java - Udacity Android 开发设置导致致命异常

Question

我正在学习 Udacity Android 开发类(class)，当我点击设置时，应用程序崩溃了。我遵循此处给出的代码:( https://github.com/udacity/SunshineVersion2/blob/3.13_add_share_action_provider/app/src/main/java/com/example/android/sunshine/app/SettingsActivity.java )。

我在这里也找到了类似的帖子:( PreferenceFragment NullPointerException )。但他们都没有真正帮助。我知道 Preference Activity 中的方法已被弃用，我尝试以 API 为目标并将其转换为 fragment ，但这两个选项都不起作用。以下是我单击设置按钮时的 logcat 消息:

AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.sunshine.app, PID: 24808
java.lang.RuntimeException: Unable to start activity                                                                                                                                  
java.lang.NullPointerException: Attempt to invoke virtualmethod 'void android.preference.Preference.setOnPreferenceChangeListener(android.preference.Preference$OnPreferenceChangeListener)' on a null object reference                                                                                                                                              

下面是设置 Activity 的代码:

public class SettingsActivity extends PreferenceActivity
    implements Preference.OnPreferenceChangeListener {

@Override

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Add 'general' preferences, defined in the XML file
    addPreferencesFromResource(R.xml.pref_general);

    // For all preferences, attach an OnPreferenceChangeListener so the UI summary can be
    // updated when the preference changes.
    bindPreferenceSummaryToValue(findPreference(getString(R.string.pref_location_key)));
    bindPreferenceSummaryToValue(findPreference(getString(R.string.pref_units_key)));
}

/**
 * Attaches a listener so the summary is always updated with the preference value.
 * Also fires the listener once, to initialize the summary (so it shows up before the value
 * is changed.)
 */

private void bindPreferenceSummaryToValue(Preference preference) {
    // Set the listener to watch for value changes.
    preference.setOnPreferenceChangeListener(this);

    // Trigger the listener immediately with the preference's
    // current value.
    onPreferenceChange(preference,
            PreferenceManager
                    .getDefaultSharedPreferences(preference.getContext())
                    .getString(preference.getKey(), ""));
}

@Override

public boolean onPreferenceChange(Preference preference, Object value) {
    String stringValue = value.toString();

    if (preference instanceof ListPreference) {
        // For list preferences, look up the correct display value in
        // the preference's 'entries' list (since they have separate labels/values).
        ListPreference listPreference = (ListPreference) preference;
        int prefIndex = listPreference.findIndexOfValue(stringValue);
        if (prefIndex >= 0) {
            preference.setSummary(listPreference.getEntries()[prefIndex]);
        }
    } else {
        // For other preferences, set the summary to the value's simple string representation.
        preference.setSummary(stringValue);
    }
    return true;
}
}

下面是我的 strings.xml:

<resources>

<string name="app_name">Sunshine</string>
<string name="hello_world">Hello world!</string>
<string name="action_settings">Settings</string>
<string name="action_refresh">Refresh</string>
<string name="title_activity_detail">DetailActivity</string>
<string name="title_activity_settings">SettingsActivity</string>
<string name = "pref_location_key">location</string>
<string name="pref_location_label">Location</string>
<string name="pref_location_default">82072</string>
<!-- Label for the temperature units preference [CHAR LIMIT=30] -->
<string name="pref_units_label">Temperature Units</string>

<!-- Label for metric option in temperature unit preference [CHAR LIMIT=25] -->
<string name="pref_units_label_metric">Metric</string>

<!-- Label for imperial option in temperature unit preference [CHAR LIMIT=25] -->
<string name="pref_units_label_imperial">Imperial</string>

<!-- Key name for temperature unit preference in SharedPreferences [CHAR LIMIT=NONE] -->
<string name="pref_units_key" translatable="false">units</string>

<!-- Value in SharedPreferences for metric temperature unit option [CHAR LIMIT=NONE] -->
<string name="pref_units_metric" translatable="false">metric</string>

<!-- Value in SharedPreferences for imperial temperature unit option [CHAR LIMIT=NONE] -->
<string name="pref_units_imperial" translatable="false">imperial</string>

这是我的 pref_general.xml:

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
>

<EditTextPreference
    android:key = "Location"
    android:title = "Location"
    android:defaultValue= "82072"
    android:inputType = "text"
    android:singleLine="true"
    />

    <ListPreference
        android:title= "@string/pref_units_label"
        android:key = "@string/pref_units_key"
        android:defaultValue="@string/pref_units_metric"
        android:entryValues="@array/pref_units_values"
        android:entries="@array/pref_units_options"
        />

</PreferenceScreen>

提前感谢您抽出宝贵时间提供帮助。

Answer 1

大多数编程语言和操作系统都区分大小写。 android:key = "Location"不匹配 <string name = "pref_location_key">location</string> .