在Android中使用Gson解析json

public class GeRen extends Activity {
 private ArrayList s_list;
 private TextView type,opendate,name,sex,enddate;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 // TODO Auto-generated method stub
 super.onCreate(savedInstanceState);
 setContentView(R.layout.geren);
 initview();
 //str为网络请求到的json字符串
 String str=this.getIntent().getStringExtra("key");
 Log.e("TAG2", str);
 Gson gson = new Gson();
 Datas d = gson.fromJson(str, Datas.class);
 setS_list(d.getResult());
 // for(int i=0;i<s_list.size();i++){
 // type.setText(s_list.get(i).getType());
 // opendate.setText(s_list.get(i).getOpendate());
 // name.setText(s_list.get(i).getName());
 // sex.setText(s_list.get(i).getSex());
 // enddate.setText(s_list.get(i).getEnddate());
 // }
 }
 private void initview() {
 type=(TextView) findViewById(R.id.type);
 opendate=(TextView) findViewById(R.id.opendate);
 name=(TextView) findViewById(R.id.name);
 sex=(TextView) findViewById(R.id.sex);
 enddate=(TextView) findViewById(R.id.enddate);
 }

}

为什么Datas d = gson.fromJson(str, Datas.class);为空