如何在按下背部的情况下结束应用程序，而不返回到前一个活动

@Override
public void onBackPressed() {
    finish();
}

@Override
public void onBackPressed() {
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle("确认退出");
    builder.setMessage("确定要退出应用程序吗？");
    builder.setPositiveButton("确定", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            finish();
        }
    });
    builder.setNegativeButton("取消", null);
    builder.show();
}