아래처럼 여기저기 찾아보고 해봤는데요.
"저장실패" 메세지 쪽으로만 빠지네요...음..
혹시 제가 뭘 잘못한건지 고수님들의 조언을 부탁드려요 ^^

Bitmap mBitmap = null;               
              
        mPage = findViewById(R.id.page);
        mPage.setDrawingCacheEnabled(true);
        mPage.buildDrawingCache();
        mBitmap = Bitmap.createBitmap(300, 300, Config.ARGB_8888);
        
        if(mBitmap!= null) {          
                     try {  
                       File captureFile = new File("/sdcard/dcim/camera/createCapture.png");
                       final FileOutputStream outStream = new FileOutputStream(captureFile);
                       if(mBitmap.compress(Bitmap.CompressFormat.PNG, 100, outStream) == true)
                       {
                        Toast.makeText(androidFirst.this, "write bitmap 성공", Toast.LENGTH_SHORT).show();
                       }
                       else
                       {
                        Toast.makeText(androidFirst.this, "write bitmap 실패", Toast.LENGTH_SHORT).show();
                       }
                       Toast.makeText(androidFirst.this, "저장 성공", Toast.LENGTH_SHORT).show();
                      } catch (FileNotFoundException e) {
                       Toast.makeText(androidFirst.this, "저장 실패", Toast.LENGTH_SHORT).show();
                       e.printStackTrace();
                      }                     
              }


p.s) manifest에서 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>
        sdcard 접근 권한 없으려고 이것도 추가했거든요..ㅜㅠ
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