java - Android:使用 ActionBar 时出现 NullPointerException

Question

我收到一个错误提示

java.lang.RuntimeException: Unable to start activity ComponentInfo{...}: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity...
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.myapp.contactList.MainPage.onCreate(MainPage.java:41)

然后我查找了几个与此类似的堆栈溢出问题并尝试了所有解决方案但仍然有这个问题。下面是我的代码:

MainPage.java

public class MainPage extends AppCompatActivity{
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_page);

    android.support.v7.app.ActionBar ab = getSupportActionBar();
    assert ab != null;
    ab.setDisplayHomeAsUpEnabled(true);  // I'M GETTING THE ERROR HERE AT THIS LINE
    ab.setBackgroundDrawable(new ColorDrawable(Color.parseColor("#51c2f2")));
    ab.setDisplayUseLogoEnabled(true);

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.myapp.contactList">

 <application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity
        android:name=".MainPage"
        android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
 </application>
</manifest>

styles.xml

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>
</style>

activity_main_page.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 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:layout_width="match_parent"
   android:layout_height="match_parent"
   android:fitsSystemWindows="true"
   tools:context="com.myapp.contactList.MainPage">

   <android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">
   </android.support.design.widget.AppBarLayout>
   <include layout="@layout/content_main_page" />
</android.support.design.widget.CoordinatorLayout>

Answer 1

基本上你的代码中有两个错误:

首先，Android 不支持 assert 关键字，当您在代码中使用它时，什么也不会发生。您应该检查这段代码:

if(ab  ==  null) {
   throw new IllegalStateException();
}

或者你也可以使用

Assert.notNull(ab);

来自一些库或框架。

您也可以通过 adb 打开断言，但仅限于您的设备:

adb shell setprop debug.assert 1

其次，您应该使用 Toolbar 作为支持操作栏。更新您的 MainActivity 类

public class MainPage extends AppCompatActivity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_page);
        android.support.v7.widget.Toolbar toolbar 
             = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar)

        setSupportActionBar(toolbar)
        android.support.v7.app.ActionBar ab = getSupportActionBar();
        ab.setDisplayHomeAsUpEnabled(true);  
        ...

并更新布局 xml，如下面的代码所示:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 
    ....
   tools:context="com.myapp.contactList.MainPage">

   <android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

      <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        app:popupTheme="@style/AppTheme.PopupOverlay"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        android:theme="?attr/actionBarTheme"
        android:minHeight="?attr/actionBarSize">

   </android.support.design.widget.AppBarLayout>
   <include layout="@layout/content_main_page" />
</android.support.design.widget.CoordinatorLayout>

您还必须为您的 Activity 设置样式"@style/AppTheme.NoActionBar"

您可以在 style.xml 中定义它，如下所示:

<style name="AppTheme.NoActionBar">
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>
</style>