Framework for creating Java learning exercises based on 2-D grids.
There are many introductory programming assignments that involve objects in a two-dimensional data structure. They include games, like tic-tac-toe, checkers, and chess; maze programs; simulations, like Conway's Game of Life or the AP® Marine Biology Simulation (MBS) case study; and simpler programs that use a grid as graph paper for drawing histograms or bit-mapped drawings. These projects lend themselves to graphical representations, but the overhead involved in implementing graphical user interfaces for such assignments, especially interfaces that support user interaction, is non-trivial. The Grid Package provides a set of simple Java classes for modeling objects in a two-dimensional grid, and provides a library of other classes that make it easy to create graphical user interfaces to display and interact with such models.
The Grid Package was inspired by, and evolved from, the AP® Marine Biology Simulation (MBS) case study. The MBS case study introduced a two-dimensional data structure called an Environment, which represents the marine environment (lake, bay, or pond) for fish in a simulation. The Grid Package introduces a similar Grid data structure that models a two-dimensional grid made up of rows and columns and a GridObject class that represents objects in a grid. Each cell in a grid may be empty or may contain one GridObject object. A GridObject object keeps track of its own row/column location in the grid and provides methods for checking and changing an object's location. Subclasses of the GridObject class represent different kinds of objects that can be placed in a grid, each of which may have different behavior.
The example below creates a simple grid with two objects (subclasses of GridObject) in it.
Grid grid = new BoundedGrid(3, 3);
grid.add(new TextCell("A"), new Location(0, 0));
grid.add(new ColorBlock(Color.RED), new Location(2, 2));
The Grid Package supports five basic types of graphical user interfaces for applications that use grid objects. An application might have:
- a simple GUI containing a display of the contents of a grid (Figure 2a in the DraftOverview document),
- a simple GUI containing an animated display of the changing contents of a grid, with a slider bar to control the speed of the animation (Figure 2b in the DraftOverview),
- an interactive GUI that allows users to control the behavior of the application by clicking on control buttons (Figure 2c in the DraftOverview),
- a specialized interactive GUI that operates on "stepped" applications (applications such as simulations that run in discrete steps), with control buttons such as Start/Restart, Step, or Run (Figure 2d in the DraftOverview), or
- an interactive GUI that allows users to control the behavior of the application by clicking on grid cells (the appearance of this kind of GUI is similar to those above).
The heart of any graphical user interface for a grid-based application is the display of the grid contents. The Grid Package provides a ScrollableGridDisplay class that knows how to display a grid. The application, though, needs to specify how to display the individual objects in the grid. Several display classes, such as ColorBlockDisplay, TextCellDisplay, and DefaultDisplay (displays a question mark), are provided in the Grid Package. Furthermore, there are several classes for displaying images, such as ScaledImageDisplay, that provide an easy way to display other kinds of grid objects without writing graphics code.
The DraftOverview.html document is a draft overview of the Grid Package. There are sections that are incomplete (and possibly sections that are somewhat out-of-date), but it still provides a detailed introduction to the Model, View, and Controller components of the package, and provides examples and instructions for using the package to create passive grid applications (e.g., a histogram), animated grid applications (e.g., moving a queen on a chessboard), grid applications driven by control buttons (e.g., New Grid and Reset buttons), generating buttons automatically from the methods of a class, creating grid applications driven by Step/Run/Stop control buttons, and creating grid applications driven by mouse clicks in grid cells.