Minecraft's target selectors (e.g. @a or @e) are implemented using the EntitySelectorArgument class. This allows you to select specific entities based on certain attributes.

The EntitySelectorArgument constructor requires an EntitySelector argument to determine what type of data to return. There are 4 types of entity selections which are available:

• EntitySelector.ONE_ENTITY - A single entity, which returns a Entity object.
• EntitySelector.MANY_ENTITIES - A collection of many entities, which returns a Collection<Entity> object.
• EntitySelector.ONE_PLAYER - A single player, which returns a Player object.
• EntitySelector.MANY_PLAYERS - A collection of players, which returns a Collection<Player> object.

The return type is the type to be cast when retrieved from the Object[] args in the command declaration.

//LinkedHashMap to store arguments for the command
LinkedHashMap<String, Argument> arguments = new LinkedHashMap<>();

//Using a collective entity selector to select multiple entities
arguments.put("entities", new EntitySelectorArgument(EntitySelector.MANY_ENTITIES));

CommandAPI.getInstance().register("kill", arguments, (sender, args) -> {
    
    //Parse the argument as a collection of entities (as stated above in the documentation)
    Collection<Entity> entities = (Collection<Entity>) args[0];
    sender.sendMessage("killed " + entities.size() + "entities");
    for(Entity e : entity)
        e.remove();
});

Example command usage for the above code would be:

• Kill all cows:

  /kill @e[type=cow]
  

• Kill the 10 furthest pigs from the command sender:

  /kill @e[type=pig,limit=10,sort=furthest]
  

The PlayerArgument class is very similar (almost identical) to EntitySelectorArgument, with the EntitySelector ONE_PLAYER. It also allows you to select a player based on their UUID.

Developer's Note:

I've not tested the PlayerArgument enough to recommend using it over the EntitySelectorArgument(EntitySelector.ONE_PLAYER). There may be other advantages to using this than the regular EntitySelectorArgument, but as of writing this documentation, I know not of the advantages nor disadvantages to using this argument type. Internally, the PlayerArgument uses the GameProfile class from Mojang's authlib, which may be able to retrieve offline players (untested).

The EntityTypeArgument class is used to retrieve a type of entity as defined in the EntityType enum. In other words, this is an entity type, for example a pig or a zombie.

LinkedHashMap<String, Argument> arguments = new LinkedHashMap<>();

arguments.put("entity", new EntityTypeArgument());
arguments.put("amount", new IntegerArgument(1, 100)); //Prevent spawning too many entities

CommandAPI.getInstance().register("spawnmob", arguments, (sender, args) -> {
    Player player = (Player) sender;
    for(int i = 0; i < (int) args[1]; i++) {
        player.getWorld().spawnEntity(player.getLocation(), (EntityType) args[0]);
    }
});