java - 空指针异常的未知原因

Question

我有以下代码，将 JLabels 添加到 JPanel，然后将此 JPanel 添加到 JFrame，但是我得到了一个 NullPointerException 并且我不知道为什么。我的代码如下:

public void createImage(String view) {

            String [] viewSplit = view.split("(?!^)");

            if(viewSplit.length == 25) {
                fiveByFiveLook.setLayout(null);
                fiveByFiveLook.setBackground(Color.WHITE);
                fiveByFiveLook.setBounds(48, 75, 404, 404);
                fiveByFiveLook.setVisible(true);


                ArrayList<String> viewArray = new ArrayList<String>(Arrays.asList(viewSplit));



                JLabel fiveByFiveLabels[][] = new JLabel[5][5];

                for(int i = 0, k = 0; i <= 4; i++, k = k + 80) {
                    for(int j = 0, l = 0; j <= 4; j++, l = l + 80) {


                        if(viewArray.get(0).equals("X")) {
                            fiveByFiveLabels[i][j].setIcon(new ImageIcon("Blank.jpg"));
                            fiveByFiveLook.add(fiveByFiveLabels[i][j]); 
                            viewArray.remove(0);
                        }
                        else if(viewArray.get(0).equals("#")) {
                            fiveByFiveLabels[i][j].setIcon(new ImageIcon("Wall.jpg"));
                            fiveByFiveLook.add(fiveByFiveLabels[i][j]); 
                            viewArray.remove(0);
                        }
                        else if(viewArray.get(0).equals(".")) {
                            fiveByFiveLabels[i][j].setIcon(new ImageIcon("Floor.jpg"));
                            fiveByFiveLook.add(fiveByFiveLabels[i][j]); 
                            viewArray.remove(0);
                        }
                        else if(viewArray.get(0).equals("E")) {
                            fiveByFiveLabels[i][j].setIcon(new ImageIcon("Exit.jpg"));
                            fiveByFiveLook.add(fiveByFiveLabels[i][j]); 
                            viewArray.remove(0);
                        }
                        else if(viewArray.get(0).equals("G")) {
                            fiveByFiveLabels[i][j].setIcon(new ImageIcon("Gold.png"));
                            fiveByFiveLook.add(fiveByFiveLabels[i][j]); 
                            viewArray.remove(0);
                        }
                        fiveByFiveLabels[i][j].setBounds(l, k, 85, 85);
                    }
                }
                humanGameWindow.add(fiveByFiveLook);
            }

            else if(viewSplit.length == 49) {

                sevenBySevenLook.setLayout(null);
                sevenBySevenLook.setBackground(Color.WHITE);
                sevenBySevenLook.setBounds(48, 75, 404, 404);
                sevenBySevenLook.setVisible(true);

                JButton sevenBySevenLabels[][] = new JButton[7][7];

                for(int i = 0, k = 0; i <= 6; i++, k = k + 57) {
                    for(int j = 0, l = 0; j <= 6; j++, l = l + 57) {
                        sevenBySevenLabels[i][j] = new JButton();
                        sevenBySevenLabels[i][j].setBounds(l, k, 62, 62);
                        //sevenBySevenLabels[i][j].setIcon(new ImageIcon("Sword copy.jpg"));
                        sevenBySevenLook.add(sevenBySevenLabels[i][j]);
                    }
                }

                humanGameWindow.add(sevenBySevenLook);
            }
        }

错误是:

java.lang.NullPointerException
    at dodGUI.createImage(dodGUI.java:324)
    at dodGUI.handleResponse(dodGUI.java:61)
    at dodGUI$1.run(dodGUI.java:79)
    at java.lang.Thread.run(Thread.java:744)
Null Pointer2: java.lang.NullPointerException

并出现在这一行:

fiveByFiveLabels[i][j].setBounds(l, k, 85, 85);

请有人告诉我这是为什么:)

Answer 1

线

JLabel fiveByFiveLabels[][] = new JLabel[5][5];

创建一个可以容纳 5x5 标签的空数组。但是里面没有标签。所有数组条目都是 null。

插入这一行

for(int i = 0, k = 0; i <= 4; i++, k = k + 80) {
    for(int j = 0, l = 0; j <= 4; j++, l = l + 80) {

        fiveByFiveLabels[i][j] = new JLabel(); // <--- This one...

并阅读http://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html