Hey there! As someone who's been through the Android newbie grind, I’ll walk you through exactly how to load all files from a specific folder and display them in a RecyclerView—step by step, no jargon overload.

First things first: you need permission to access files on the device. The rules vary a bit based on Android version:

• App Private Folder: If your target folder is inside your app's private storage (like getExternalFilesDir(null)), you don't need any extra permissions—just access it directly.
• Public Storage (e.g., Downloads, DCIM): For Android 10+, you'll need the READ_EXTERNAL_STORAGE permission. For Android 13+, there are more granular permissions, but let's start with the universal read permission for simplicity.

Add Permission to Manifest #

Add this line inside your AndroidManifest.xml (before the <application> tag):

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" android:maxSdkVersion="32" />
<!-- For Android 13+, add specific permissions if needed, e.g., READ_MEDIA_IMAGES for photos -->

Request Permission Dynamically #

Android requires you to ask the user for permission at runtime (not just in the manifest). We'll handle this in your Activity/Fragment later.

Write a helper function to get all files from your target folder. We'll add checks to avoid crashes if the folder doesn't exist:

private fun getFilesFromFolder(folderPath: String): List<File> {
    val targetFolder = File(folderPath)
    return if (targetFolder.exists() && targetFolder.isDirectory) {
        // Filter out nulls (in case some files can't be accessed)
        targetFolder.listFiles()?.filterNotNull()?.toList() ?: emptyList()
    } else {
        // Show a toast if the folder doesn't exist
        Toast.makeText(this, "Target folder doesn't exist!", Toast.LENGTH_SHORT).show()
        emptyList()
    }
}

A RecyclerView needs two things: an item layout (to show each file) and an adapter (to bind data to the layout).

Create Item Layout (res/layout/item_file.xml) #

This layout will display a file icon and the file name:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="16dp"
    android:orientation="horizontal"
    android:gravity="center_vertical">

    <ImageView
        android:id="@+id/iv_file_icon"
        android:layout_width="48dp"
        android:layout_height="48dp"
        android:src="@drawable/ic_file_default"/> <!-- Replace with your default icon -->

    <TextView
        android:id="@+id/tv_file_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:textSize="16sp"
        android:textStyle="bold"/>
</LinearLayout>

Create File Adapter Class #

This adapter will take your list of files and populate the RecyclerView:

class FileAdapter(private val fileList: List<File>) : RecyclerView.Adapter<FileAdapter.FileViewHolder>() {

    // ViewHolder to hold references to item views
    inner class FileViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        val fileNameText: TextView = itemView.findViewById(R.id.tv_file_name)
        val fileIcon: ImageView = itemView.findViewById(R.id.iv_file_icon)
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): FileViewHolder {
        // Inflate the item layout
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.item_file, parent, false)
        return FileViewHolder(view)
    }

    override fun onBindViewHolder(holder: FileViewHolder, position: Int) {
        val currentFile = fileList[position]
        // Set file name
        holder.fileNameText.text = currentFile.name
        // Set icon based on file type (customize this with your own icons!)
        when (currentFile.extension.lowercase()) {
            "pdf" -> holder.fileIcon.setImageResource(R.drawable.ic_file_pdf)
            "jpg", "jpeg", "png" -> holder.fileIcon.setImageResource(R.drawable.ic_file_image)
            "mp4", "avi", "mov" -> holder.fileIcon.setImageResource(R.drawable.ic_file_video)
            "docx", "doc" -> holder.fileIcon.setImageResource(R.drawable.ic_file_word)
            else -> holder.fileIcon.setImageResource(R.drawable.ic_file_default)
        }
    }

    override fun getItemCount(): Int = fileList.size
}

Now let's put everything together in your main Activity (we'll use Kotlin here, but you can adapt it to Java easily):

class FileViewerActivity : AppCompatActivity() {

    private lateinit var fileRecyclerView: RecyclerView
    private lateinit var fileAdapter: FileAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_file_viewer)

        // Initialize RecyclerView
        fileRecyclerView = findViewById(R.id.rv_files)
        fileRecyclerView.layoutManager = LinearLayoutManager(this)

        // Check permission before loading files
        checkStoragePermission()
    }

    private fun checkStoragePermission() {
        if (ContextCompat.checkSelfPermission(
                this,
                Manifest.permission.READ_EXTERNAL_STORAGE
            ) == PackageManager.PERMISSION_GRANTED
        ) {
            // Permission granted, load files
            loadFiles()
        } else {
            // Request permission from user
            ActivityCompat.requestPermissions(
                this,
                arrayOf(Manifest.permission.READ_EXTERNAL_STORAGE),
                100 // Request code (can be any integer)
            )
        }
    }

    private fun loadFiles() {
        // Replace this path with your target folder's absolute path
        // Example: Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).absolutePath
        val targetFolderPath = "/storage/emulated/0/YourCustomFolder"

        // Load files off the main thread to avoid UI freezes
        lifecycleScope.launch {
            val files = withContext(Dispatchers.IO) {
                getFilesFromFolder(targetFolderPath)
            }
            // Set up adapter with the file list
            fileAdapter = FileAdapter(files)
            fileRecyclerView.adapter = fileAdapter
        }
    }

    // Handle permission request result
    override fun onRequestPermissionsResult(
        requestCode: Int,
        permissions: Array<out String>,
        grantResults: IntArray
    ) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults)
        if (requestCode == 100 && grantResults.isNotEmpty()) {
            if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                loadFiles()
            } else {
                Toast.makeText(this, "Permission denied—can't access files!", Toast.LENGTH_SHORT).show()
            }
        }
    }

    // Reuse the helper function from Step 2
    private fun getFilesFromFolder(folderPath: String): List<File> {
        val targetFolder = File(folderPath)
        return if (targetFolder.exists() && targetFolder.isDirectory) {
            targetFolder.listFiles()?.filterNotNull()?.toList() ?: emptyList()
        } else {
            Toast.makeText(this, "Target folder doesn't exist!", Toast.LENGTH_SHORT).show()
            emptyList()
        }
    }
}

• Avoid UI Freezes: Always load files on a background thread (we used Kotlin Coroutines here—lifecycleScope is perfect for Activities/Fragments).
• Handle Edge Cases: Add checks for empty file lists, missing folders, and permission denials to make your app robust.
• Customize Icons: Replace the placeholder icons with your own drawables for a polished look.
• Android 10+ Scoped Storage: If you're targeting Android 10 or higher and need access to non-media files, you might need to use MANAGE_EXTERNAL_STORAGE permission (but only for specific use cases—Google discourages it for most apps).

内容的提问来源于stack exchange，提问作者Tayyab Husain