一旦程序在设备安装后，data/data/包名/ 即为内部存储空间，对外保密。
Context提供了2个方法来打开输入、输出流

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是字节流，如果是写文本的话，需要进一步把FileOutputStream包装 UTF-8是编码
                    OutputStreamWriter osw = new OutputStreamWriter(fos, "UTF-8");
                    //写
                    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);
                    //当输入输出都指定字符集编码的时候，就不会出现乱码的情况
                    InputStreamReader isr = new InputStreamReader(fis, "UTF-8");
                    //获取文件的可用长度，构建一个字符数组
                    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就是我们写入的文件