重新启动应用程序时，我的Sharedpreference无法保存

我只有2个活动，分别是Main和Second活动，但是sharedpreference在2个活动之间起作用，但是当我重新启动它时，它永远不会进入if语句（Main活动）if（prefs！= null），这意味着当我重新启动它时，prefs始终为null 。关闭应用程序时，sharedpreference是否实际保存数据？提前谢谢:)

MainActivity

package com.example.hongsukchoi.rentaltruck;

import android.content.Intent;
import android.provider.MediaStore;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.content.SharedPreferences;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;


public class MainActivity extends AppCompatActivity {

    private EditText mile_edittext;
    private RadioGroup radio;
    private Button submit_button;
    double truck_price;
    double mile_price;
    double total_price;
    private int truck_type;
    private RadioButton ten;
    private RadioButton seventeen;
    private RadioButton twentysix;


    SharedPreferences prefs;
    SharedPreferences.Editor editor;

    Set<String> mySet = new HashSet<>();


    @Override
    protected void onCreate(Bundle savedInstanceState) {

        ten = (RadioButton) findViewById(R.id.tenfeettruck);
        seventeen = (RadioButton) findViewById(R.id.seventeenfeettruck);
        twentysix = (RadioButton) findViewById(R.id.twentysixfeettruck);


        //ten.setChecked(true);

        if(prefs != null) {
            System.out.println("This is what I am looking for " + prefs.getAll());
            //R.id.checked
            ten.setChecked(true);


        }
        System.out.println("Application is now started");

        //ArrayList<String> allPrefs = new ArrayList(prefs.getStringSet("total_set", null));
       // System.out.println("HERE ARE ALL THE PREFERENCES(Main acivity): " + allPrefs.toString());





        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        prefs = getSharedPreferences("type", MODE_PRIVATE);
        editor = prefs.edit();

        radio = (RadioGroup) findViewById(R.id.truckRadio);
        submit_button = (Button) findViewById(R.id.submit_button);
        mile_edittext = (EditText) findViewById(R.id.mile_edittext);



        submit_button.setOnClickListener(new View.OnClickListener(){


            @Override
            public void onClick(View v) {

                String temp3 = mile_edittext.getText().toString();
                mile_price = Double.parseDouble(temp3);




                total_price = truck_price + (mile_price*2);


                String total2 = String.valueOf(total_price);

                //editor.putString("total", total2);
                //editor.putStringSet("total_set", mySet);
                editor.putInt("type", truck_type);
                editor.commit();



                Intent i = new Intent(MainActivity.this, SecondActivity.class);
                i.putExtra("total",total2);
                //i.putExtra("type", truck_type);
                startActivity(i);







            }
        });








    }
    public void onRadioButtonClicked(View view) {
        // Is the button now checked?
        boolean checked = ((RadioButton) view).isChecked();

        // Check which radio button was clicked
        switch(view.getId()) {
            case R.id.tenfeettruck:
                if (checked)
                    truck_price = 19.99;
                truck_type = 10;

                mySet.add(String.valueOf(truck_type));



                break;
            case R.id.seventeenfeettruck:
                if (checked)
                    truck_price = 29.99;
                truck_type = 17;


                mySet.add(String.valueOf(truck_type));


                break;
            case R.id.twentysixfeettruck:
                if (checked)
                    truck_price = 39.99;
                truck_type = 26;


                mySet.add(String.valueOf(truck_type));


                break;
        }
    }




}


SecondActivity

package com.example.hongsukchoi.rentaltruck;

import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.widget.ImageView;
import android.widget.TextView;

import org.w3c.dom.Text;

import java.util.ArrayList;

public class SecondActivity extends Activity {
    private double total_price;
    private ImageView myImgView;
    private TextView Total_displaying;
    private TextView whichtruck;
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);

        myImgView = (ImageView)findViewById(R.id.truckimage);
        whichtruck = (TextView)findViewById(R.id.whichtruck);
        SharedPreferences prefs = getSharedPreferences("type", MODE_PRIVATE);

        SharedPreferences prefs_set = getSharedPreferences("total_set", MODE_PRIVATE);

        if(prefs.getStringSet("total_set", null) != null) {

            ArrayList<String> allPrefs = new ArrayList(prefs.getStringSet("total_set", null));
            System.out.println("HERE ARE ALL THE PREFERENCES(Second acivity): " + allPrefs.toString());
        }

        //SharedPreferences type = getSharedPreferences("type", MODE_PRIVATE);
        Intent i = getIntent();
        String temp = i.getExtras().getString("total");

        int type_temp = prefs.getInt("type", 0);


        if(temp != null) {
             total_price = Double.parseDouble(temp);
        }



        if(type_temp == 10)
        {
            myImgView.setImageDrawable(getResources().getDrawable(R.drawable.truckten));
            whichtruck.setText("is 10 feet truck!");

        }
        else if(type_temp == 17)
        {
            myImgView.setImageDrawable(getResources().getDrawable(R.drawable.truckseventeen));
            whichtruck.setText("is 17 feet truck!");

        }
        else if(type_temp == 26)
        {
            myImgView.setImageDrawable(getResources().getDrawable(R.drawable.trucktwentysix));
            whichtruck.setText("is 26 feet truck!");

        }

        Total_displaying = (TextView) findViewById(R.id.Total_displaying);
        String display = Double.toString(total_price);
        Total_displaying.setText(display);
    }


}