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);
}

}