int[] x = {150, 60,280, 20,240,150, -1, 20,280,280, 20, 20};
int[] y = { 20,280,100,100,280, 20, -1, 20, 20,280,280, 20};

void setup() {
  size(300,300);
  background(255);
}

void draw() {
  for( int i = 0 ; i < x.length-1 ; i++ ) {
    if(0 <= x[i] && 0 <= x[i+1] ) {
      stroke(0); // black
      line(x[i],y[i],x[i+1],y[i+1]);
    }
  }
}