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 public class MainActivity extends AppCompatActivity {
    private ArrayList[] getFileList() {
        ArrayList[] arr = new ArrayList[2];
        ArrayList<String> files = new ArrayList<>();
        ArrayList<String> infor = new ArrayList<>();

        String sdcard_path = Environment.getExternalStorageDirectory().getAbsolutePath();

        Log.d("TAG", sdcard_path);
        File target_dic = new File(sdcard_path);
        File[] target_files = target_dic.listFiles();

        if(target_files.length != 0) {
            for(File file : target_files) {
                files.add(file.getName());
                infor.add(file.length()   "B");
            }
        }

        arr[0] = files;
        arr[1] = infor;

        return arr;
    }

    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
    private void setRecyclerView(ArrayList<String> list, ArrayList<String> list2) {
        RecyclerView recyclerView = findViewById(R.id.recyclerView);
        RecyclerAdapter recyclerAdapter = new RecyclerAdapter(list, list2, getDeviceWidth(), MainActivity.this);

        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setAdapter(recyclerAdapter);
    }

            @Override
    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        initMenuButton();
        setupPermission();
    }

    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
    private void setupPermission() {
        int chk = checkCallingOrSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (chk == PackageManager.PERMISSION_DENIED) {
                ActivityCompat.requestPermissions(this, new String[]{ Manifest.permission.READ_EXTERNAL_STORAGE }, 0);
                return;
            }
        }

        if(chk == PackageManager.PERMISSION_GRANTED) {
            ArrayList[] arr = getFileList();
            setRecyclerView(arr[0], arr[1]);
        }

        return;
    }

    @Override
    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        Log.d("TAG", "권한결과 체크함수 호출됨.");

        if(requestCode == 0 amp;amp; grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            ArrayList[] arr = getFileList();
            setRecyclerView(arr[0], arr[1]);

        } else {
            Toast.makeText(this, "권한을 설정해주세요.", Toast.LENGTH_SHORT).show();
            finish();
        }
    }
} 

 <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.moonsung.todaymusic">

    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>