At start of program while creating frame:

KeyListener listener = new KeyboardInput(players);
        frame.addKeyListener(listener);
        frame.setFocusable(true);

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Add this class:


    public static class KeyboardInput implements KeyListener {

        Player[] players;

        KeyboardInput(Player[] players){
            this.players = players;
        }


        @Override
        public void keyTyped(KeyEvent e) {

        }

        @Override
        public void keyPressed(KeyEvent e) {


            switch (e.getKeyCode()){
                case(KeyEvent.VK_W):{
                    players[0].moveUp();
                    break;
                }

                case(KeyEvent.VK_S):{
                    players[0].moveDown();
                    break;
                }

                case(KeyEvent.VK_UP):{
                    players[1].moveUp();
                    break;
                }
                case(KeyEvent.VK_DOWN):{
                    players[1].moveDown();
                    break;
                }
            }

        }

        @Override
        public void keyReleased(KeyEvent e) {

        }
    }

}