java - 尝试在空对象引用上调用接口(interface)方法'boolean java.util.Set.addAll(java.util.Collection)'

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What is a NullPointerException, and how do I fix it?
                                
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我正在尝试为我的Sharedpreference创建一个“集合”。我需要这个来保存笔记。但是总是有一个错误。

Relevant code is below:

主要活动：

    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：

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();

    }
}

以下是Logcat中的实际错误：

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.

我不了解我在填充Set“ setSharedPreference”时如何获取空对象。

任何指针将不胜感激。

最佳答案

else子句仅在setSharedPreferences == null时执行。因此，它不应包含对该空值（例如setSharedPreferences.addAll(headlines)）的任何方法调用。

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