安卓开发onclicklistener中，findinbyview为何Button cannot be resolved a type？

Question

问题如上，包我也导入了。package com.example.second;import android.widget.*;import android.app.Activity;import android.os.Bundle;import android.view.Menu;import android.widget.Button;import android.widget.CompoundButton.OnCheckedChangeListener;public class MainActivity extends Activity {     private Button loginButton;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        loginButton=findViewById(R.id.button1);                            }    @Override    public boolean onCreateOptionsMenu(Menu menu) {        // Inflate the menu; this adds items to the action bar if it is present.        getMenuInflater().inflate(R.menu.activity_main, menu);        return true;    }    }xml如下<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    tools:context=".MainActivity" >    <Button        android:id="@+id/button1"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_alignParentLeft="true"        android:layout_alignParentTop="true"        android:text="Button" /></RelativeLayout>