我目前正在尝试在我的应用中实现抽屉式导航。我遇到了工具栏为空的问题。我在网上做了很多搜索，并尝试了许多不同的解决方案，但无济于事。我还没有完成所有代码的编写，但我无法启动我的应用程序来检查抽屉导航。谁能看到我哪里出错了？

如果我还有什么需要发布的，请告诉我。

我在所有 Activity 屏幕上都包含了这个

<include
    android:id="@+id/app_bar"
    layout="@layout/app_bar"/>

app_bar.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar
   xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:app="http://schemas.android.com/apk/res-auto"
   app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:minHeight="?attr/actionBarSize"
   android:background="?attr/colorPrimary"
   app:popupTheme="@style/ThemeOverlay.AppCompat.Dark" >
</android.support.v7.widget.Toolbar>

MainActivity 的第 72 行

setSupportActionBar(mToolbar);

主要 Activity

public class Trans_List extends ActionBarActivity {
  private Toolbar mToolbar;
  private CharSequence mTitle;

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_trans_list);

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

        setSupportActionBar(mToolbar);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
        NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.left_drawer);
        drawerFragment.setUp((DrawerLayout) findViewById(R.id.drawer_layout), mToolbar);

样式.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="AppBaseTheme" parent="@style/Theme.AppCompat.Light.DarkActionBar" >
        <item name="windowActionBar">false</item>
    </style>

    <style name="Theme.split" parent="AppBaseTheme">

    </style>

    <style name="dialog_custom" parent="@style/Theme.AppCompat.Dialog">
        <item name="@android:listPreferredItemPaddingLeft">20dip</item>
        <item name="@android:listPreferredItemPaddingRight">20dip</item>
        <item name="@android:listPreferredItemPaddingStart">20dip</item>
        <item name="@android:listPreferredItemPaddingEnd">20dip</item>
    </style>
</resources>

LogCat

01-07 01:14:29.702    4811-4811/com.lee.yuchan.splt E/AndroidRuntime﹕
FATAL EXCEPTION: main
        Process: com.lee.yuchan.splt, PID: 4811
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lee.yuchan.splt/com.lee.yuchan.splt.Trans_List}:
java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()'
on a null object reference
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
                at android.app.ActivityThread.access$800(ActivityThread.java:144)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
                at android.os.Handler.dispatchMessage(Handler.java:102)
                at android.os.Looper.loop(Looper.java:135)
                at android.app.ActivityThread.main(ActivityThread.java:5221)
                at java.lang.reflect.Method.invoke(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:372)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence
android.support.v7.widget.Toolbar.getTitle()' on a null object
reference
                at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:95)
                at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:88)
                at android.support.v7.internal.app.ToolbarActionBar.<init>(ToolbarActionBar.java:84)
                at android.support.v7.app.ActionBarActivityDelegateBase.setSupportActionBar(ActionBarActivityDelegateBase.java:175)
                at android.support.v7.app.ActionBarActivity.setSupportActionBar(ActionBarActivity.java:92)
                at com.lee.yuchan.splt.Trans_List.onCreate(Trans_List.java:72)
                at android.app.Activity.performCreate(Activity.java:5933)
                at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
                at android.app.ActivityThread.access$800(ActivityThread.java:144)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
                at android.os.Handler.dispatchMessage(Handler.java:102)
                at android.os.Looper.loop(Looper.java:135)
                at android.app.ActivityThread.main(ActivityThread.java:5221)
                at java.lang.reflect.Method.invoke(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:372)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

主要 Activity XML

 <?xml version="1.0" encoding="utf-8"?>

    <android.support.v4.widget.DrawerLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/drawer_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <!-- The main content view -->
    <LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

        <include
            layout="@layout/app_bar"/>

        <TextView
            android:id="@+id/selectItems"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/select_instr1"
            android:textSize="20sp"
            android:gravity="center"
            android:paddingTop="4sp"
            android:paddingBottom="10sp" >
        </TextView>

        <ListView
            android:id="@+id/listView1"
            android:layout_width="match_parent"
            android:layout_height="0dip"
            android:layout_weight="1" >
        </ListView>

        <Button
            android:id="@+id/buttonChecked"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="0dip"
            android:background="@drawable/custom_button"
            android:textColor="#ffffff"
            android:text="@string/next_button" />

    </LinearLayout>

    <!-- The navigation drawer -->
    <fragment
        android:id="@+id/left_drawer"
        android:layout_width="@dimen/nav_dr"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        app:layout="@layout/fragment_navigation_drawer"
        android:name="com.lee.yuchan.splt.NavigationDrawerFragment"
        tools:layout="@layout/fragment_navigation_drawer" />

    </android.support.v4.widget.DrawerLayout>

最佳答案

在 Toolbar xml 中设置 id 属性，如下所示:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/app_bar"
    app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:minHeight="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Dark" >
</android.support.v7.widget.Toolbar>

然后将其从 include block 中删除，使其看起来像这样:

<include
    layout="@layout/app_bar"/>

为了进一步简化，您可以完全删除 include block ，并将 Toolbar 直接插入到 include 所在的 xml 中。

关于java - NullPointerException 与 android.support.v7.widget.Toolbar，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/27810184/