Android退出程序时，“确定”还是"取消"

public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
new AlertDialog.Builder(this)
//添加图片
.setIcon(android.R.drawable.ic_dialog_alert)
//标题
.setTitle(R.string.title)
//提示内容
.setMessage(R.string.content)
.setPositiveButton(
R.string.confirm,
new android.content.DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog,
int which) {
Intent startMain = new Intent(
Intent.ACTION_MAIN);
startMain.addCategory(Intent.CATEGORY_HOME);
startMain
.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(startMain);
System.exit(0);
}
}).setNegativeButton(R.string.cancel, null)
.create().show();//最后不要忘记show()显示
return false;
}
return false;
}