PointerException异常在getDefaultSha

PointerException异常在getDefaultSha

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NullPointerException异常在getDefaultShared preferences

本文介绍了NullPointerException异常在getDefaultShared preferences的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我创建一个 preferenceFragment 基础上，开发人员指南，但我在获得 NullPointerException异常在的onCreate getDefaultShared preferences（）（）调用当我试图恢复已保存的用户preferences。我从XML充气 preferenceFragment 我设置默认值了。唯一的例外是在 getDefaultShared preferencesName抛出（）在 getDefaultShared preferences（）。

下面是 preferenceFragment 类：

 公共类SettingsFragment扩展preferenceFragment工具
    OnShared $ P $ {pferenceChangeListener公共静态最后弦乐KEY_MAX_WALK_DISTANCE_ preFERENCE =max_walk_distance_ preference
公共静态最后弦乐KEY_MAX_SEARCH_TIME_ preFERENCE =max_search_time_ preference上下文的背景下= getActivity（）;@覆盖
公共无效的onCreate（捆绑savedInstanceState）{
    super.onCreate（savedInstanceState）;    //从XML资源负载的preferences
    加preferencesFromResource（R.xml preferences）;    //加载用户preferences
    共享preferences共享preferences = preferenceManager
            .getDefaultShared preferences（背景）;
    如果（共享preferences！= NULL）{
        preference walkDistance =寻找preference（KEY_MAX_WALK_DISTANCE_ preFERENCE）;
        如果（walkDistance！= NULL）{
            walkDistance.setSummary（共享preferences.getString（
                    KEY_MAX_WALK_DISTANCE_ preFERENCE，）
                    + getResources（）。的getString（
                            R.string.max_walk_distance_postfix））;
        }
        preference searchTime =找到preference（KEY_MAX_SEARCH_TIME_ preFERENCE）;
        如果（searchTime！= NULL）{
            sea​​rchTime.setSummary（共享preferences.getString（
                    KEY_MAX_SEARCH_TIME_ preFERENCE，）
                    + getResources（）。的getString（
                            R.string.max_search_time_postfix））;
        }
    }
}@覆盖
公共无效onShared preferenceChanged（共享preferences共享preferences，
        字符串键）{
    如果（key.equals（KEY_MAX_WALK_DISTANCE_ preFERENCE））{
        preference walkDistance =寻找preference（键）;
        //设置总结为用户描述所选值
        walkDistance.setSummary（共享preferences.getString（键，）
                + getResources（）。的getString（
                        R.string.max_walk_distance_postfix））;
    }否则如果（key.equals（KEY_MAX_SEARCH_TIME_ preFERENCE））{
        preference searchTime =找到preference（键）;
        //设置总结为用户描述所选值
        sea​​rchTime.setSummary（共享preferences.getString（键，）
                + getResources（）
                        .getString（R.string.max_search_time_postfix））;
    }
}@覆盖
公共无效onResume（）{
    super.onResume（）;
    GET preferenceScreen（）。getShared preferences（）
            .registerOnShared preferenceChangeListener（本）;
}@覆盖
公共无效的onPause（）{
    super.onPause（）;
    GET preferenceScreen（）。getShared preferences（）
            .unregisterOnShared preferenceChangeListener（本）;
}

}

下面是XML：

 ＆LT;？XML版本=1.0编码=UTF-8＆GT？;
＆LT; preferenceScreen的xmlns：机器人=htt​​p://schemas.android.com/apk/res/android＆GT;＆LT; EditText上preference
    机器人：DIALOGTITLE =@字符串/ max_walk_distance_dialogtitle
    机器人：键=max_walk_distance_ preference
    机器人：总结=@字符串/ max_walk_distance_summary
    机器人：标题=@字符串/ max_walk_distance_title
    安卓的inputType =数字
    机器人：设置defaultValue =500/＆GT;＆LT; EditText上preference
    机器人：DIALOGTITLE =@字符串/ max_search_time_dialogtitle
    机器人：键=max_search_time_ preference
    机器人：总结=@字符串/ max_search_time_summary
    机器人：标题=@字符串/ max_search_time_title
    安卓的inputType =数字
    机器人：设置defaultValue =10/＆GT;＆LT; / preferenceScreen＆GT;


解决方案

 上下文的背景下= getActivity（）;

当对象实例化和 getActivity（）收益空您的片段不与任何活动相关。因此，试图获得preferences用一个空当NPE 上下文。

推迟 getActivity（）来如你的的onCreate（）（这是后 onAttach称为（））。

I'm creating a PreferenceFragment based on the developer guide but I get a NullPointerException at the getDefaultSharedPreferences() call in onCreate() when I try to restore saved user preferences. I inflate the PreferenceFragment from xml and I set default values too. The exception is thrown at getDefaultSharedPreferencesName() within getDefaultSharedPreferences().

Here is the PreferenceFragment class:

public class SettingsFragment extends PreferenceFragment implements
    OnSharedPreferenceChangeListener {

public static final String KEY_MAX_WALK_DISTANCE_PREFERENCE = "max_walk_distance_preference";
public static final String KEY_MAX_SEARCH_TIME_PREFERENCE = "max_search_time_preference";

Context context = getActivity();

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Load the preferences from an XML resource
    addPreferencesFromResource(R.xml.preferences);

    // load user preferences
    SharedPreferences sharedPreferences = PreferenceManager
            .getDefaultSharedPreferences(context);
    if (sharedPreferences != null) {
        Preference walkDistance = findPreference(KEY_MAX_WALK_DISTANCE_PREFERENCE);
        if (walkDistance != null) {
            walkDistance.setSummary(sharedPreferences.getString(
                    KEY_MAX_WALK_DISTANCE_PREFERENCE, "")
                    + getResources().getString(
                            R.string.max_walk_distance_postfix));
        }
        Preference searchTime = findPreference(KEY_MAX_SEARCH_TIME_PREFERENCE);
        if (searchTime != null) {
            searchTime.setSummary(sharedPreferences.getString(
                    KEY_MAX_SEARCH_TIME_PREFERENCE, "")
                    + getResources().getString(
                            R.string.max_search_time_postfix));
        }
    }
}

@Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences,
        String key) {
    if (key.equals(KEY_MAX_WALK_DISTANCE_PREFERENCE)) {
        Preference walkDistance = findPreference(key);
        // Set summary to be the user-description for the selected value
        walkDistance.setSummary(sharedPreferences.getString(key, "")
                + getResources().getString(
                        R.string.max_walk_distance_postfix));
    } else if (key.equals(KEY_MAX_SEARCH_TIME_PREFERENCE)) {
        Preference searchTime = findPreference(key);
        // Set summary to be the user-description for the selected value
        searchTime.setSummary(sharedPreferences.getString(key, "")
                + getResources()
                        .getString(R.string.max_search_time_postfix));
    }
}

@Override
public void onResume() {
    super.onResume();
    getPreferenceScreen().getSharedPreferences()
            .registerOnSharedPreferenceChangeListener(this);
}

@Override
public void onPause() {
    super.onPause();
    getPreferenceScreen().getSharedPreferences()
            .unregisterOnSharedPreferenceChangeListener(this);
}

}

Here is the XML:

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android" >

<EditTextPreference
    android:dialogTitle="@string/max_walk_distance_dialogtitle"
    android:key="max_walk_distance_preference"
    android:summary="@string/max_walk_distance_summary"
    android:title="@string/max_walk_distance_title"
    android:inputType="number"
    android:defaultValue="500" />

<EditTextPreference
    android:dialogTitle="@string/max_search_time_dialogtitle"
    android:key="max_search_time_preference"
    android:summary="@string/max_search_time_summary"
    android:title="@string/max_search_time_title"
    android:inputType="number"
    android:defaultValue="10"/>

</PreferenceScreen>
解决方案 
Context context = getActivity();

Your fragment is not associated with any activity when the object is instantiated and getActivity() returns null. Hence the NPE when trying to get preferences with a null Context.

Postpone the getActivity() to e.g. your onCreate() (which is called after onAttach()).

这篇关于NullPointerException异常在getDefaultShared preferences的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！

07-30 18:43
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