无法在MineSweeper中更改父类变量

Question

我在父类中有三个变量可以改变游戏的难度。

public class MineSweeper implements ActionListener{
    int gridlength = 20;
    int gridheight = 20;    
    int numMines = 30;

当用户点击新游戏时，会出现一个提示，允许用户更改难度。它确实运行以下提示，但它始终恢复为上面的值。如何更改新游戏的值？

public void actionPerformed(ActionEvent e) {
    if (e.getSource() == newGame){
        String gridl = JOptionPane.showInputDialog(subframe, "Please enter in the amount of columns.");
        gridlength = Integer.parseInt(gridl);
        String gridh = JOptionPane.showInputDialog(subframe, "Please enter in the amount of rows.");
        gridheight = Integer.parseInt(gridh);
        String amountOfMines = JOptionPane.showInputDialog(subframe, "Please enter in the amount of mines.");   
        numMines = Integer.parseInt(amountOfMines);
        new MineSweeper();