该行提供了一个空指针，我不知道为什么

Question

I'm trying to add buttons to all these panels, so I can check if they're clicked. I'm still new to Java and this is how we were taught how to do it.

Right now I'm making a big panel and adding 48 new panels onto it and then adding buttons on each of those panels, so I can make an action event. If there is a way for me to check if I clicked the panel then I could do that, but I have no idea how.

I'm getting the NullPointerException on the line "panel[x].add(click[x]);"

package CatchTheMouse;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class CatchTheMouse extends JFrame implements ActionListener, MouseListener{
    final int ROWS = 8;
    final int COLS = 6;
    final int GAP = 2;
    final int MAX_PANELS = ROWS * COLS;
    int clicks;
    int hits;
    int percentage = 0;
    int width;
    int height;
    int panelX;
    int panelY;
    int whichPanel = (int)(Math.random() * 47 + 1);

    JButton[] click = new JButton[MAX_PANELS];
    JLabel grats = new JLabel("");
    JLabel spot = new JLabel("X");
    JPanel[] panel = new JPanel[MAX_PANELS];
    JPanel pane = new JPanel(new GridLayout(ROWS, COLS, GAP, GAP));
    Font xFont = new Font("Ariel", Font.BOLD, 20);
    Font font = new Font("Ariel", Font.PLAIN, 12);

    public CatchTheMouse() {
        super("Catch the Mouse");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(300,300);
        add(spot);
        spot.setFont(xFont);
        add(grats);
        grats.setFont(font);
        add(pane);
        for(int x = 0; x < MAX_PANELS; ++x) {
            panel[x] = new JPanel();
            pane.add(panel[x]);
            panel[x].setBackground(Color.RED);
            panel[x].add(click[x]);
            click[x].addActionListener(this);
            click[x].setVisible(false);
        }
        pane.setBackground(Color.BLACK);
        panel[whichPanel].add(spot);
    }

    public void mouseClicked(MouseEvent e) {
        clicks = e.getClickCount();
    }

    public void mouseEntered(MouseEvent e) {

    }

    public void mouseExited(MouseEvent e) {
    }

    public void mousePressed(MouseEvent e) {
    }

    public void mouseReleased(MouseEvent e) {
    }

    public void actionPerformed(ActionEvent e) {
        Object src = e.getSource();
        if(src == click[whichPanel]) {
            hits++;
            grats.setText("You have made " + Integer.toString(hits) + " hits");
        }
    }

    public static void main(String[] args) {
        CatchTheMouse frame = new CatchTheMouse();
        frame.setVisible(true);
    }
}

Answer 1

A guess, this line:

panel[x].add(click[x]);

You're trying to add JButton's that have not yet been constructed. Construct them first before adding!

click[x] = new JButton("something");
panel[x].add(click[x]);

In the future though, when asking for help here, please include all relevant information, including and especially the line that throws any exceptions that you're stuck on.

Answer 2

You are missing click[x] = new JButton() prior to using click[x] . You got it right with the initialization of panel[x] .

    for(int x = 0; x < MAX_PANELS; ++x) {
        panel[x] = new JPanel();
        pane.add(panel[x]);
        panel[x].setBackground(Color.RED);           
        click[x] = new JPanel(); // add this
        panel[x].add(click[x]);
        click[x].addActionListener(this);
        click[x].setVisible(false);
    }