对自定义 ArrayList 使用 SharedPreferences -- 适配器或编辑器错误？

Question

我正在尝试将自定义 ArrayList 保存到 SharedPreferences。理想情况下，这个应用程序第一次运行时，应该有一个空的 SharedPreferences，但如果我需要用一个项目填充它，那么我可以使用“示例”来完成。 我尝试使用调试断点并将问题隔离到下面标记的 setAdapter 行。

应用程序在 setAdapter 行崩溃，但可能是由于编辑器未初始化？调试器告诉我“editor = null”

public class MainActivity extends AppCompatActivity {
    ArrayList<PurchaseOrders> purchaseOrders = new ArrayList<PurchaseOrders>();
    PurchaseOrderAdapter adapter;

    public SharedPreferences sharedPref;
    public SharedPreferences.Editor editor;

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

        purchaseOrders = new ArrayList<PurchaseOrders>();

        String[] items = new String[]{"1", "2", "3"};

        purchaseOrders.add(new PurchaseOrders(false, "purchaseNumber", "Customer", "Date 1", "Date 2", items));

        save();

        sharedPref = this.getSharedPreferences("com.lochdownstudios.simplePurchaseOrderer", Context.MODE_PRIVATE);
        editor = sharedPref.edit();

        load();

        adapter = new PurchaseOrderAdapter(this, purchaseOrders);
        final ListView listView = (ListView) findViewById(R.id.listView);
        listView.setAdapter(adapter); // Break Point here.
    }

    private void save() {
        Gson gson = new Gson();
        String json = gson.toJson(purchaseOrders);
        editor.putString("savedPurchaseOrders", json); //editor = null .... hmmm....
        editor.apply();
    }

    private void load() {
        Gson gson = new Gson();
        String json = sharedPref.getString("savedPurchaseOrders", "");
        Type type = new TypeToken<List<PurchaseOrders>>(){}.getType();
        purchaseOrders = gson.fromJson(json, type);
    }
}

在断点处（在 cmets 中提到），我的调试器告诉我“editor = null”。然后在接下来的步骤中，它会因以下 logcat 而崩溃。

08-23 19:06:02.799 23974-23974/com.lochdownstudios.simplePurchaseOrderer E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.lochdownstudios.simplePurchaseOrderer, PID: 23974
java.lang.RuntimeException: Unable to start activity
ComponentInfo{com.lochdownstudios.simplePurchaseOrderer/com.lochdownstudios.simplePurchaseOrderer.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'android.content.SharedPreferences$Editor android.content.SharedPreferences$Editor.putString(java.lang.String, java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3253)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3349)
at android.app.ActivityThread.access$1100(ActivityThread.java:221)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:7224)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'android.content.SharedPreferences$Editor android.content.SharedPreferences$Editor.putString(java.lang.String, java.lang.String)' on a null object reference
at com.lochdownstudios.simplePurchaseOrderer.MainActivity.save(MainActivity.java:119)
at com.lochdownstudios.simplePurchaseOrderer.MainActivity.onCreate(MainActivity.java:39)
at android.app.Activity.performCreate(Activity.java:6876)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1135)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3206)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3349)
at android.app.ActivityThread.access$1100(ActivityThread.java:221) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:158) 
at android.app.ActivityThread.main(ActivityThread.java:7224) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

想法？谢谢！

Answer 1

在实际初始化 editor 之前，您正在调用使用 editor 的 save

也许改变顺序

    sharedPref = this.getSharedPreferences("com.lochdownstudios.simplePurchaseOrderer", 
                                                                   Context.MODE_PRIVATE);
    editor = sharedPref.edit();

    save();