android两次点击返回键退出和按钮退出对话框

<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="${relativePackage}.${activityClass}" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="82dp"
        android:text="TextView" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="164dp"
        android:text="@string/hello_world" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView2"
        android:layout_below="@+id/textView2"
        android:layout_marginLeft="16dp"
        android:layout_marginTop="74dp"
        android:text="退出" />

</RelativeLayout>

package com.example.andtest_1;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity {

	private TextView tv1, tv2;
	private Button btn;
	private long firstTime = 0;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		// tv1,tv2;
		tv1 = (TextView) findViewById(R.id.textView1);
		tv2 = (TextView) findViewById(R.id.textView2);
		btn = (Button) findViewById(R.id.button1);
		tv1.setText("tv1");

		tv2.setText("tv2");

		btn.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View v) {
				// TODO 自动生成的方法存根
				dialog();
			}
		});
	}// //onCreate

	private void dialog() {
		AlertDialog.Builder builder = new Builder(MainActivity.this);

		builder.setTitle("提示");
		builder.setMessage("您确认退出吗？");
		builder.setPositiveButton("确定",
				new android.content.DialogInterface.OnClickListener() {

					@Override
					public void onClick(DialogInterface dialog, int which) {
						// TODO 自动生成的方法存根
						dialog.dismiss();
						MainActivity.this.finish();

					}
				});
		builder.setNegativeButton("取消",
				new android.content.DialogInterface.OnClickListener() {

					@Override
					public void onClick(DialogInterface dialog, int which) {
						// TODO 自动生成的方法存根

						dialog.dismiss();

					}
				});

		builder.create().show();

	}// /dialog

	@Override
	public boolean onKeyUp(int keyCode, KeyEvent event) {
		// TODO 自动生成的方法存根
		switch (keyCode) {
		case KeyEvent.KEYCODE_BACK:
			long secondTime = System.currentTimeMillis();
			if (secondTime - firstTime > 2000) {
				Toast.makeText(MainActivity.this, "再按一次退出程序",
						Toast.LENGTH_SHORT).show();
				firstTime = secondTime;// //这句千万别忘了！！！！
				return true;

			} else {
				System.exit(0);
			}
			break;
		}
		return super.onKeyUp(keyCode, event);
	}

}