Adding subclass object to superclass list

Question

I have a method that takes a list of a super class (Specifically my class Mob). It looks like this:

List<? extends Mob> mobs

I want to be able to add any object which extends the super class Mob to this list, like this:

spawnMob(new Zombie(World.getInstance(), 0, 0), World.getInstance().getZombies());

Where this is the method in question:

public static void spawnMob(Mob mob, List<? extends Mob> mobs){
    mobs.add(mob);
}

This line of code World.getInstance().getZombies() returns a List of the object Zombie. Zombie extends Mob.

However, this line of code:

mobs.add(mob);

Throws this error:

The method add(capture#1-of ? extends Mob) in the type List<capture#1-of ? extends Mob> is not applicable for the arguments (Mob)

What can I do to resolve this?

Edit, after changing the method to except List<Mob> I receive this error:

The method spawnMob(Mob, List<Mob>) in the type MobSpawner is not applicable for the arguments (Zombie, List<Zombie>)

Why don't define your list as List<Mob> mobs? You can still add objects extending Mob to that list. — exception1
– exception1, Commented Feb 19, 2014 at 22:38

rgettman · Accepted Answer · 2014-02-19 22:38:00Z

5

You cannot add anything but null to a List specified with an upper bounded wildcard. A List<? extends Mob> could be of anything that extends Mob. It could be a List<Mafia> for all the compiler knows. You shouldn't be able to add a Zombie to a List that could be a List<Mafia>. To preserve type safety, the compiler must prevent such calls.

To add to such a list, you must remove the wildcard.

public static void spawnMob(Mob mob, List<Mob> mobs){

If you might have to pass in Lists with specific subclasses, then consider making the method generic:

public static <T extends Mob> void spawnMob(T mob, List<T> mobs){

answered Feb 19, 2014 at 22:38

rgettman

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Comments

Jean Logeart · Accepted Answer · 2014-02-19 22:37:44Z

1

Try with List<Mob> mobs.

answered Feb 19, 2014 at 22:37

Jean Logeart

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2 Comments

user3316633 Over a year ago

This makes sense, but I receive this error now The method spawnMob(Mob, List<Mob>) in the type MobSpawner is not applicable for the arguments (Zombie, List<Zombie>)

Jean Logeart Over a year ago

Don't change the method, just change the mobs declaration

saiarcot895 · Accepted Answer · 2014-02-19 22:38:19Z

1

You can just declare the list as List<Mob> mobs, and any subclass of Mob will be accepted. Note that when you get items from the list, you will only know for certain that they are of type Mob. You will have to do some testing to see what type it is.

answered Feb 19, 2014 at 22:38

saiarcot895

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Comments

Aragorn · Accepted Answer · 2014-02-19 22:42:24Z

0

Just create mobs list as

List<Mob> mobs;

answered Feb 19, 2014 at 22:42

Aragorn

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