Creating multiple subclass objects

Question

I'm trying to achieve a 'wave' of enemies through a for loop. Basically when a wave object is called it accepts an int that sets the number of enemies in the wave. Each enemy has it's own class that is a subclass of 'Enemy'. What I'm stuck on is how I can go about passing in a second parameter in the wave constructor to set which enemy subclass is created eg 25 'Orcs' created or 13 'Trolls' in one method. Any help will be greatly appreciated.

I'm not so sure what you mean but, you can make your constructor accepts two parameter ie: public void Enemy(int a, int b) — noob
– noob, Commented Feb 16, 2013 at 12:26

Hovercraft Full Of Eels · Accepted Answer · 2013-02-16 12:33:00Z

1

It sounds like you want to create a static factory method of the Enemy class that creates new Enemy objects based on parameter. Something like:

// EnemyType is an enum
public static Enemy createEnemy(EnemyType enemyType) {
   switch (enemyType) {
     case BASIC_MONSTER:
       return new BasicMonster();
     case ORC:
       return new Orc();
     case TROLL:
       return new Troll();
     case ..... // etc...
   }
}

Note, I would use something cleaner for the parameter such as an enum, not an int so as to be sure that the parameter passed in is correct. Otherwise you risk having a nonsense int such as -24232 being passed in.

answered Feb 16, 2013 at 12:33

Hovercraft Full Of Eels

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Boris the Spider · Accepted Answer · 2013-02-16 12:48:13Z

You can use an Enum

public enum EnemyType {
  ORC{
   @override
   public Enemy create() {
    return new Orc();
   }
  },
  TROLL{
   @override
   public Enemy create() {
    return new Troll();
   }
  }...etc;


  public abstract Enemy create();
}

Then pass the relevant enum into your wave method:

public Collection<Enemy> createWave(final int num, final EnemyType enemyType) {
 final Collection<Enemy> enemies = new ArrayList<>(num);
 for(int i=0;i<num;i++) {
  enemies.put(enemyType.create());
 }
 return enemies;
}

If you have lots of differenet enemy types consider a generic factory

public interface EmemyFactory<E extends Enemy> {
 E create();
}

Then create an implementation for each enemy type and store them in the enum instead

public enum EnemyType {
  ORC(new OrcFactory()),
  TROLL(new TrollFactory()),
  ...etc;

  private final EnemyFactory enemyFactory;
  public EnemyType(final EnemyFactory enemyFactory) {
   this.enemyFactory = enemyFactory;
  }

  public Enemy create() {
   return enemyFactory.create();
  }
}

And last and least you could use a little reflection, assuming your Enemies have a noargs constructor:

public Collection<Enemy> createWave(final int num, final Class<? extends Enemy> enemyClass) {
 final Collection<Enemy> enemies = new ArrayList<>(num);
 for(int i=0;i<num;i++) {
  enemies.put(enemyClass.newInstance());
 }
 return enemies;
}

This is messy and prone to runtime errors however...

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