android 加一个按钮，退出程序

package com.example.yanlei.yl;

import android.graphics.Color;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import android.text.Html;
import android.text.Html.ImageGetter;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

import android.text.Editable;
import android.text.TextWatcher;
import android.widget.EditText;
import android.widget.Button;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;

import android.app.Activity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;

import android.widget.Button;
import android.widget.EditText;

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        View btn1 = this.findViewById(R.id.button1);
        btn1.setOnClickListener(new ButtonClickListener());

    }

    private class ButtonClickListener implements View.OnClickListener {

        public void onClick(View v) {
            //System.exit(0);
            android.os.Process.killProcess(android.os.Process.myPid());
        }
    }

 <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="退出程序"
        android:id="@+id/button1"
        android:layout_marginTop="157dp"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginLeft="96dp"
        android:layout_marginStart="96dp" />