我正在嘗試在JPanel上顯示圖片，但我不斷收到錯誤消息：java.lang.IllegalArgumentException：輸入== null！我不明白發生了什么。這是我正在使用的代碼：public void actionPerformed(ActionEvent e) {    try {        Image image=ImageIO.read(getClass().getResource("img/" +num.getText()+".jpg"));        Image resized = image.getScaledInstance(200, 200, 100);        pictureFrame.setIcon(new ImageIcon(resized));    } catch (Exception ex){        ex.printStackTrace();    }}這只是導致我得到錯誤！堆棧跟蹤產生以下內容：Java.lang.IllegalArgumentException: input == null!    at javax.imageio.ImageIO.read(ImageIO.java:1362)    at work.Item.actionPerformed(Item.java:96)    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)    at java.awt.Component.processMouseEvent(Component.java:6297)    at javax.swing.JComponent.processMouseEvent(JComponent.java:3275)    at java.awt.Component.processEvent(Component.java:6062)    at java.awt.Container.processEvent(Container.java:2039)    at java.awt.Component.dispatchEventImpl(Component.java:4660)    at java.awt.Container.dispatchEventImpl(Container.java:2097)    at java.awt.Component.dispatchEvent(Component.java:4488)    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4575)    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4236)    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4166)    at java.awt.Container.dispatchEventImpl(Container.java:2083)我該如何解決？我檢查了圖像的位置，并嘗試從不同的位置進行操作，總是得到相同的錯誤！我正在使用Netbeans IDE。