TextField yourTextField; // <-- global variable (initialized somewhere)

public void keyPressed(KeyEvent e) {
    // ... all the other stuff

    textField.setText( Double.toString(velx) );
}

void updateUI() {
    // ... whatever else needs doing

    if( !textField.getText().equals( Double.toString(velx) ) ) {
        textField.setText( Double.toString(velx) );
    }
}

void mainLoop() {
    updateUI()
}

boolean velx_changed = false;

velx++;
velx_changed = true;

void mainLoop() {
    if( velx_changed ) {
        textField.setText( Double.toString(velx) );
    }
}