then you could not see modifications to an object in two different scopes. With the exception of primitives, the "value" being passed is a *reference* to an object, i.e. passing the reference. So I consider it pass by reference with an asterisk for primitives. – MadConan Nov 04 '13 at 13:53
@MadConan While pass-by-value is confusing, just pretending it is pass-by-reference is misleading, because if it were pass-by-reference this would be possible: ` void swap(T a, T b) { T tmp=a; a=b; b=t; }`... – Axel Nov 04 '13 at 14:04

How to create multiple lists with common elements in java

4 Answers4