NullPointerException：null对象引用上的setDisplayHomeAsUpEnabled（boolean）

Question

我有AppCompatActivity的标准设置。要启用父级activity的“向上按钮”，我需要调用getSupportActionBar().setDisplayHomeAsUpEnabled(true);

有很多类似的qazxsw poi。不幸的是我无法找到我的错误，因为我使用qazxsw poi而不是threads和AppCompatActivity而不是Activity

活动：

getSupportActionBar()

activity_gallery_view.xml

getActionBar()

styles.xml

public class GalleryActivity extends AppCompatActivity {

   (...)

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        setSupportActionBar(toolbar);

        (...)
    }
}

堆栈跟踪：

<?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">

    <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_gallery"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

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

</android.support.design.widget.CoordinatorLayout>

Answer 1

您已经在xml和java文件中为工具栏指定了不同的ID。使它们相同，并且很可能您的错误应该消失。这是你的错误

<resources>

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

    <style name="AppTheme.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" />

    <style name="AppTheme.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" />

</resources>

在Activity java文件中你已经完成了

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.stack.overflow/com.stack.overflow.reviewmaschine.gallery.GalleryActivity}: 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:2416)
                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                                     at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                     at android.os.Looper.loop(Looper.java:148)
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                  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.stack.overflow.reviewmaschine.gallery.GalleryActivity.onCreate(GalleryActivity.java:56)
                                                                     at android.app.Activity.performCreate(Activity.java:6251)
                                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                     at android.os.Looper.loop(Looper.java:148) 
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                     at java.lang.reflect.Method.invoke(Native Method) 
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

解决这个问题

Answer 2

在你得到它之前设置它。

android:id="@+id/toolbar_gallery"

Answer 3

像这样更改setToolbarActionbar和getSupportActionbar的顺序：

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

在以编程方式设置之前，您只需尝试接收supportActionbar。这就是Nul​​lPointerException的原因

Answer 4

得到同样的问题。在审查了Android Manifest之后解决了

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

有关它的更多信息，请访问：

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar_gallery);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);