This unit's readings introduce the concept of constructors, methods that share the same name as a class that are run whenever objects of that class are instantiated. Constructors allow you to define and build the World in a specific way. Each time you press the Reset button in the Greenfoot workspace, you force the World to redraw itself to its initial state, invoking the constructor methods of the actors.
In this Application, you place Actor subclasses called Dog into an empty subclass of World called DogWorld. You build a world of dogs and govern their behavior according to specific methods, some already defined and some that you define yourself. Download, unzip, and open the good-dog.zip scenario and complete each of the following exercises.
Methods that do not return a value essentially issue commands to objects. List the command methods defined for Dog, and briefly explain how the Dog object responds to each one. A full description of each method should not require more than two or three sentences. Hint: Invoke these methods by right-clicking on a Dog object.
When a method returns a value, the purpose of that method call is often (not always) to obtain information about the object. List the methods of the Dog class that return values, identify the type of value that each one returns, and briefly explain the purpose of each method. A full description of each method should not require more than two or three sentences.
Identify the instance variable(s) for Dog and briefly describe the purpose of each. Identify the initial values and describe how they are affected by the methods within the class.
Write and call the following constructor methods within the DogWorld class. Include appropriate comments to describe each method:
drawDog(int x, int y): Instantiates a Dog object at horizontal location x and vertical location y.
randomDogs(int population): Introduces a number of Dog objects into the World at random locations. The total number of objects is dictated by the parameter population.
Design the default DogWorld to contain three Dog objects at random locations and one player-controllable Dog object precisely at the center of the DogWorld.
Write a method (and call the method) within the appropriate class that allows you to control the Dog object in the center of DogWorld according to the following control scheme. Assume that the top of the screen represents north, the right side of the screen represents east, the bottom of the screen represents south, and the left side of the screen represents west:
Up arrow: Move north
Down arrow: Move south
Right arrow: Move east
Left arrow: Move west
Up + right arrow pressed simultaneously: Move northeast
Up + left arrow pressed simultaneously: Move northwest
Down + right arrow pressed simultaneously: Move southeast
Down + left arrow pressed simultaneously: Move southwest
Write code that forces the Dog to greet the stationary Dog objects it encounters as it moves around.
Write a loop that subtracts one from the Dog's energy for every 30 steps that the Dog takes.
Make the player-controlled Dog move more slowly whenever the method isTired returns a value of true.
Force the player-controlled Dog to fall asleep when its energy reaches 0, and once it wakes up, set its energy to 100.
Make your own modifications to the program, according to the following specifications:
Create a new instance variable for the Dog object that in some way governs the Dog's actions. (Consider exercise 9 above as an example.)
Introduce a new Actor class with which the player-controlled Dog has meaningful interaction. In other words, the new class should affect one of the Dog's instance variables and/or cause the Dog to invoke a command method.
Create a new method that includes an if/else statement that invokes one of the unused methods listed in exercise 1 above.
When you have satisfied the requirements of exercises 4-11 in this Application, open the File menu of Greenfoot and select Close. This saves your finished scenario.