Adapted from Cay Horstmann — thanks!

Critters are included in GridWorld to emphasize design. The act method calls five methods, and subclasses override some or all of them to achieve some desired behavior. (For those of you interested in digging deeper into programming and software engineering, the act method is an example of the “template method” design pattern. Design patterns are a powerful way of expressing common best-practices about code structure. Google it!)

When to use Critter

Not every actor can or should be represented as a
Critter. In the context of GridWorld, a Critter is not a warm and fuzzy
creature, but an actor that first processes actors and then makes a move. Trying to make Actors that don’t follow this pattern into Critters will generally result in overly-complicated logic.

Critter rules

Here are a few simple rules to remember when working with Critters (these don’t apply to bugs or other Actors!). They will be discussed in a little more detail below.

• Don’t override act
• A critter can change its state only in processActors or
  makeMove
• A critter can change the state of other actors in processActors

Critter methods

Each of the five methods called from act has postconditions that restrict what it can do. The following table shows the postconditions as they are stated in the GridWorld documentation.

Here, the state of an actor includes

• the location
• the direction
• the contents of any other instance variables

If a critter wants to change its own state, it can only do so in processActors and makeMove. If it wants to change
its location (or remove itself), it can only do so in makeMove (Removing an actor changes its state: its location becomes null).

If a critter wants to change the state of another critter, it can only do so in processActors. (There is a teensy exception: makeMove removes the critter at the target location, and it may add a new critter to the old location.)

Note that getActors, getMoveLocations, and selectMoveLocation can’t change the state of any actor.

Also note that processActors can only mutate actors that were handed to it, and selectMoveLocations can only select from the locations that were handed to it. The makeMove method has even less choice—it must move to the location given in the
parameter.

These conditions significantly restrict what a critter can do, and how it can do it. When you design a particular critter, you need to distribute
responsibilities among the five methods that are called by act.

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