使用 SharedPreferences 获取 java.lang.nullPointerException [英] Getting java.lang.nullPointerException using SharedPreferences

问题描述

我正在尝试保存文件解析的日期，以便下次用户打开应用程序时，可以根据上次解析日期检查日期.

I am trying to save the date of file parsing, so that when next time user, opens the application, the date can be checked against the last parsing date.

我正在使用共享首选项来保存数据并检索它，但出现错误.这是代码:

I am using shared preference to save the data and retrieve it, but getting error. Here is the code :

SharedPreferences settings =  getPreferences(0);
String today = new Date(System.currentTimeMillis()).toString();
SharedPreferences.Editor edit = settings.edit();
System.out.println("******** Today : " + today);
edit.putString("lastdate", today);

String fetch = settings.getString("lastdate", "0");
System.out.println("******** Fetch : " + fetch);
txtTest.setText(fetch);

但我收到空指针错误，我是否遗漏了什么?

But I am getting null pointer error, am I missing something?

推荐答案

许多应用程序可能会提供一种方法来捕获用户对特定应用程序或活动设置的偏好.为了支持这一点，Android 提供了一组简单的 API.

Many applications may provide a way to capture user preferences on the settings of a specific application or an activity. For supporting this, Android provides a simple set of APIs.

首选项通常是名称值对.它们可以跨应用程序中的各种活动存储为共享首选项"(注意目前它不能跨进程共享).或者它可以是需要存储特定于活动的东西.

Preferences are typically name value pairs. They can be stored as "Shared Preferences" across various activities in an application (note currently it cannot be shared across processes). Or it can be something that needs to be stored specific to an activity.

1. 共享首选项:应用程序之外的所有组件(活动、服务等)都可以使用共享首选项.

1. Shared Preferences: The shared preferences can be used by all the components (activities, services etc) off the applications.

活动处理首选项:这些首选项只能在活动中使用，不能由应用程序的其他组件使用.

Activity handled preferences: These preferences can only be used with in the activity and can not be used by other components of the application.

共享偏好:

在Context 类的getSharedPreferences 方法的帮助下管理共享首选项.首选项存储在默认文件 (1) 中，或者您可以指定用于引用首选项的文件名 (2).

The shared preferences are managed with the help of getSharedPreferences method of the Context class. The preferences are stored in a default file(1) or you can specify a file name(2) to be used to refer to the preferences.

(1) 这里是指定文件名时获取实例的方法

(1) Here is how you get the instance when you specify the file name

public static final String PREF_FILE_NAME = "PrefFile";
   SharedPreferences preferences = getSharedPreferences(PREF_FILE_NAME, MODE_PRIVATE);

MODE_PRIVATE 是首选项的操作模式.这是默认模式，意味着只有调用应用程序才能访问创建的文件.支持的其他两种模式是 MODE_WORLD_READABLE 和 MODE_WORLD_WRITEABLE.在MODE_WORLD_READABLE 中，其他应用程序可以读取创建的文件但不能修改它.在 MODE_WORLD_WRITEABLE 的情况下，其他应用程序也对创建的文件具有写入权限.

MODE_PRIVATE is the operating mode for the preferences. It is the default mode and means the created file will be accessed by only the calling application. Other two mode supported are MODE_WORLD_READABLE and MODE_WORLD_WRITEABLE. In MODE_WORLD_READABLE other application can read the created file but can not modify it. In case of MODE_WORLD_WRITEABLE other applications also have write permissions for the created file.

(2) 推荐方式使用默认模式，不指定文件名

(2) The recommended way is to use by the default mode, without specifying the file name

SharedPreferences preferences = PreferencesManager.getDefaultSharedPreferences(context);

最后，获得首选项实例后，您可以通过以下方式从首选项中检索存储的值:

Finally, once you have the preferences instance, here is how you can retrieve the stored values from the preferences:

 int storedPreference = preferences.getInt("storedInt", 0);

要在首选项文件中存储值，必须使用SharedPreference.Editor 对象.Editor 是 SharedPreference 类的嵌套接口.

To store values in the preference file SharedPreference.Editor object has to be used. Editor is the nested interface of the SharedPreference class.

SharedPreferences.Editor editor = preferences.edit();
editor.putInt("storedInt", storedPreference); // value to store
editor.commit();

Editor 还支持诸如 remove() 和 clear() 之类的方法来从文件中删除首选项值.

Editor also support methods like remove() and clear() to delete the preference value from the file.

活动偏好:

共享首选项可以被其他应用程序组件使用.但是如果您不需要与其他组件共享首选项并希望拥有活动的私人首选项.您可以在活动的 getPreferences() 方法的帮助下做到这一点.getPreference 方法使用 getSharedPreferences() 方法和活动类的名称作为首选项文件名.

The shared preferences can be used by other application components. But if you do not need to share the preferences with other components and want to have activities private preferences. You can do that with the help of getPreferences() method of the activity. The getPreference method uses the getSharedPreferences() method with the name of the activity class for the preference file name.

以下是获取偏好的代码

SharedPreferences preferences = getPreferences(MODE_PRIVATE);
int storedPreference = preferences.getInt("storedInt", 0);

存储值的代码也与共享首选项的情况相同.

The code to store values is also same as in case of shared preferences.

SharedPreferences preferences = getPreference(MODE_PRIVATE);
SharedPreferences.Editor editor = preferences.edit();
editor.putInt("storedInt", storedPreference); // value to store
editor.commit();

您还可以使用其他方法，例如将活动状态存储在数据库中.注意 Android 还包含一个名为 android.preference 的包.该包定义了实现应用程序首选项 UI 的类.

You can also use other methods like storing the activity state in database. Note Android also contains a package called android.preference. The package defines classes to implement application preferences UI.

要查看更多示例，请查看 Android 的 Data在开发者网站上存储帖子.

To see some more examples check Android's Data Storage post on developers site.

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