Question

我正在尝试在Java中为JButton创建自定义游标。所以我使用Toolkit类。但是Toolkit方法无法以某种方式加载我的图像。这是我的代码：

public class ButtonPerso extends JButton{

 private Toolkit toolkit; 

 private Cursor myCursor; 

 private Point hotSpot;

 private Image image;

 public ButtonPerso(String label) {

  super(label);

  Toolkit.getDefaultToolkit();

  image = toolkit.createImage("candle.gif" );

  hotSpot = new Point(0, 0);  

  //myCursor = toolkit.createCustomCursor(image, hotSpot, "Candle");
  myCursor = toolkit.createCustomCursor(image, hotSpot, "Candle");

  addMouseListener(new MouseAdapter() {
    public void mouseEntered(MouseEvent e) {
      //setCursor (Cursor.getPredefinedCursor(Cursor.HAND_CURSOR));
      setCursor(myCursor);
    }
      }
  );

 }
}

每次我收到错误：

 Exception in thread "main" java.lang.NullPointerException
    at GUI.ButtonPerso.<init>(ButtonPerso.java:36)
    at GUI.Menu.<init>(Menu.java:61)
    at GUI.Fenetre.<init>(Fenetre.java:18)
    at Main.main(Main.java:34)

所以我猜这些例外情况正在发生：IndexOutOfBoundsException,HeadlessException.我试图找到问题所在：

try {
  myCursor = toolkit.createCustomCursor(Menu.image, hotSpot, "Candle");
  }
  catch (HeadlessException h) {

  }

  catch (IndexOutOfBoundsException i) {
    System.out.println("index except");
  }

但我仍然得到同样的警告，我不知道该怎么办你能帮我一把吗？也许它来自我的形象

Answer 1

而不是这一行：

Toolkit.getDefaultToolkit();

你想写：

toolkit = Toolkit.getDefaultToolkit();

否则toolkit将保持null，当您尝试在其上调用方法时，您将获得NullPointerException。

Answer 2

private Toolkit toolkit; 
\\ toolkit is not initialized and hence it is a null reference.
\\ Below line will throw NullPointerException
image = toolkit.createImage("candle.gif" );

\\ You need to assign the toolkit first.
toolkit = Toolkit.getDefaultToolkit();

java中的Toolkit类

2 个答案: