int x = 0;
int y = 0;

boolean state = true;
void setup()
{
size(500,500);  
}

void draw()
{
background(122,122,122);
if (state == true)
{
if (x < 450 && y < 450)
{
rect(x,y,50,50);
x+=5;
}

if (x == 450 && y < 450 )
{
rect(x,y,50,50);
y+=5;
}

if (y == 450)
{
rect(x,y,50,50);
x-=5;
}

}

if (x == 0)
{
state = false;
rect(x,y,50,50);
y-=5;
}

}