java - 是什么导致我的java代码的这部分出现空指针异常？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

我需要创建一个按钮数组，每个按钮都有自己的 Card，来自另一个程序(我在其中称为 Deck)。实例化卡片，然后实例化按钮后，我不断收到空指针异常，我不知道它来自哪里。如果您不明白我在说什么，请随时询问；我对编程还比较陌生。

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

    public class TenCard extends JFrame implements ActionListener
{
// a window with borders, title bar, buttons and panel.
private static JFrame f;

// panel that will hold the contents
private JPanel        p;

// declare deck and displayed cards
private Deck        myDeck;     //Deck
private Card[]      playerCards1;
private Card[]      playerCards2;


/**********components**********/

//JButtons
private JButton[]    cardbuttons1;
private JButton[]    cardbuttons2;
private JButton      sortbutton;
private JButton      shufflebutton;
private JButton     flipbutton;


//Textfields
private JTextField text1;
private JTextField text2;


//Menu
private JMenuBar   menuBar;
private JMenuItem  item1, item2;
/*****ending the components*****/

/**
* constructor that instatiates
* the frame, panel, layout, etc and
* adds two cards to the panel
*/
public TenCard()
{
    f = new JFrame("High Card Game");

    // create the content panel
    p = new JPanel();

    // set the layout
    GridLayout layout = new GridLayout(2,3);

    // instantiate game and assign cards
    myDeck  = new Deck();
    myDeck.shuffle();


    Card playerCards1[]= new Card[5];
    for (int i=0; i<5; i++)
    {
        playerCards1[i]= myDeck.dealOneCard();
    }


    Card playerCards2[]= new Card[5];
    for (int i=0; i<5; i++)
    {
        playerCards2[i]= myDeck.dealOneCard();
    }





    // set layout
    p.setLayout(layout);
    addCardsToPanel();
    addFieldToContentPanel();

    // add the content to the window
    f.setContentPane(p);
}

/**
* Create and add three buttons to content panel
*/
public void addCardsToPanel()
{
    // instantiate buttons with images
    sortbutton = new JButton("sort cards");
    shufflebutton = new JButton("shuffle cards");
    flipbutton = new JButton("flip cards");


    JButton cardbuttons1[] = new JButton[5];
    for (int i = 0;  i < 5; i++)
    {
        cardbuttons1[i] = new JButton("player 1, card" +i, playerCards1[i].getBackImage());

    }

    JButton cardbuttons2[] = new JButton[5];
    for (int i = 0;  i < 5; i++)
     {
         cardbuttons2[i] = new JButton("player 2, card" +i, playerCards2[i].getBackImage());
     }


    // add buttons

    p.add(sortbutton);
    p.add(shufflebutton);
    p.add(flipbutton);


}

Answer 1

private Card[]      playerCards1;
private Card[]      playerCards2;

这两个变量永远不会初始化，因此会出现 NullPointerException。在 public TenCard() {} 中，您重新声明具有相同名称的变量，因此只有局部变量被初始化

Card playerCards1[]= new Card[5];   // --> Remove this line to work with the "outside" playerCards1
    for (int i=0; i<5; i++)
    {
        playerCards1[i]= myDeck.dealOneCard();
    }

对playerCards2做同样的事情