你MainActivity 继承的是 fragment 清单文件里面不能注册fragment ,
1.你可以MainActivity extends FragmentActivity
2.或者
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_fragment);
FragmentManager fm = getSupportFragmentManager();
Fragment fragment = fm.findFragmentById(R.id.xxxx);
if(fragment == null){
fragment = new Fragment();
fm.beginTransaction().add(R.id.xxxx, fragment).commit();
}
}