一旦程序在設備安裝后，data/data/包名/ 即為內部存儲空間，對外保密。
Context提供了2個方法來打開輸入、輸出流

FileInputStream openFileInput(String name)
FileOutputStream openFileOutput(String name, int mode)

public class MainActivity extends Activity {

    private TextView show;
    private EditText et;
    private String filename = "test";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        show = (TextView) findViewById(R.id.show);
        et = (EditText) findViewById(R.id.et);

        findViewById(R.id.write).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    FileOutputStream fos = openFileOutput(filename, Context.MODE_PRIVATE);
                    //FileOutputStream是字節流，如果是寫文本的話，需要進一步把FileOutputStream包裝 UTF-8是編碼
                    OutputStreamWriter osw = new OutputStreamWriter(fos, "UTF-8");
                    //寫
                    osw.write(et.getText().toString());
                    osw.flush();
                    fos.flush();
                    osw.close();
                    fos.close();
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (UnsupportedEncodingException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }); 
        
        findViewById(R.id.read).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    FileInputStream fis = openFileInput(filename);
                    //當輸入輸出都指定字符集編碼的時候，就不會出現亂碼的情況
                    InputStreamReader isr = new InputStreamReader(fis, "UTF-8");
                    //獲取文件的可用長度，構建一個字符數組
                    char[] input = new char[fis.available()];
                    isr.read(input);
                    isr.close();
                    fis.close();
                    String readed = new String(input);
                    show.setText(readed);
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (UnsupportedEncodingException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        });         
    }   
}

data/data/packagename/files/test就是我們寫入的文件