本文介绍了getActionBar（）setDisplayHomeAsUpEnabled（真）。空指针在preferenceActivity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
我搜索了很多针对此问题，但并没有发现什么相关
I searched a lot for this issue, but didn't found nothing relevant.
我'更新我的应用程序的项目使用API 21，现在当我运行preferenceactivity我得到该行一个空指针。
I' updated my app project to use api 21, now when i run preferenceactivity i get a nullpointer on that row.
这是我的code（这是从所提供的同一code的蚀的创建一个新的settingsactivity时）。
This is my code (it's the same code provided from eclipse when creating a new settingsactivity).
package com.example.prova;
import android.annotation.TargetApi;
import android.content.Context;
import android.content.res.Configuration;
import android.media.Ringtone;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.preference.ListPreference;
import android.preference.Preference;
import android.preference.PreferenceActivity;
import android.preference.PreferenceCategory;
import android.preference.PreferenceFragment;
import android.preference.PreferenceManager;
import android.preference.RingtonePreference;
import android.text.TextUtils;
import android.view.MenuItem;
import android.support.v4.app.NavUtils;
import java.util.List;
/**
* A {@link PreferenceActivity} that presents a set of application settings. On
* handset devices, settings are presented as a single list. On tablets,
* settings are split by category, with category headers shown to the left of
* the list of settings.
* <p>
* See <a href="http://developer.android.com/design/patterns/settings.html">
* Android Design: Settings</a> for design guidelines and the <a
* href="http://developer.android.com/guide/topics/ui/settings.html">Settings
* API Guide</a> for more information on developing a Settings UI.
*/
public class SettingsActivity extends PreferenceActivity {
/**
* Determines whether to always show the simplified settings UI, where
* settings are presented in a single list. When false, settings are shown
* as a master/detail two-pane view on tablets. When true, a single pane is
* shown on tablets.
*/
private static final boolean ALWAYS_SIMPLE_PREFS = false;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setupActionBar();
}
/**
* Set up the {@link android.app.ActionBar}, if the API is available.
*/
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void setupActionBar() {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
// Show the Up button in the action bar.
getActionBar().setDisplayHomeAsUpEnabled(true);
}
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == android.R.id.home) {
// This ID represents the Home or Up button. In the case of this
// activity, the Up button is shown. Use NavUtils to allow users
// to navigate up one level in the application structure. For
// more details, see the Navigation pattern on Android Design:
//
// http://developer.android.com/design/patterns/navigation.html#up-vs-back
//
// TODO: If Settings has multiple levels, Up should navigate up
// that hierarchy.
NavUtils.navigateUpFromSameTask(this);
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
setupSimplePreferencesScreen();
}
/**
* Shows the simplified settings UI if the device configuration if the
* device configuration dictates that a simplified, single-pane UI should be
* shown.
*/
private void setupSimplePreferencesScreen() {
if (!isSimplePreferences(this)) {
return;
}
// In the simplified UI, fragments are not used at all and we instead
// use the older PreferenceActivity APIs.
// Add 'general' preferences.
addPreferencesFromResource(R.xml.pref_general);
// Add 'notifications' preferences, and a corresponding header.
PreferenceCategory fakeHeader = new PreferenceCategory(this);
fakeHeader.setTitle(R.string.pref_header_notifications);
getPreferenceScreen().addPreference(fakeHeader);
addPreferencesFromResource(R.xml.pref_notification);
// Add 'data and sync' preferences, and a corresponding header.
fakeHeader = new PreferenceCategory(this);
fakeHeader.setTitle(R.string.pref_header_data_sync);
getPreferenceScreen().addPreference(fakeHeader);
addPreferencesFromResource(R.xml.pref_data_sync);
// Bind the summaries of EditText/List/Dialog/Ringtone preferences to
// their values. When their values change, their summaries are updated
// to reflect the new value, per the Android Design guidelines.
bindPreferenceSummaryToValue(findPreference("example_text"));
bindPreferenceSummaryToValue(findPreference("example_list"));
bindPreferenceSummaryToValue(findPreference("notifications_new_message_ringtone"));
bindPreferenceSummaryToValue(findPreference("sync_frequency"));
}
/** {@inheritDoc} */
@Override
public boolean onIsMultiPane() {
return isXLargeTablet(this) && !isSimplePreferences(this);
}
/**
* Helper method to determine if the device has an extra-large screen. For
* example, 10" tablets are extra-large.
*/
private static boolean isXLargeTablet(Context context) {
return (context.getResources().getConfiguration().screenLayout & Configuration.SCREENLAYOUT_SIZE_MASK) >= Configuration.SCREENLAYOUT_SIZE_XLARGE;
}
/**
* Determines whether the simplified settings UI should be shown. This is
* true if this is forced via {@link #ALWAYS_SIMPLE_PREFS}, or the device
* doesn't have newer APIs like {@link PreferenceFragment}, or the device
* doesn't have an extra-large screen. In these cases, a single-pane
* "simplified" settings UI should be shown.
*/
private static boolean isSimplePreferences(Context context) {
return ALWAYS_SIMPLE_PREFS
|| Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB
|| !isXLargeTablet(context);
}
/** {@inheritDoc} */
@Override
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public void onBuildHeaders(List<Header> target) {
if (!isSimplePreferences(this)) {
loadHeadersFromResource(R.xml.pref_headers, target);
}
}
/**
* A preference value change listener that updates the preference's summary
* to reflect its new value.
*/
private static Preference.OnPreferenceChangeListener sBindPreferenceSummaryToValueListener = new Preference.OnPreferenceChangeListener() {
@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.
ListPreference listPreference = (ListPreference) preference;
int index = listPreference.findIndexOfValue(stringValue);
// Set the summary to reflect the new value.
preference
.setSummary(index >= 0 ? listPreference.getEntries()[index]
: null);
} else if (preference instanceof RingtonePreference) {
// For ringtone preferences, look up the correct display value
// using RingtoneManager.
if (TextUtils.isEmpty(stringValue)) {
// Empty values correspond to 'silent' (no ringtone).
preference.setSummary(R.string.pref_ringtone_silent);
} else {
Ringtone ringtone = RingtoneManager.getRingtone(
preference.getContext(), Uri.parse(stringValue));
if (ringtone == null) {
// Clear the summary if there was a lookup error.
preference.setSummary(null);
} else {
// Set the summary to reflect the new ringtone display
// name.
String name = ringtone
.getTitle(preference.getContext());
preference.setSummary(name);
}
}
} else {
// For all other preferences, set the summary to the value's
// simple string representation.
preference.setSummary(stringValue);
}
return true;
}
};
/**
* Binds a preference's summary to its value. More specifically, when the
* preference's value is changed, its summary (line of text below the
* preference title) is updated to reflect the value. The summary is also
* immediately updated upon calling this method. The exact display format is
* dependent on the type of preference.
*
* @see #sBindPreferenceSummaryToValueListener
*/
private static void bindPreferenceSummaryToValue(Preference preference) {
// Set the listener to watch for value changes.
preference
.setOnPreferenceChangeListener(sBindPreferenceSummaryToValueListener);
// Trigger the listener immediately with the preference's
// current value.
sBindPreferenceSummaryToValueListener.onPreferenceChange(
preference,
PreferenceManager.getDefaultSharedPreferences(
preference.getContext()).getString(preference.getKey(),
""));
}
/**
* This fragment shows general preferences only. It is used when the
* activity is showing a two-pane settings UI.
*/
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public static class GeneralPreferenceFragment extends PreferenceFragment {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
addPreferencesFromResource(R.xml.pref_general);
// Bind the summaries of EditText/List/Dialog/Ringtone preferences
// to their values. When their values change, their summaries are
// updated to reflect the new value, per the Android Design
// guidelines.
bindPreferenceSummaryToValue(findPreference("example_text"));
bindPreferenceSummaryToValue(findPreference("example_list"));
}
}
/**
* This fragment shows notification preferences only. It is used when the
* activity is showing a two-pane settings UI.
*/
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public static class NotificationPreferenceFragment extends
PreferenceFragment {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
addPreferencesFromResource(R.xml.pref_notification);
// Bind the summaries of EditText/List/Dialog/Ringtone preferences
// to their values. When their values change, their summaries are
// updated to reflect the new value, per the Android Design
// guidelines.
bindPreferenceSummaryToValue(findPreference("notifications_new_message_ringtone"));
}
}
/**
* This fragment shows data and sync preferences only. It is used when the
* activity is showing a two-pane settings UI.
*/
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public static class DataSyncPreferenceFragment extends PreferenceFragment {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
addPreferencesFromResource(R.xml.pref_data_sync);
// Bind the summaries of EditText/List/Dialog/Ringtone preferences
// to their values. When their values change, their summaries are
// updated to reflect the new value, per the Android Design
// guidelines.
bindPreferenceSummaryToValue(findPreference("sync_frequency"));
}
}
}
我没有得到任何错误，但是当运行此我得到：
I get no errors, but when run this i get:
12-03 17:00:53.627: E/AndroidRuntime(10654): FATAL EXCEPTION: main
12-03 17:00:53.627: E/AndroidRuntime(10654): Process: com.example.prova, PID: 10654
12-03 17:00:53.627: E/AndroidRuntime(10654): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.prova/com.example.prova.SettingsActivity}: java.lang.NullPointerException
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.ActivityThread.access$800(ActivityThread.java:139)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.os.Handler.dispatchMessage(Handler.java:102)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.os.Looper.loop(Looper.java:136)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.ActivityThread.main(ActivityThread.java:5097)
12-03 17:00:53.627: E/AndroidRuntime(10654): at java.lang.reflect.Method.invokeNative(Native Method)
12-03 17:00:53.627: E/AndroidRuntime(10654): at java.lang.reflect.Method.invoke(Method.java:515)
12-03 17:00:53.627: E/AndroidRuntime(10654): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
12-03 17:00:53.627: E/AndroidRuntime(10654): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
12-03 17:00:53.627: E/AndroidRuntime(10654): at dalvik.system.NativeStart.main(Native Method)
12-03 17:00:53.627: E/AndroidRuntime(10654): Caused by: java.lang.NullPointerException
12-03 17:00:53.627: E/AndroidRuntime(10654): at com.example.prova.SettingsActivity.setupActionBar(SettingsActivity.java:57)
12-03 17:00:53.627: E/AndroidRuntime(10654): at com.example.prova.SettingsActivity.onCreate(SettingsActivity.java:47)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.Activity.performCreate(Activity.java:5248)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
12-03 17:00:53.627: E/AndroidRuntime(10654): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162)
12-03 17:00:53.627: E/AndroidRuntime(10654): ... 11 more
那么，如何创建一个环境活动，但保持向后兼容性？另外，如果我删除该行应用程序的工作，但我没有在顶栏。
So, how to create a setting activity but keeping backward compatibility? Also, if i remove that line the app works but i don't have the topbar.
我的的AndroidManifest.xml ：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.prova"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="21" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".SettingsActivity"
android:label="@string/title_activity_settings"
android:parentActivityName=".MainActivity" >
<meta-data
android:name="android.support.PARENT_ACTIVITY"
android:value="com.example.prova.MainActivity" />
</activity>
</application>
</manifest>
我的值/ styles.xml ：
<resources>
<!--
Base application theme, dependent on API level. This theme is replaced
by AppBaseTheme from res/values-vXX/styles.xml on newer devices.
-->
<style name="AppBaseTheme" parent="Theme.AppCompat.Light">
<!--
Theme customizations available in newer API levels can go in
res/values-vXX/styles.xml, while customizations related to
backward-compatibility can go here.
-->
</style>
<!-- Application theme. -->
<style name="AppTheme" parent="AppBaseTheme">
<!-- All customizations that are NOT specific to a particular API-level can go here. -->
</style>
</resources>
编译采用Android 5.0与Android支持库V7。
Compiling with android 5.0 with android support library v7.
推荐答案
使用 preferenceActivcity 和 Theme.AppCompat.Light 的方法 getActionBar（）返回null。您应该使用不同的样式（仅适用于API> = 11）。
Using a PreferenceActivcity and the Theme.AppCompat.Light the method getActionBar() returns null.You should use a different style (only for API>=11).
还有一个另外的（更好的选择）。
There is an alternative (a better alternative).
建议在谷歌官方的：
我如何使用AppCompat用preferences？您可以继续在一个API V11 +设备运行时要使用preferenceFragment在ActionBarActivity。对于在这之前的设备，你需要提供一个正常的preferenceActivity未料风格。
How do I use AppCompat with Preferences?You can continue to use PreferenceFragment in your ActionBarActivity when running on an API v11+ device. For devices before that, you will need to provide a normal PreferenceActivity which is not material-styled.
然后就可以使用 preferenceFragment 和 ActionBarActivity 的API> 11。通过这种方式，你可以使用Theme.AppCompat.Light主题。
Then you can use a PreferenceFragment and a ActionBarActivity for api>11.In this way you can use the Theme.AppCompat.Light theme.
注意使用 getSupportActionBar（）而不是 getActionBar（）。
这篇关于getActionBar（）setDisplayHomeAsUpEnabled（真）。空指针在preferenceActivity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！