public class MainActivity extends AppCompatActivity {

    final String fileName = new String("myFile.txt");

    TextView mTextView;

    EditText mEditText;

    Button mSaveButton;

    Button mLoadButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTextView = findViewById(R.id.textView);

        mEditText = findViewById(R.id.editText);

        mSaveButton = findViewById(R.id.saveFile);

        mLoadButton = findViewById(R.id.loadFile);

        final File newFile = new File(getFilesDir(), fileName);

        final String mTextToSave = mEditText.getText().toString();

        mSaveButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                try{
                    FileOutputStream outputStream = new FileOutputStream(newFile);
                    outputStream.write(mTextToSave.getBytes());
                    outputStream.close();
                } catch (Exception e) {
                    Log.e("SaveButton", "Exception");
                }
            }
        });

        mLoadButton.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View view) {
                mTextView.setText(new String (ReadFile(newFile)));
            }
        });
    }

    private String ReadFile(File newFile){
        String result = "";
        try{
            String message;
            FileInputStream inputstream = new FileInputStream(newFile);
            InputStreamReader inputStreamReader = new InputStreamReader(inputstream);
            BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
            StringBuffer stringBuffer = new StringBuffer();

            while((message = bufferedReader.readLine()) != null){

                stringBuffer.append(message + '\n');
            }
            result = stringBuffer.toString();
            inputstream.close();
        } catch (Exception e) {
            Log.e("LoadButton", "Exception");
        }

        return result;
    }
}