Understanding Jogl Window Frame - opengl

Question

I just ran my first java program running JOGL. I have worked on c++ using the Glut.h library. I have a question as I am trying to understand the coordinate system on the graphic window displayed using JOGL.
Now, in c++, the window coordinate system begins at the top left corner of the window (that is where the origin is). However, when I run the below JOGL program, the origin is not at the top left corner. The below program will draw 3 lines, all of which start at the origin:
import com.sun.opengl.util.Animator;
import java.awt.Frame;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import javax.media.opengl.GL;
import javax.media.opengl.GLAutoDrawable;
import javax.media.opengl.GLCanvas;
import javax.media.opengl.GLEventListener;
import javax.media.opengl.glu.GLU;
public class Lab2 implements GLEventListener {
public static void main(String[] args) {
Frame frame = new Frame("My First JOGL Project");
GLCanvas canvas = new GLCanvas();
canvas.addGLEventListener(new Lab2());
frame.add(canvas);
frame.setSize(640, 480);
final Animator animator = new Animator(canvas);
frame.addWindowListener(new WindowAdapter() {
#Override
public void windowClosing(WindowEvent e) {
// Run this on another thread than the AWT event queue to
// make sure the call to Animator.stop() completes before
// exiting
new Thread(new Runnable() {
public void run() {
animator.stop();
System.exit(0);
}
}).start();
}
});
// Center frame
frame.setLocationRelativeTo(null);
frame.setVisible(true);
animator.start();
}
public void init(GLAutoDrawable drawable) {
// Use debug pipeline
// drawable.setGL(new DebugGL(drawable.getGL()));
GL gl = drawable.getGL();
System.err.println("INIT GL IS: " + gl.getClass().getName());
// Enable VSync
gl.setSwapInterval(1);
// Setup the drawing area and shading mode
gl.glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
gl.glShadeModel(GL.GL_SMOOTH); // try setting this to GL_FLAT and see what happens.
}
public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) {
GL gl = drawable.getGL();
GLU glu = new GLU();
if (height <= 0) { // avoid a divide by zero error!
height = 1;
}
final float h = (float) width / (float) height;
gl.glViewport(0, 0, width, height);
gl.glMatrixMode(GL.GL_PROJECTION);
gl.glLoadIdentity();
//glu.gluPerspective(45.0f, h, 1.0, 20.0);
glu.gluPerspective(45.0f, h, 2.0, 20.0);
gl.glMatrixMode(GL.GL_MODELVIEW);
gl.glLoadIdentity();
}
public void display(GLAutoDrawable drawable) {
GL gl = drawable.getGL();
// Clear the drawing area
gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
// Reset the current matrix to the "identity"
gl.glLoadIdentity();
// Move the "drawing cursor" around
gl.glTranslatef(-1.5f, 0.0f, -6.0f);
// Drawing Lines
gl.glBegin(GL.GL_LINES);
gl.glColor3f(0.0f, 1.0f, 0.0f); // Set the current drawing color to green
gl.glVertex3f(0.0f, 0.0f, 0.0f); //Set first vertex of first line
gl.glVertex3f(1.0f, 0.0f, 0.0f); //Set second vertex of first line
gl.glColor3f(0.0f, 0.0f, 1.0f); // Set the current drawing color to blue
gl.glVertex3f(0.0f, 0.0f, 0.0f); //Set first vertex of second line
gl.glVertex3f(0.0f, 1.0f, 0.0f); //Set second vertex of second line
gl.glColor3f(1.0f, 0.0f, 0.0f); // Set the current drawing color to red
gl.glVertex3f(0.0f, 0.0f, 0.0f); //Set first vertex of third line
gl.glVertex3f(0.0f, 0.0f, 1.0f); //Set second vertex of third line
gl.glEnd();
// Flush all drawing operations to the graphics card
gl.glFlush();
}
public void displayChanged(GLAutoDrawable drawable, boolean modeChanged, boolean
deviceChanged) {}
}
Can someone epxlain?