顯示java.lang.NullPointerException（除非方法是靜態使用！）

Question

在我的MIDLet我叫anImg的Java類ImageFetcher的一個實例。同樣在我的MIDLet中，我有一個簡單地說是獲取的命令，一個CommandListener在檢測到抓取被點擊時運行下面的函數。該函數應該簡單地從類ImageFetcher的anImg實例運行public getImage（），該實例返回一個圖像，然後將該圖像附加/設置到顯示器上的窗體上。 （您可能識別來自諾基亞的JavaME維基的getImage（）函數!!!）

代替任何圖像的顯示此被寫入到輸出端子在NetBeans： 信息：顯示java.lang.NullPointerException

但是，如果我將public getImage（）更改爲public static getImage（）並將ImageImage.getImage（）替換爲ImageFetcher.getImage（），則圖像將成功顯示！

謝謝您的答覆對這個問題:) 我期待着這個嚴酷的考驗後去我的頭髮回來了！

FetchImageApp.java

... 
... 
public class FetchImageApp() 
extends MIDlet implements CommandListener { 

    private ImageFetcher anImg; //this is my ImageFetcher instance, it is assigned within the constructor 

    public FetchImageApp(){ 
     anImg = new ImageFetcher(); //NO IT WASN'T, I knew it was something simple... I feel a fool... but I know we all do it! 
    } 
... 
    private doThis(){ 
     try { 
      Image im; 
      if ((im = anImg.getImage()) != null) { 
       ImageItem ii = new ImageItem(null, im, ImageItem.LAYOUT_DEFAULT, null); 
       // If there is already an image, set (replace) it 
       if (form.size() != 0) { 
        form.set(0, ii); 
       } else // Append the image to the empty form 
       { 
        form.append(ii); 
       } 
      } else { 
       form.append("Unsuccessful download."); 
      } 
      // Display the form with the image 
      display.setCurrent(form); 
     } catch (Exception e) { 
      System.err.println("Msg: " + e.toString()); 
     } 
    } 
... 
... 
... 

ImageFetcher.java

... 
... 
... 
    /*-------------------------------------------------- 
    * Open connection and download png into a byte array. 
    *-------------------------------------------------*/ 
    public Image getImage() throws IOException { 
     String url = "http://kenai.com/attachments/wiki_images/chessgame/java-duke-logo.png"; 
     ContentConnection connection = (ContentConnection) Connector.open(url); 

     // * There is a bug in MIDP 1.0.3 in which read() sometimes returns 
     // an invalid length. To work around this, I have changed the 
     // stream to DataInputStream and called readFully() instead of read() 
// InputStream iStrm = connection.openInputStream(); 
     DataInputStream iStrm = connection.openDataInputStream(); 

     ByteArrayOutputStream bStrm = null; 
     Image im = null; 

     try { 
      // ContentConnection includes a length method 
      byte imageData[]; 
      int length = (int) connection.getLength(); 
      if (length != -1) { 
       imageData = new byte[length]; 

       // Read the png into an array 
//  iStrm.read(imageData); 
       iStrm.readFully(imageData); 
      } else // Length not available... 
      { 
       bStrm = new ByteArrayOutputStream(); 

       int ch; 
       while ((ch = iStrm.read()) != -1) { 
        bStrm.write(ch); 
       } 

       imageData = bStrm.toByteArray(); 
       bStrm.close(); 
      } 

      // Create the image from the byte array 
      im = Image.createImage(imageData, 0, imageData.length); 
     } finally { 
      // Clean up 
      if (iStrm != null) { 
       iStrm.close(); 
      } 
      if (connection != null) { 
       connection.close(); 
      } 
      if (bStrm != null) { 
       bStrm.close(); 
      } 
     } 
     return (im == null ? null : im); 
    } 
... 
... 
... 

這裏是聽者代碼爲每個請求:)

public void commandAction(Command c, Displayable d) { 
    if (c == doThisCommand) { 
     if (c.getLabel().equals("Start")) { 
      System.out.println("Started..."); 
      begin(); 
      //doThisCommand = new Command("Stop", Command.OK, 2); //ERROR:: After the command is changed to exit the program throws and unhandled excaption. 
     } else { 
      System.out.println("Stopped..."); 
      doThisCommand = new Command("Start", Command.OK, 2); 
     } 
    } else if (c == exitCommand) { 
     notifyDestroyed(); 
    } else { 
     throw new UnsupportedOperationException("Not supported yet."); 
    } 
} 

Answer 1

如果您在anImg.getImage()上得到NullPointerException，那麼它只是表示anImg是null。做一個System.out.println(anImg);，你會看到它打印null。

爲了解決這個問題，您需要實例anImg莫名其妙。例如。

ImageFetcher anImg = new ImageFetcher(); 

只有這樣你才能夠訪問它並調用它的方法。

Answer 2

你可以發佈你的聽衆？我猜測NPE來自於監聽器使用的ImageFetcher實例是null。當引用它時引發一個NullPointerException。

這當您更改爲靜態方法，因爲沒有介入的情況下是不會發生。