MainActivity.java

public class MainActivity extends AppCompatActivity {

    TextView readText;
    EditText writeText;
    String name = "Test";
    String TAG = "Main";

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

        readText = findViewById(R.id.textView);
        writeText = findViewById(R.id.editText2);

    }

    public void Read(View view) {
        String text = readTextOnInternalStorage(name);
        readText.setText(text);
    }

    public void Write(View view) {
        String text = writeText.getText().toString();
        writeInternalStorage(name, text);
    }

    /*
     * Function: Write Text to Internal Storage
     * Input: file name and data (String type) */
    private void writeInternalStorage(String fileName, String data){
        try {
            FileOutputStream fOut = openFileOutput(fileName, MODE_PRIVATE);
            fOut.write(data.getBytes());
            fOut.close();
            Toast.makeText(getBaseContext(),"file saved",Toast.LENGTH_SHORT).show();
        } catch (java.io.IOException e) {
            e.printStackTrace();
        }
    }

    /* Function: Read Text from Internal Storage
     * Input: file name
     * @return: text */
    private String readTextOnInternalStorage(String fileName){
        String data = "";
        try {
            FileInputStream fin = openFileInput(fileName);
            BufferedReader reader = new BufferedReader(new InputStreamReader(fin, "UTF-8"));
            String aDataRow = "";
            int n = 0;
            while ((aDataRow=reader.readLine()) != null){
                n++;
                if (n > 1){
                    //More line
                    data += "\n" + aDataRow;
                }else {
                    //One line
                    data = aDataRow;
                }
            }
            Toast.makeText(getBaseContext(),"file read",Toast.LENGTH_SHORT).show();
        } catch (java.io.IOException e) {
            e.printStackTrace();
        }
        return data;
    }