How to use KeyListener for moving Rectangles

I am trying to move a rectangle while the key is pressed and to stop it on release like the game "Snake". As reference I followed this tutorial.

I tried to adjust a few things in my code:

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import java.awt.Graphics;
import java.awt.Color;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.event.KeyListener;
import java.awt.event.KeyEvent;    
public class f3 extends JPanel implements ActionListener, KeyListener { 
static JFrame frame;
static Timer t;
static int x, y, velx, vely, c;
f3(){
t = new Timer(5, this);
x = 0;
y = 0;
velx = 0;
vely = 0;
frame = new JFrame();
frame.addKeyListener(this);
frame.setFocusable(true);
frame.setFocusTraversalKeysEnabled(false);
frame.setSize(500,400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(this);
frame.setVisible(true);
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.BLUE);
g.fillRect(x, y, 50, 30);
t.start();
}
public void actionPerformed(ActionEvent e) {
x = x + velx;
y = y + vely;
repaint();
}
public void keyPressed(KeyEvent e) {
c = e.getKeyCode();
if(c == KeyEvent.VK_RIGHT) {
velx = 1;
vely = 0;
}
if(c == KeyEvent.VK_LEFT) {
velx = -1;
vely = 0;
}
if(c == KeyEvent.VK_UP) {
velx = 0;
vely = -1;
}
if(c == KeyEvent.VK_DOWN) {
velx = 0;
vely = 1;
}
}
public void keyTyped(KeyEvent e) {}
public void keyReleased(KeyEvent e) {}
public static void main(String args[]) {
new f3();
}
}

The value of velx and vely are set after a key event is triggered. Since you are using a Timer here, the GUI will continuously update because actionPerformed is repeatedly triggered.

Remove the Timer, then put the change section of keyPressed to this form and you will get a desired result.

public void keyPressed(KeyEvent e){
if(e.getKeyCode() == KeyEvent.VK_RIGHT){
velx = 1;
vely = 0;
x = x + velx;
y = y + vely;
repaint();
}     
}

Since velx and vely indicate the velocity of the movement (as opposed to moving the player one space at a time, for instance), you will want to also ensure keyReleased returns the appropriate velocity to 0 when the specific key associated with that axis is released.

You do not have a handler for keyReleased method, where you should set velocities to zero as such:

public void keyReleased(KeyEvent e) {
velx = 0;
vely = 0;
}