android系统中*.bin文件读取时乱码问题

       大家好，我最近想通过程序查看android系统中的*.bin文件里的数据信息，我试过用notepad和ue查看软件打开后看到都是乱码。
       后来我想通过android程序来读取查看不知是什么原因看到的也全部是些乱码，我的查看方法如下：
       从android系统中复制出*.bin文件放到我项目工程中assets文件中，读取代码如下：        private AssetManager am;
        private InputStream is;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.main);                readerBin();
        }        private void readerBin() {                am = this.getResources().getAssets();
                try {
                        is = am.open("xtra.bin");
                        if (is != null) {
                                Log.e("TAG", "It worked!");
                        } 
                        String str=readTextFile(is);
                         Log.e("TAG", str);             
                } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                }
        }  private String readTextFile(InputStream inputStream) {                ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
                byte buf[] = new byte[1024];
                int len;
                try {
                        while ((len = inputStream.read(buf)) != -1) {                                String text = new String(buf, "UTF-8");
                                Log.i("TAG", text);
                                outputStream.write(buf, 0, len);                        }                        outputStream.close();                        inputStream.close();                } catch (IOException e) {                }
                return outputStream.toString();
        }     但最后读取出来的数据也是些乱码，不知道是何原因？请大家帮忙一些