int n = 4;
int[] xArray = new int[n]; //allocate memory
int[] yArray = new int[n];
xArray[0] = 20; //first point
yArray[0] = 20;
xArray[1] = 20; //second point
yArray[1] = 80;
xArray[2] = 80; //third point
yArray[2] = 80;
xArray[3] = 80; //fourth point
yArray[3] = 20;
beginShape(POLYGON);
for(int i = 0; i < n; i++)
  vertex(xArray[i],yArray[i]); //retrieve in an ordered fashion
endShape(CLOSE);