Android mkdir()でフォルダが作れない

パーミッションの付与及び権限リクエストを行っているにも関わらずmkDir()でFalseとなりフォルダが作成できません。
何が足りないのでしょうか。
ご教示頂けると幸いです。よろしくお願い致します

AndroidManifest.xml
1<?xml version="1.0" encoding="utf-8"?>
2<manifest xmlns:android="http://schemas.android.com/apk/res/android"
3    xmlns:tools="http://schemas.android.com/tools"
4    package="com.fujitsu.dfxmobile">
5    <uses-permission android:name="android.permission.VIBRATE"/>
6    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
7    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
8    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
9    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
10    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
11    <uses-permission android:name="android.permission.INTERNET"/>
12    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
13

MainActivity.java
1    protected void onCreate(Bundle savedInstanceState) {
2        super.onCreate(savedInstanceState);
3        setContentView(R.layout.alram_list);
4        firtTime = true;
5        context = getApplicationContext();
6
7
8        if (Build.VERSION.SDK_INT >= 23) {
9            if(ActivityCompat.checkSelfPermission(this,Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED
10                    || ActivityCompat.checkSelfPermission(this,Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED)
11            {
12                ActivityCompat.requestPermissions(this,
13                        new String[]{
14                                Manifest.permission.WRITE_EXTERNAL_STORAGE,
15                                Manifest.permission.READ_EXTERNAL_STORAGE,
16                        },
17                        PERMISSION_WRITE_EX_STR);
18            }
19        }
20
21        //音を鳴らす
22        Alarm.play(AlramList.this, R.raw.sound1);
23        //バイブレーション（要調整）
24        Vibrator vibrator = (Vibrator)getSystemService(VIBRATOR_SERVICE);
25        long[] pattern = {3000,1000,2000};
26        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
27            vibrator.vibrate(VibrationEffect.createWaveform(pattern, 2));
28        } else {
29            vibrator.vibrate(pattern, 2);
30        }
31        try {
32            Thread.sleep(7000);
33        } catch (InterruptedException e) {
34        }
35        vibrator.cancel();
36       OnCreate以下略
37   }
38    @Override
39    public void onRequestPermissionsResult(
40            int requestCode, String[] permission, int[] grantResults
41    ) {
42        super.onRequestPermissionsResult(requestCode, permission, grantResults);
43        if (grantResults.length <= 0) {
44            return;
45        }
46        switch (requestCode) {
47            case PERMISSION_WRITE_EX_STR: {
48                if (grantResults[0] == PackageManager.PERMISSION_GRANTED && grantResults[1] == PackageManager.PERMISSION_GRANTED) {
49                    /// 許可が取れた場合・・・
50                    //アプリケーションフォルダがなければ作る
51                    LogController.getInstance();
52                } else {
53                    /// 許可が取れなかった場合・・・
54                    Toast.makeText(this,
55                            "アプリを起動できません....", Toast.LENGTH_LONG).show();
56                    finish();
57                }
58            }
59            return;
60        }
61    }
62

LogController.java
1
2	private final static String LOG_DIR = "test";
3
4	/** コンストラクタ */
5	public LogController() {
6        if(!Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))
7            return;
8		String dirPath = Environment.getExternalStorageDirectory().getPath();
9
10		mkdir(dirPath, LOG_DIR);
11	}
12
13
14	/** フォルダ作成 */
15	private void mkdir(String dirPath, String name) {
16		File dir = new File(dirPath, "/" + name);
17		if (!dir.exists()) {
18			    String a;
19				if(!dir.mkdir()){
20					a = "1";
21				}else{
22					a = "2";
23				}
24		}
25
26	}
27