Simple Problem with Rendering

I’m having trouble with Rendering using JOGL.

I am trying to create a program that reads a string of characters, drawing something different for each character. For example, if the program reads ‘F’ then I want to draw a line. Am I supposed to do ALL drawing inside the display() function? And if so, I tried using a while loop that exited after all the characters were read inside the display function and for some reason my drawing only appears half the time that I execute the program. (e.g. every time I press run, only once in awhile I will see the line.) I assume it has something to do with the display() function getting called in a loop…take a look at my code:

public void display(GLAutoDrawable glDrawable) {
		
final GL gl = glDrawable.getGL();
 GLU glu = new GLU();
 gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);

		      gl.glLoadIdentity();
		      gl.glTranslatef(0, 0, -6.0f);
		      
		    gl.glColor3f(1.0f, 0.0f, 0.0f);
		     while(i < l.sLength()){
		  	gl.glBegin(GL.GL_LINES);
		    	 switch(l.readNext()){
		  		
		  		case 'F':
		  			up += 0.5f;
		  	gl.glVertex3f(curX, curY, 0.0f);
		        gl.glVertex3f(0.0f, up, 0.0f);	
		  			break;
		  		case '+':
		  			System.out.println("+");
		  			break;
		  		case '-':
		  			System.out.println("-");
		  			break;
		  		}
		  		i++;
		  		curY += up;
		  		}
		      gl.glEnd();
		  

I apologize if that formatted incorrectly!