Java Games Continued: Fun with Polygons - Pro Android Games - page 83

These are the most important methods for drawing shapes in the Canvas class, but we need more.
For example, for a polygon-based game, a basic technique is to inscribe shapes within rectangles. In this
way, operations such as collision detection and rendering can be applied. Thus, we need three basic
shapes for our Asteroids game that are missing from the Android API: a rectangle, a polygon, and a
polygon sprite.

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Tip The Android API defines the classes Rect and RectF for rectangles, but it does not define polygons or
polygon sprites.

Understanding the Caveats of Drawing Rectangles
From the drawing methods in the previous section, we could use Canvas.drawLines to draw our polygons
directly. However, we also need extra functionally:

A way to detect if a polygon is inside another in the (X, Y) coordinate system,
which is necessary to detect collisions within the game

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A way to assign style and color to the polygon
Thus, using Canvas.drawLines is insufficient to manipulate polygons directly. We need a more
elegant solution; we need to create Rectangle , Polygon , and PolygonSprite classes.
Android already has the classes Rect and RectF that could be reused. Nevertheless these classes lack
the functionality to check if the rectangle is inscribed within another (a critical requirement for
Asteroids).
Listing 4-1 shows the class Rectangle capable of remembering its (X, Y) coordinates plus width and
height. It can also check if its bounds contain or are inside another rectangle. As a matter of fact this
code is taken from the Java SE java.awt.Rectangle class (taking advantage of the high portability of
Java).
The Rectangle class in Listing 4-1 is the basis for the polygon sprite we need for Asteroids.

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Listing 4-1. The Rectangle Class Used by Polygon

package ch04.common;

public class Rectangle {

public int x;
public int y;
public int width;
public int height;

public Rectangle() {
this(0, 0, 0, 0);
}