尝试查找ViewById时抛出NullpointerException

从片段编写代码以初始化按钮，因为您的按钮进入片段布局而不是活动布局。

@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container,        Bundle savedInstanceState) {    View rootView = inflater.inflate(R.layout.fragment_main, container, false);    Button login = (Button) rootView.findViewById(R.id.loginButton);    login.setonClickListener(new View.onClickListener() {        @Override        public void onClick(View arg0) { Intent intent = new Intent(MainActivity.this,         LoginActivity.class); startActivity(intent);        }    });    return rootView;}

而从删除登录按钮相关的代码

onCreate

Activity