ヌルオブジェクトリファレンスでインタフェースメソッド 'boolean java.util.Set.addAll（java.util.Collection）'を呼び出そうとしました

Question

私のSharedpreferenceの "Set"を作成しようとしています。私はメモを保存するためにこれが必要です。しかし、常にエラーがあります。

Relevant code is below: 

MainActivity：

package com.example.miguelgaucho.notes; 

import android.content.Context; 
import android.content.DialogInterface; 
import android.content.Intent; 
import android.content.SharedPreferences; 
import android.os.Bundle; 
import android.support.design.widget.FloatingActionButton; 
import android.support.design.widget.Snackbar; 
import android.support.v7.app.AlertDialog; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.Toolbar; 
import android.util.Log; 
import android.view.View; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.widget.AdapterView; 
import android.widget.ArrayAdapter; 
import android.widget.ListView; 
import java.util.ArrayList; 
import java.util.Set; 

public class MainActivity extends AppCompatActivity { 

    Intent intentEditNote = null; 
    static ListView savedNotes = null; 
    static ArrayList<String> headlines = new ArrayList<String>(); 
    static ArrayAdapter arrayAdapter = null; 
    static Set<String> setSharedPreferences = null; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 

     FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
     fab.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
         .setAction("Action", null).show(); 
      } 
     }); 
     SharedPreferences sharedPreferences = this.getSharedPreferences("com.example.miguelgaucho.notes", 
       Context.MODE_PRIVATE); 
     intentEditNote = new Intent(getApplicationContext(), EditNote.class); 
     savedNotes = (ListView) findViewById(R.id.savedNotes); 
     setSharedPreferences = sharedPreferences.getStringSet("headlinesSP", null); 
     headlines.add("Example note"); 

     headlines.clear(); 
     if (setSharedPreferences != null){ 
      headlines.addAll(setSharedPreferences); 
     }else { 
      headlines.add("Example Notes"); 
      setSharedPreferences.addAll(headlines); //Here is the error!! 
      sharedPreferences.edit().putStringSet("headlinesSP", setSharedPreferences).apply(); 
     } 

     arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, headlines); 
     savedNotes.setAdapter(arrayAdapter); 

     savedNotes.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
      @Override 
      public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
       intentEditNote.putExtra("noteID", (position)); 
       startActivity(intentEditNote); 

      } 
     }); 
     savedNotes.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() { 

      @Override 
      public boolean onItemLongClick(final AdapterView<?> arg0, View arg1, 
              final int position, long arg3) { 
       AlertDialog.Builder alert = new AlertDialog.Builder(MainActivity.this); 
       alert.setMessage("Are you sure to delete the note?"); 
       alert.setCancelable(false); 
       alert.setPositiveButton("Yes", new DialogInterface.OnClickListener() { 

        @Override 
        public void onClick(DialogInterface dialog, int which) { 
         headlines.remove(position); 
         arrayAdapter.notifyDataSetChanged(); 
        } 
       }); 
       alert.setNegativeButton("Cancel", new DialogInterface.OnClickListener() { 

        @Override 
        public void onClick(DialogInterface dialog, int which) { 
         dialog.cancel(); 

        } 
       }) 
       .show(); 
       return true; 
      } 
     }); 
    } 

    @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; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.addNote) { 
      startActivity(intentEditNote); 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 
} 

EditNote.class：以下

package com.example.miguelgaucho.notes; 

import android.content.Context; 
import android.content.Intent; 
import android.content.SharedPreferences; 
import android.os.Bundle; 
import android.support.design.widget.FloatingActionButton; 
import android.support.design.widget.Snackbar; 
import android.support.v7.app.ActionBar; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.Toolbar; 
import android.text.Editable; 
import android.text.TextWatcher; 
import android.util.Log; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.ArrayAdapter; 
import android.widget.EditText; 

import java.text.DateFormat; 
import java.util.Date; 

public class EditNote extends AppCompatActivity implements TextWatcher{ 

    ActionBar actionBar; 
    static EditText headline; 
    Intent mainActivity; 
    int noteID; 
    //EditText makeANote; 





    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_edit_note); 
     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 
     actionBar = getSupportActionBar(); 
     actionBar.setDisplayHomeAsUpEnabled(true); 

     FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
     fab.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
         .setAction("Action", null).show(); 
      } 
     }); 
     mainActivity = new Intent(getApplicationContext(), MainActivity.class); 
     headline = (EditText) findViewById(R.id.headline); 
     //makeANote = (EditText) findViewById(R.id.makeANote); 
     Intent intent = getIntent(); 
     noteID = intent.getIntExtra("noteID", -1); 

     if (noteID != -1){ 
      headline.setText(MainActivity.headlines.get(noteID)); 
     } 
     if (headline == null){ 
      Date date = new Date(); 
      String label = DateFormat.getDateTimeInstance().format(date); 
      Log.i("Time: ", label); 
      MainActivity.headlines.add(label); 
      Log.i("Headlines", MainActivity.headlines.toString()); 
      MainActivity.arrayAdapter.notifyDataSetChanged(); 

     }else{ 
      headline.addTextChangedListener(this); 
     } 
    } 
    @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_edit_note, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     int id = item.getItemId(); 
     if (id == R.id.save) { 

      startActivity(mainActivity); 
      return true; 
     } 

     switch (item.getItemId()) { 
      case android.R.id.home: 
       onBackPressed(); 
       return true; 
      default: 
       return super.onOptionsItemSelected(item); 
     } 

    } 

    @Override 
    public void beforeTextChanged(CharSequence s, int start, int count, int after) { 

    } 

    @Override 
    public void onTextChanged(CharSequence s, int start, int before, int count) { 


    } 

    @Override 
    public void afterTextChanged(Editable s) { 
     if (noteID >= 0){ 
      MainActivity.headlines.set(noteID, String.valueOf(s)); 
     }else { 
      MainActivity.headlines.add(String.valueOf(s)); 
     } 
     MainActivity.arrayAdapter.notifyDataSetChanged(); 
     SharedPreferences sharedPreferences = this.getSharedPreferences 
       ("com.example.miguelgaucho.notes", Context.MODE_PRIVATE); 

     MainActivity.setSharedPreferences.addAll(MainActivity.headlines); 
     sharedPreferences.edit().putStringSet 
       ("headlinesSP", MainActivity.setSharedPreferences).apply(); 

    } 
} 

Logcatで実際のエラーです：

E/AndroidRuntime: FATAL EXCEPTION: main 
        Process: com.example.miguelgaucho.notes, PID: 6498 
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.miguelgaucho.notes/com.example.miguelgaucho.notes.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.Set.addAll(java.util.Collection)' on a null object reference 
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646) 
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
         at android.app.ActivityThread.-wrap12(ActivityThread.java) 
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
         at android.os.Handler.dispatchMessage(Handler.java:102) 
         at android.os.Looper.loop(Looper.java:154) 
         at android.app.ActivityThread.main(ActivityThread.java:6077) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
        Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.Set.addAll(java.util.Collection)' on a null object reference 
         at com.example.miguelgaucho.notes.MainActivity.onCreate(MainActivity.java:59) 
         at android.app.Activity.performCreate(Activity.java:6662) 
         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) 
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599) 
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)  
         at android.app.ActivityThread.-wrap12(ActivityThread.java)  
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)  
         at android.os.Handler.dispatchMessage(Handler.java:102)  
         at android.os.Looper.loop(Looper.java:154)  
         at android.app.ActivityThread.main(ActivityThread.java:6077)  
         at java.lang.reflect.Method.invoke(Native Method)  
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)  
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)  
Application terminated. 

私は私がどのように理解していません私がSet "setShaを埋めるときにnullオブジェクトを取得するredPreference。

いずれのポインタも大歓迎です。

Answer 1

else節は、setSharedPreferences == nullの場合にのみ実行されます。そのため、null値（たとえば、setSharedPreferences.addAll(headlines)など）でメソッド呼び出しを含むべきではありません。

if (setSharedPreferences != null){ 
     headlines.addAll(setSharedPreferences); 
    }else { 
     headlines.add("Example Notes"); 
     setSharedPreferences.addAll(headlines); // setSharedPreferences is null here 
     sharedPreferences.edit().putStringSet("headlinesSP", setSharedPreferences).apply(); 
    }