* onPostExecute(Result) 此方法在主线程执行，任务执行的结果作为此方法的参数返回。
PageTask扩展了AsyncTask，在 doInBackground()方法中读取网页内容。PageTask的源代码如下所示：
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// 设置三种类型参数分别为String,Integer,String
class PageTask extends AsyncTask<String, Integer, String> {
// 可变长的输入参数，与AsyncTask.exucute()对应
@Override
protected String doInBackground(String... params) {
try {
HttpClient client = new DefaultHttpClient();
// params[0] 代表连接的url
HttpGet get = new HttpGet(params[0]);
HttpResponse response = client.execute(get);
HttpEntity entity = response.getEntity();
long length = entity.getContentLength();
InputStream is = entity.getContent();
String s = null;
if (is != null) {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte[] buf = new byte[128];
int ch = -1;
int count = 0;
while ((ch = is.read(buf)) != -1) {
