即使退出应用程序后，如何显示变量totalCost

Question

我正在尝试扫描QR码并将其结果添加到变量totalCost。如果我退出应用程序，则totalCost将被重置。如何进行更改，如果我在totalCost为100时退出应用程序并重新启动它，则totalCost为100而不是0？

package com.mycompany.smartshoppingcart;

import android.net.Uri;
import android.os.Bundle;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.ActivityNotFoundException;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

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

public class MainActivity extends Activity {

private static final int ACTIVITY_RESULT_QR_DRDROID = 0;

TextView report;
Button scan;
int totalCost = 0;

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

    report = (TextView) findViewById(R.id.textView2);
    scan = (Button) findViewById(R.id.button1);

    scan.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {
            // TODO Auto-generated method stub

            Intent i = new Intent("la.droid.qr.scan");

            try {

                startActivityForResult(i, ACTIVITY_RESULT_QR_DRDROID);
            }
            catch (ActivityNotFoundException activity) {

                MainActivity.qrDroidRequired(MainActivity.this);
            }
        }
    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    // TODO Auto-generated method stub

    super.onActivityResult(requestCode, resultCode, data);

    if( ACTIVITY_RESULT_QR_DRDROID == requestCode
            && data != null && data.getExtras() != null ) {

      String result = data.getExtras().getString("la.droid.qr.result");

        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < result.length(); i++) {
            char c = result.charAt(i);
            if (Character.isDigit(c)) {
                builder.append(c);
            }
        }

        String res = builder.toString();

      int cost = Integer.parseInt(res);
      //  int cost = Integer.parseInt(result);
       totalCost += cost;
      report.setText("Your bill amount is Rs" + totalCost);
      //  report.setText(result);
        report.setVisibility(View.VISIBLE);
    }
}

/*
 *
 * If we don't have QRDroid Application in our Device,
 * It will call below method (qrDroidRequired)
 *
 */

protected static void qrDroidRequired(final MainActivity activity) {
    // TODO Auto-generated method stub

    AlertDialog.Builder AlertBox = new AlertDialog.Builder(activity);

    AlertBox.setMessage("QRDroid Missing");

    AlertBox.setPositiveButton("Direct Download", new OnClickListener() {

        @Override
        public void onClick(DialogInterface arg0, int arg1) {
            // TODO Auto-generated method stub

            activity.startActivity(new Intent(Intent.ACTION_VIEW,
                    Uri.parse("http://droid.la/apk/qr/")));
        }
    });

    AlertBox.setNeutralButton("From Market", new OnClickListener() {

        @Override
        public void onClick(DialogInterface dialog, int which) {
            // TODO Auto-generated method stub

            activity.startActivity(new Intent(Intent.ACTION_VIEW,
                    Uri.parse("http://market.android.com/details?id=la.droid.qr")));
        }
    });

    AlertBox.setNegativeButton("Cancel", new OnClickListener() {

        @Override
        public void onClick(DialogInterface dialog, int which) {
            // TODO Auto-generated method stub

            dialog.cancel();
        }
    });

    AlertBox.create().show();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);


    return true;
}

}