我有一个带有抽屉导航的 Activity ，它在 fragment 之间轻弹。没什么特别的。

但是我想要的是操作栏出现在 Activity 的顶部。问题是我不断收到 Actionbar 的 nullpointerexception。

我试过以下页面的答案:

getActionBar returns null

getActionBar() returns Null (AppCompat-v7 21)

这些都没有用，所以我想我会用我自己的代码提交我自己的问题。

主 Activity .java

package com.jampez.smalltalk;


import com.jampez.smalltalk.fragments.HomeFragment;
import com.jampez.smalltalk.fragments.MessageFragment;
import com.jampez.smalltalk.fragments.PreferencesFragment;
import com.jampez.smalltalk.fragments.SettingsFragment;
import com.jampez.smalltalk.helper.SessionManager;

import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBarDrawerToggle;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class MainActivity extends FragmentActivity {
    private DrawerLayout mDrawerLayout;
    private ListView mDrawerList;
    private ActionBarDrawerToggle mDrawerToggle;
    private String[] items;
    int selectedPosition;
    private SessionManager session;

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

    setContentView(R.layout.activity_main);
    getActionBar().setTitle("");

    // Session manager
    session = new SessionManager(getApplicationContext());

    if (!session.isLoggedIn()) {
        // User is already logged in. Take him to main activity
        Intent intent = new Intent(MainActivity.this, LoginActivity.class);
        startActivity(intent);
        finish();
    }

    items = getResources().getStringArray(R.array.menus);
    // Getting reference to the DrawerLayout 
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.drawer_list);

    /* Creating an ArrayAdapter to add items to mDrawerList */
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.drawer_list_item, items);
    mDrawerList.setAdapter(adapter);
    mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
            R.drawable.ic_drawer, 
            R.string.drawer_open) {

        /* Called when drawer is closed */
        public void onDrawerClosed(View view) {
            //Put your code here
        }

        /* Called when a drawer is opened */
        public void onDrawerOpened(View drawerView) {
            //Put your code here
        }
    };
    mDrawerLayout.setDrawerListener(mDrawerToggle);


    // Enabling Home button
    getActionBar().setHomeButtonEnabled(true);
    getActionBar().setDisplayHomeAsUpEnabled(true);

    // Setting item click listener for the listview mDrawerList
    mDrawerList.setOnItemClickListener(new OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            selectedPosition = position;

            /* Replace fragment content */
            displayView(position);

           /* Closing the drawer */
            mDrawerLayout.closeDrawer(mDrawerList);
        }
    });

    /* Setting default fragment */
    selectedPosition = 0;
    displayView(0);
}

private void displayView(int position) {
    // update the main content by replacing fragments
    android.app.Fragment fragment = null;
    Log.i("position", position+"");
    switch (position) {
    case 0:
        fragment = new HomeFragment();
        break;
    case 1:
        fragment = new MessageFragment();
        break;
    case 2:
        fragment = new PreferencesFragment();
        break;
    case 3:
        fragment = new SettingsFragment();
        break;
    case 4:
        Intent sendIntent = new Intent();
        sendIntent.setAction(Intent.ACTION_SEND);
        sendIntent.putExtra(Intent.EXTRA_TEXT, "This is my text to send.");
        sendIntent.setType("text/plain");
        startActivity(sendIntent);
        break;
    case 5:
        String url = "http://www.example.com";
        Intent i = new Intent(Intent.ACTION_VIEW);
        i.setData(Uri.parse(url));
        startActivity(i);
        break;
        default:
            break;
    }

    if (fragment != null) {

        android.app.FragmentManager fragmentManager = getFragmentManager();
        android.app.FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.content_frame, fragment);
        fragmentTransaction.commit();


        // update selected item and title, then close the drawer
        mDrawerList.setItemChecked(position, true);
        mDrawerList.setSelection(position);

        //setTitle(navMenuTitles[position]);
        mDrawerLayout.closeDrawer(mDrawerList);
    } else {
        // error in creating fragment
        Log.e("MainActivity", "Error in creating fragment");
    }
}

@Override
protected void onPostCreate(Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);
    mDrawerToggle.syncState();
}
}

activity_main.xml

<android.support.v4.widget.DrawerLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:theme="@android:style/Theme.WithActionBar" >

<!-- The main content view -->
<FrameLayout
    android:id="@+id/content_frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<!-- The navigation drawer list-->
<ListView
    android:id="@+id/drawer_list"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:background="#111"
    android:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:dividerHeight="0dp" />
</android.support.v4.widget.DrawerLayout>

首页 fragment .java

package com.jampez.smalltalk.fragments;

import com.jampez.smalltalk.R;

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class HomeFragment extends Fragment {

public HomeFragment(){}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

    View rootView = inflater.inflate(R.layout.fragment_home, container, false);

    //getActivity().getActionBar().setTitle(R.string.home);

    return rootView;
}
}

fragment _home.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#e5e5e5"
android:orientation="vertical" >

<TextView
    android:id="@+id/home_text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp"
    android:layout_gravity="start|center_vertical"
    android:text="@string/home"/>    
</LinearLayout>

Android list

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.jampez.smalltalk"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="14"
    android:targetSdkVersion="23" />

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.GET_ACCOUNTS" />
<uses-permission android:name="android.permission.USE_CREDENTIALS" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

<!-- To auto-complete the email text field in the login form with the user's emails -->
<uses-permission android:name="android.permission.READ_PROFILE" />
<uses-permission android:name="android.permission.READ_CONTACTS" />

<application
    android:name=".app.AppController"
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <meta-data
        android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />
    <meta-data
        android:name="com.facebook.sdk.ApplicationId"
        android:value="@string/facebook_app_id" />

    <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=".LoginActivity"
        android:label="@string/app_name"
        android:windowSoftInputMode="adjustResize|stateHidden" >
    </activity>
    <activity
        android:name="com.facebook.FacebookActivity"
        android:configChanges="keyboard|keyboardHidden|screenLayout|screenSize|orientation"
        android:label="@string/app_name"
        android:theme="@android:style/Theme.Translucent.NoTitleBar" />
</application>

样式.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="android:Theme.Holo.Light.DarkActionBar">
    <!--
        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. -->
    <item name="android:actionBarStyle">@style/MyActionBar</item>
    <item name="android:actionOverflowButtonStyle">@style/MyActionButtonOverflow</item>
     <item name="drawerArrowStyle">@style/DrawerArrowStyle</item>
</style>

<style name="DrawerArrowStyle" parent="Widget.AppCompat.DrawerArrowToggle">
<item name="spinBars">true</item>
<item name="color">@color/white</item>
</style>

<style name="MyActionButtonOverflow"     parent="android:style/Widget.Holo.ActionButton.Overflow">
<item name="android:color">@color/white</item>
</style>

 <style name="MyActionBar" parent="@android:style/Widget.Holo.Light.ActionBar">
    <item name="android:background">#25a0da</item>
</style>

Logcat 输出

11-08 19:17:38.618: E/AndroidRuntime(6347): FATAL EXCEPTION: main
11-08 19:17:38.618: E/AndroidRuntime(6347): Process: com.jampez.smalltalk, PID: 6347
11-08 19:17:38.618: E/AndroidRuntime(6347): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.jampez.smalltalk/com.jampez.smalltalk.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2658)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2723)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.ActivityThread.access$900(ActivityThread.java:172)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.os.Handler.dispatchMessage(Handler.java:102)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.os.Looper.loop(Looper.java:145)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.ActivityThread.main(ActivityThread.java:5832)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at java.lang.reflect.Method.invoke(Native Method)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at java.lang.reflect.Method.invoke(Method.java:372)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
11-08 19:17:38.618: E/AndroidRuntime(6347): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
11-08 19:17:38.618: E/AndroidRuntime(6347):     at com.jampez.smalltalk.MainActivity.onCreate(MainActivity.java:37)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.Activity.performCreate(Activity.java:6221)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
11-08 19:17:38.618: E/AndroidRuntime(6347):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2611)
11-08 19:17:38.618: E/AndroidRuntime(6347):     ... 10 more

最佳答案

*首先在你的gradle文件中包含这个依赖

compile 'com.android.support:appcompat-v7:22.2.0'

*您应该将 Activity 扩展到 AppCompactActivity 并开始使用 toolbar 而不是 ActionBar。

*这就是您在 MainActivity

public class MainActivity extends AppCompatActivity {

    private Toolbar mToolbar;

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

        mToolbar = (Toolbar) findViewById(R.id.toolbar);

        setSupportActionBar(mToolbar);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
    }
}

*这是在 activity_main.xml 中添加工具栏的方法

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:minHeight="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    local:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
    local:popupTheme="@style/ThemeOverlay.AppCompat.Light" />

*你的主题应该看起来像这样 styles.xml

<style name="MyMaterialTheme" parent="MyMaterialTheme.Base">

    </style>

    <style name="MyMaterialTheme.Base" parent="Theme.AppCompat.Light.DarkActionBar">
        <item name="windowNoTitle">true</item>
        <item name="windowActionBar">false</item>
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>

确保在您的应用程序的 list 中添加此样式名称。

*如果你使用v4.app.Fragment而不是使用Fragment会更好。

*这是你应该如何从你的 fragment 类设置标题

((AppCompatActivity)getActivity()).getSupportActionBar().setTitle(R.string.home);

引用this清晰的教程，它一定会帮助您设置 Material 抽屉导航。

如果您想快速开始而不会有太多痛苦，那么您可以尝试整合 this第三方库。

关于Android ActionBar 始终为空，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33597986/