游戏中判断一群点可否构成凸多边形

下面的代码需要box2d！！！！！！！！！！！
import org.jbox2d.common.vec2;
public class polyonutil {<br> private static polyonutil instance;<br> private float a;<br> private float b;<br> private boolean x_axis;<br> private boolean y_axis;<br> private float axis;<br> float i;
 public static polyonutil getinstance() {<br>  if (instance == null) {<br>   instance = new polyonutil();<br>  }<br>  return instance;<br> }
 public void setab(vec2 point1, vec2 point2) {<br>  if (point1.x == point2.x) {<br>   if (point1.y != point2.y) {<br>    y_axis = true;<br>    axis = point1.x;<br>   }<br>  } else if (point1.y == point2.y) {<br>   if (point1.x != point2.x) {<br>    x_axis = true;<br>    axis = point1.y;<br>   }<br>  } else {<br>   y_axis = false;<br>   x_axis = false;<br>   a = (point1.y - point2.y) / (point1.x - point2.x);<br>   b = point1.y - point1.x * a;<br>  }<br> }
 public byte getflag(vec2 point) {<br>  if (y_axis) {<br>   if (point.x > axis) {<br>    return 1;<br>   } else if (point.x < axis) {<br>    return -1;<br>   } else {<br>    return 0;<br>   }
  } else if (x_axis) {<br>   if (point.y > axis) {<br>    return 1;<br>   } else if (point.y < axis) {<br>    return -1;<br>   } else {<br>    return 0;<br>   }<br>  } else {<br>    i = point.y - a * point.x - b;<br>   if ((point.y - a * point.x - b) < 0) {<br>    return -1;<br>   } else if ((point.y - a * point.x - b) > 0) {<br>    return 1;<br>   } else {<br>    return 0;<br>   }<br>  }<br> }<br>}
//下面的函数判断一群点是否构成凸多边形
public boolean ishull(vec2[] mousestroke){
int mousestrokelength=mousestroke.length;
for (int i = 0; i < mousestrokelength; i+=2) {<br>   polyonutil.getinstance().setab(mousestroke[(i) % mousestrokelength],<br>     mousestroke[(i + 1) % mousestrokelength]);<br>   for (int j = 0, k = 0; j < mousestrokelength; j++) {<br>    if (j == i || j == (i + 1)) {<br>     continue;<br>    } else {<br>     flag[k] = polyonutil.getinstance().getflag(mousestroke[j<br>       % mousestrokelength]);<br>     k++;<br>    }<br>   }<br>   for (int l = 0; l < flag.length - 1; l++) {<br>    if (flag[l] * flag[l + 1] < 0) {<br>     // ao 当是凹多边形时候<br>          return false;<br>    }<br>   }<br>  }
return true;
}