用多边形开发Java游戏：小行星游戏实现指南

### 用多边形开发 Java 游戏：小行星游戏实现指南 #### 1. 为小行星游戏创建多边形类 标准的 J2SE API 中有很实用的 `Polygon` 类，可用于开发小行星游戏。遗憾的是，Google 未将 `java.awt` 中的这些实用类纳入 Android API。以下是修改后的 `Polygon` 类，它是 Java SE `Polygon` 类的精简版本，适用于 Android 和小行星游戏。 ##### 1.1 `Polygon` 类的主要方法 - **构造函数**：默认构造函数创建一个四边形。也可通过提供代表顶点 X 和 Y 坐标的两个数组以及边数来定义一个 N 边形。 - **calculateBounds(int xpoints[], int ypoints[], int npoints)**：根据一组 X 和 Y 坐标以及点数，计算多边形的边界。 - **addPoint(int x, int y)**：根据给定的 X 和 Y 坐标，为多边形添加一个点。 - **updateBounds(int x, int y)**：更新多边形的边界，使其包含新点 (X, Y)。 - **getBoundingBox()**：返回多边形的矩形边界。 - **contains(int x, int y)**：检查多边形是否包含给定的点 (X, Y)。 - **float[] getPoints()**：返回多边形顶点的 X 和 Y 坐标。 ```java package ch04.common; public class Polygon { public int npoints; public int[] ypoints; public int[] xpoints; protected Rectangle bounds; public Polygon() { xpoints = new int[4]; ypoints = new int[4]; } public Polygon(int xpoints[], int ypoints[], int npoints) { if (npoints > xpoints.length || npoints > ypoints.length) { throw new IndexOutOfBoundsException( "npoints > xpoints.length || npoints > ypoints.length"); } this.npoints = npoints; this.xpoints = new int[npoints]; this.ypoints = new int[npoints]; System.arraycopy(xpoints, 0, this.xpoints, 0, npoints); System.arraycopy(ypoints, 0, this.ypoints, 0, npoints); } void calculateBounds(int xpoints[], int ypoints[], int npoints) { int boundsMinX = Integer.MAX_VALUE; int boundsMinY = Integer.MAX_VALUE; int boundsMaxX = Integer.MIN_VALUE; int boundsMaxY = Integer.MIN_VALUE; for (int i = 0; i < npoints; i++) { int x = xpoints[i]; boundsMinX = Math.min(boundsMinX, x); boundsMaxX = Math.max(boundsMaxX, x); int y = ypoints[i]; boundsMinY = Math.min(boundsMinY, y); boundsMaxY = Math.max(boundsMaxY, y); } bounds = new Rectangle(boundsMinX, boundsMinY, boundsMaxX - boundsMinX, boundsMaxY - boundsMinY); } public void reset() { npoints = 0; bounds = null; } public void addPoint(int x, int y) { if (npoints == xpoints.length) { int tmp[]; tmp = new int[npoints * 2]; System.arraycopy(xpoints, 0, tmp, 0, npoints); xpoints = tmp; tmp = new int[npoints * 2]; System.arraycopy(ypoints, 0, tmp, 0, npoints); ypoints = tmp; } xpoints[npoints] = x; ypoints[npoints] = y; npoints++; if (bounds != null) { updateBounds(x, y); } } void updateBounds(int x, int y) { if (x < bounds.x) { bounds.width = bounds.width + (bounds.x - x); bounds.x = x; } else { bounds.width = Math.max(bounds.width, x - bounds.x); } if (y < bounds.y) { bounds.height = bounds.height + (bounds.y - y); bounds.y = y; } else { bounds.height = Math.max(bounds.height, y - bounds.y); } } public Rectangle getBoundingBox() { if (npoints == 0) { return new Rectangle(); } if (bounds == null) { calculateBounds(xpoints, ypoints, npoints); } return bounds; } public boolean contains(int x, int y) { if (npoints <= 2 || !getBoundingBox().contains(x, y)) { return false; } int hits = 0; int lastx = xpoints[npoints - 1]; int lasty = ypoints[npoints - 1]; int curx, cury; for (int i = 0; i < npoints; lastx = curx, lasty = cury, i++) { curx = xpoints[i]; cury = ypoints[i]; if (cury == lasty) { continue; } int leftx; if (curx < lastx) { if (x >= lastx) { continue; } leftx = curx; } else { if (x >= curx) { continue; } leftx = lastx; } double test1, test2; if (cury < lasty) { if (y < cury || y >= lasty) { continue; } if (x < leftx) { hits++; continue; } test1 = x - curx; test2 = y - cury; } else { if (y < lasty || y >= cury) { continue; } if (x < leftx) { hits++; continue; } test1 = x - lastx; test2 = y - lasty; } if (test1 < (test2 / (lasty - cury) * (lastx - curx))) { hits++; } } return ((hits & 1) != 0); } @Override public String toString() { if (npoints == 0) return null; String s = ""; for (int i = 0; i < xpoints.length; i++) { s += "(" + xpoints[i] + "," + ypoints[i] + ") "; } return s; } public float[] getPoints() { int size = npoints * 4; float[] points = new float[size]; int j = 1; if (size == 0 || xpoints == null || ypoints == null) return null; points[0] = xpoints[0]; points[1] = ypoints[0]; for (int i = 2; i < points.length - 2; i += 4) { points[i] = xpoints[j]; points[i + 1] = ypoints[j]; points[i + 2] = xpoints[j]; points[i + 3] = ypoints[j]; j++; } poi ```