.add() returns a boolean in Java, how to return as a list

Question

I've been reading about this for a while now and I can't figure it out. Consider these classes, both in their own file.

public class World {
    private List<Entity> entities = new ArrayList<>();

    public List<Entity> getEntities() {
        return entities;
    }

    public void setEntities(List<Entity> entities) {
        this.entities = entities;
    }
}

public Class Entity { 
    public Entity() {
        List<Entity> modifiedList = World.getEntities().add(this); // 1
        World.setEntities(modifiedList);
    }
}

There's a type mismatch: I cannot convert from boolean to List (1) How would I solve this? How could you possibly convert a boolean to a List? Also, the concept static confuses me. If anyone can direct me to some light, and accessible read about static vs non-static, please post it in the comments!

Why don't you just store the result of getEntities in the variable modifiedList, then invoke add on modifiedList? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Nov 16, 2015 at 17:36
@SotiriosDelimanolis Isn't that just moving the problem from line 1 to line 2? — Bram Vanroy
– Bram Vanroy, Commented Nov 16, 2015 at 17:38
Also, you need to realize that calling World.setEntities() is useless. All you need here is World.getEntities().add(this); — JB Nizet
– JB Nizet, Commented Nov 16, 2015 at 17:38
Forgetting, for the moment, the static issue ... as it is implemented, there is only one List in your code above. There is no need to return a "new" List, because there is no new List. The call to getEntities() returns the actual List that is in your World. It does not make a copy in any way. Adding items to this list adds them to this same List, the one that is in the World. Ergo, there is no need to return a new List, as you are modifying the only List in your program. — dcsohl
– dcsohl, Commented Nov 16, 2015 at 18:16
That's not a feature of Lists, it's how java's references to objects (such as Lists) work: stackoverflow.com/questions/40480/… — zapl
– zapl, Commented Nov 16, 2015 at 18:53

Community · Accepted Answer · 2017-05-23 11:59:02Z

You've declared a class

public class World {
    private List<Entity> entities = new ArrayList<>();

    public List<Entity> getEntities() {
        return entities;
    }

    public void setEntities(List<Entity> entities) {
        this.entities = entities;
    }
}

which declares two instance methods, getEntities and setEntities. These are instance methods, so you need an instance to invoke them. That's what that duplicate was saying.

World world = new World();

World#getEntities has a return type of List. List has an add method with a return type of boolean, where the value of the boolean typically indicates whether the value was added or not. If you don't need that information, ignore the returned value

List<Entity> entities = world.getEntities();
entities.add(this);

getEntities returns a reference to the same object referenced by the field entities in the object referenced by world in my example above. There's no point invoking setEntities to re-assign it.

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