问题

我试图获取路径中的每个文件夹。现在我只获取文件夹，但是一旦出现其他文件，就会出现JavaNullPointerException。我知道这是因为 listFiles()，但我不知道如何以其他方式让它工作。有人可以帮助我吗？

    @Override
    public Object[] getElements(Object inputElement) {
        return File.listRoots();
    }
    
    @Override
    public Object getParent(Object element) {
        return ((File) element).getParentFile();
    }
    
    @Override
    public boolean hasChildren(Object element) {
        Object[] obj = getChildren(element);       
        return obj == null ? false : obj.length > 0;
        
    }
    
    @Override
    public Object[] getChildren(Object parentElement) {
        
        List<File> files = new ArrayList<>();
            for (File file : ((File) parentElement).listFiles()) {
                if (file.isDirectory()) {
                    files.add(file);
                }
            }
        return files.toArray();
    }

<details>
<summary>英文:</summary>

im trying to get every folder of my path.
Now Im getting just folders but as soon as some other file comes up it gives me a JavaNullPointerException.
I know its beacuse of ***listFiles()*** but I dont know how to let it work otherwise.
Can someone help me?

``

    @Override
    public Object[] getElements(Object inputElement) {
        return File.listRoots();
    }
    
    @Override
    public Object getParent(Object element) {
        return ((File) element).getParentFile();
    }
    
    @Override
    public boolean hasChildren(Object element) {
        Object[] obj = getChildren(element);       
        return obj == null ? false : obj.length &gt; 0;
        
    }
    
    @Override
    public Object[] getChildren(Object parentElement) {
        
        List&lt;File&gt; files = new ArrayList&lt;&gt;();
            for (File file : ((File) parentElement).listFiles()) {
                if (file.isDirectory()) {
                    files.add(file);
                }
            }
        return ((File) parentElement).listFiles();
    }


</details>


# 答案1
**得分**: 1

File.listFiles() 如果文件不是目录，则返回 null。这会导致您的异常。

要仅显示目录，请使用类似以下方式：
```java
@Override
public Object[] getChildren(Object parentElement) {
   File parentFile = (File)parentElement;

   if (!parentFile.isDirectory()) {
      return new Object[0];
   }

   return parentFile.listFiles(File::isDirectory);
}

@Override
public Object[] getChildren(Object parentElement) {
   File parentFile = (File)parentElement;

   if (!parentFile.isDirectory()) {
      return new Object[0];
   }

   return parentFile.listFiles(File::isDirectory);
}