尝试在java中打开图像文件时发生NullPointerException

Question

我有一个内存匹配程序，允许用户向其中添加新文件夹和图像。当程序运行时默认的文件夹和图像工作正常。当它选择用户添加的文件夹时，它会添加第一个图像，然后提供NullPointerException。

这里就是我认为这个问题是

ArrayList <Card> cardsList = new ArrayList(); 

    //cboDifficulty.addActionListener(this); 
    String difficulty = cboDifficulty.getSelectedItem().toString(); 

     gameDiff = 0;//initialize gameDiff to 0 

    if(difficulty.equals("Beginner")){//Beginnner 
    /*place 3 cards and 1 copy of each card (total of 6 cards) on gamescreen*/ 
     gameDiff = 3; 
    } 
    else if(difficulty.equals("Easy")){//Easy 
    /*place 6 cards and 1 copy of each card (total of 12 cards) on gamescreen*/ 
     gameDiff = 6; 
    } 

    String userDir = cboImages.getSelectedItem().toString(); 
    File filePath = new File(baseDir + "/" + userDir + "/"); 

    comboFile = filePath.listFiles(); 


    List<File> listShuffle = Arrays.asList(comboFile); 
    Collections.shuffle(listShuffle); 
    for(int tick = 0; tick < listShuffle.size(); tick++){ 
     comboFile[tick] = listShuffle.get(tick); 
    } 

    for(int ctr = 0; ctr < comboFile.length; ctr++){ 
     if(ctr < gameDiff){ 

      Card card = new Card(); 
      card.setbackImage(new ImageIcon(cardBackImage)); 

      Image img = null; 

      if(comboFile[ctr].isFile()){ 
       JOptionPane.showMessageDialog(null, comboFile[ctr].toString()); 

下一行是代码，其中NullPointerException是

   img = new ImageIcon(this.getClass().getResource(comboFile[ctr].getPath())).getImage(); 
      } 

      Image dimg = img.getScaledInstance(144, 216, Image.SCALE_SMOOTH); 
      card.setfrontImage(new ImageIcon(dimg)); 

      // Populate card with db results 
      cardsList.add(card); 

     } 

    } 
     // Clone list of cards for game 
     for(int counter = 0; counter < gameDiff && counter < cardsList.size(); counter++){ 
      // Pull out current card from the loop 
      Card orgCard = cardsList.get(counter); 


      // Make new card to populate (clone it) 
       Card cloneCard = new Card(); 
       cloneCard.setfrontImage(orgCard.getfrontImage()); 
       cloneCard.setbackImage(orgCard.getbackImage()); 

       cardsList.add(cloneCard); 

      }     

    shuffleCard(cardsList); 

    createGameScreen(cardsList); 

如前所述，当我使用的默认文件夹此代码的工作正常只有在尝试使用用户提供的图像时才会中断。

下面是创建目录代码：

JFileChooser fc = new JFileChooser(); 


    int returnVal = fc.showOpenDialog(null); 

    fc.setFileSelectionMode(fc.DIRECTORIES_ONLY); 
    if (returnVal == fc.APPROVE_OPTION) { 
     File selectedFile = fc.getSelectedFile(); 
     imageFilename = selectedFile.getPath(); 
     JOptionPane.showMessageDialog(null, "You selected " + imageFilename); 
     ImageIcon imageView = new ImageIcon(imageFilename); 
     lblIcon.setIcon(imageView); 

这里是保存图像

String sveCaption = lblCaption.getText(); 


    // Convert text in combobox to string... 
    String newDir = cboSelectGroup.getSelectedItem().toString(); 

    // Convert path to string... 
    String src = baseDir + "/" + newDir; 

    // Create new file... 
    File javaFilePath = new File(src, "/" + lblCaption.getText() + ".png"); 
    File oldImage = new File(imageFilename); 
    if (! javaFilePath.exists()){ 

     JOptionPane.showMessageDialog(null, "Folder/Category, Image and Caption saved!!!"); 
     //oldImage.renameTo(javaFilePath); 
     FileChannel copyFile = new FileInputStream(oldImage).getChannel(); 
     FileChannel dest = new FileOutputStream(javaFilePath).getChannel(); 
     dest.transferFrom(copyFile, 0, copyFile.size()); 
    } 

的保存创建由用户选择的文件夹中的一个新的文件中的代码。游戏甚至会看到文件，因为它会进入if语句并打印我添加的消息。但是在那一点上，它会给出例外。

我忘了说在异常之前打印的消息显示预期的路径images/userfolder/userimage.png。

Answer 1

此代码：

this.getClass().getResource(comboFile[ctr].getPath()) 

...依赖而图像可作为同一类加载器作为执行类的资源上。如果它只是文件系统上的一个文件，并且该类是在一个jar文件中，那不会是这样。

如果您只是试图加载文件，请不要使用Class.getResource() - 只需将文件名（理想情况下为绝对文件名以避免歧义）传递给ImageIcon构造函数。