Actor collided = getOneIntersectingObject(Platform.class); //make a variable(in this case collided) of type actor and make it an intersecting object of the type you are running into
if(collided == null)//if you have not run into it
{
   fall(); //fall, example setLocation(getX(),getY()-5
}

private boolean atTop()
    {
        int blue = getWorld().getColorAt(getX(),getY()).getBlue(); //The amount of blue at the characters x and y
        int green = getWorld().getColorAt(getX(),getY()).getGreen(); //The amount of green at the characters x and y
        if(blue > green) //If there is more blue than green
        {
            return true; //you are at the top
        }
        else
        {
            return false; //otherwise you arent
        }
    }

collided = getOneObjectAtOffset(getX(),getY()+20,Platform.class)//if there is an object offsetting you by 20 pixels below you

if(collided != null)//do the same as before
{
      etc.
}

private static final int MAX_SPEED = 5; //declare a variable called MAX_SPEED, static final referring to as unchangeable and in all of that class, to save memory to any number you want that looks good(in this case 5)

if(speed >= MAX_SPEED)//If speed is greater or equal to MAX_SPEED
{
      speed = MAXSPEED;//set your speed to max
}