我对Android开发很安静。我一直在尝试使用导航栏和工具栏创建聊天应用程序。我使用了我在网上找到的模板来创建聊天应用程序，它正在运行。但是，当尝试手动添加工具栏时，我遇到了一些问题。我发现MainFragment被硬编码到XML上。这禁止我使用导航栏（如果片段硬编码到XML上，则看不到替换片段）。所以这引导我遵循这个：https://developer.android.com/training/basics/fragments/fragment-ui.html，如下所示：
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mToolBar = (Toolbar) findViewById(R.id.nav_act);
setSupportActionBar(mToolBar);
mDrawerLayout = (DrawerLayout) findViewById(R.id.navigation_header);
mToggle = new ActionBarDrawerToggle(this, mDrawerLayout,R.string.open, R.string.close);
mDrawerLayout.addDrawerListener(mToggle);
mToggle.syncState();
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
NavigationView navigationView = (NavigationView) findViewById(R.id.navigation_view);
navigationView.setNavigationItemSelectedListener(this);
//Set up Mainfragment
if (findViewById(R.id.fragment_container) != null) {
if (savedInstanceState != null) {
return;
}
MainFragment firstFragment = new MainFragment();
firstFragment.setArguments(getIntent().getExtras());
// Add the fragment to the 'fragment_container' FrameLayout
getSupportFragmentManager().beginTransaction().add(R.id.fragment_container, firstFragment).commit();
}
}
我删除了硬编码的MainFragment XML并将其替换为RelativeLayout，并为其指定了一个ID“Fragment_Container”。
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/navigation_header"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"
tools:ignore="MergeRootFrame">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/fragment_container"></RelativeLayout>
<android.support.design.widget.NavigationView
android:layout_height="match_parent"
android:layout_width="match_parent"
android:id="@+id/navigation_view"
app:menu="@menu/menu_navigation"
android:layout_gravity="start"
app:headerLayout="@layout/navigation_header">
</android.support.design.widget.NavigationView>
</android.support.v4.widget.DrawerLayout>
问题是，当我尝试运行我的代码时，我收到此错误：
Process: com.github.nkzawa.socketio.androidchat, PID: 8429
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.github.nkzawa.socketio.androidchat/com.github.nkzawa.socketio.androidchat.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at com.github.nkzawa.socketio.androidchat.MainActivity.onCreate(MainActivity.java:40)
at android.app.Activity.performCreate(Activity.java:6975)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
作为参考，这是MainFragment布局：
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/message_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainFragment">
<include layout="@layout/navigation_action" android:layout_width="match_parent"
android:layout_height="wrap_content"></include>
<android.support.v7.widget.RecyclerView
android:id="@+id/messages"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:scrollbars="vertical"
android:scrollbarStyle="outsideOverlay"/>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:orientation="horizontal"
android:gravity="center_vertical">
<EditText
android:id="@+id/message_input"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:hint="@string/prompt_message"
android:imeActionId="@+id/send"
android:imeActionLabel="@string/action_send"
android:imeOptions="actionSend"
android:inputType="text"
android:maxLines="1"
android:singleLine="true"
tools:ignore="InvalidImeActionId" />
<ImageButton
android:id="@+id/send_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@android:drawable/ic_menu_send"
android:contentDescription="@string/action_send"/>
</LinearLayout>
</LinearLayout>
Styles.XML是：
<resources>
<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
<!-- Customize your theme here. -->
</style>
<style name="SplashTheme" parent="Theme.AppCompat.NoActionBar">
<item name="android:windowBackground">@color/grey2</item>
</style>
</resources>
当我使用XML硬编码时，我没有收到此错误。 有人理解我为什么会收到这个错误吗？谢谢！