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>