尝试在空对象引用上调用虚方法’void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)’

概述我正面临这个错误.我花了很多时间,但唉！我陷入了这个错误.我想在我的应用程序中添加Action Bar,但它在getSupportActionBar()上不断向我显示此错误,请任何人指导我. FATAL EXCEPTION: mainProcess: com.technerdshub.vusocial, PID: 20633java.lang.RuntimeException: Unable 我正面临这个错误.我花了很多时间,但唉！我陷入了这个错误.我想在我的应用程序中添加Action bar,但它在getSupportActionbar()上不断向我显示此错误,请任何人指导我.

FATAL EXCEPTION: mainProcess: com.technerdshub.vusocial,PID: 20633java.lang.RuntimeException: Unable to start activity ComponentInfo{com.technerdshub.vusocial/com.technerdshub.vusocial.ActivitIEs.StudentDashboardActivity}: 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:2693)  at androID.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)  at androID.app.ActivityThread.access0(ActivityThread.java:177)  at androID.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)  at androID.os.Handler.dispatchMessage(Handler.java:102)  at androID.os.Looper.loop(Looper.java:145)  at androID.app.ActivityThread.main(ActivityThread.java:5942)  at java.lang.reflect.Method.invoke(Native Method)  at java.lang.reflect.Method.invoke(Method.java:372)  at com.androID.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1389)  at com.androID.internal.os.ZygoteInit.main(ZygoteInit.java:1184) 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.technerdshub.vusocial.ActivitIEs.StudentDashboardActivity.onCreate(StudentDashboardActivity.java:50)   

我的java文件是：

package com.technerdshub.vusocial.ActivitIEs;import androID.app.Activity;import androID.content.Intent;import androID.net.Uri;import androID.os.Bundle;import androID.support.design.Widget.TabLayout;import androID.support.v4.app.FragmentManager;import androID.support.v4.app.FragmentPagerAdapter;import androID.support.v4.vIEw.VIEwPager;import androID.support.v7.app.ActionBaractivity;import androID.support.v7.Widget.Toolbar;import androID.support.v7.app.AppCompatActivity;import com.Google.androID.gms.appindexing.Action;import com.Google.androID.gms.appindexing.AppIndex;import com.Google.androID.gms.common.API.Googleapiclient;import com.technerdshub.vusocial.Fragments.TaskFragment;import com.technerdshub.vusocial.Fragments.dummy.DummyContent;import com.technerdshub.vusocial.R;import java.util.ArrayList;import java.util.List;public class StudentDashboardActivity extends AppCompatActivity {    //private Toolbar toolbar;    private TabLayout tabLayout;    private VIEwPager vIEwPager;    /**     * ATTENTION: This was auto-generated to implement the App Indexing API.     * See https://g.co/AppIndexing/AndroIDStudio for more information.     */    private Googleapiclient clIEnt;    @OverrIDe    protected voID onCreate(Bundle savedInstanceState) {        displayLoginActivity();        super.onCreate(savedInstanceState);        setContentVIEw(R.layout.activity_student_dashboard);//        Parse.initialize(new Parse.Configuration.Builder(this)//                        .applicationID("J5CIV2z6xeSCXDqdOfhe0kPSikvRFPyDyOJxqJNx")//                        .clIEntKey("bEWm4nWtWiBrMczGfOvA7s4ulr2bAU3W3TtVSLDf")//                        .build()//        );        getSupportActionbar().setdisplayHomeAsUpEnabled(true);        getSupportActionbar().setElevation(2);        vIEwPager = (VIEwPager) findVIEwByID(R.ID.vIEwpager);        setupVIEwPager(vIEwPager);        tabLayout = (TabLayout) findVIEwByID(R.ID.tabs);        tabLayout.setupWithVIEwPager(vIEwPager);//        ParSEObject testObject = new ParSEObject("Task");//        testObject.put("marks",99);//        testObject.saveInBackground();        // ATTENTION: This was auto-generated to implement the App Indexing API.        // See https://g.co/AppIndexing/AndroIDStudio for more information.        clIEnt = new Googleapiclient.Builder(this).addAPI(AppIndex.API).build();    }    private voID setupVIEwPager(VIEwPager vIEwPager) {        VIEwPagerAdapter adapter = new VIEwPagerAdapter(getSupportFragmentManager());        adapter.addFragment(new TaskFragment(),"Quiz");        adapter.addFragment(new TaskFragment(),"Assignment");        adapter.addFragment(new TaskFragment(),"GDP");        vIEwPager.setAdapter(adapter);    }    private voID displayLoginActivity() {        Intent i = new Intent(this,Login.class);        startActivity(i);    }    @OverrIDe    public voID onStart() {        super.onStart();        // ATTENTION: This was auto-generated to implement the App Indexing API.        // See https://g.co/AppIndexing/AndroIDStudio for more information.        clIEnt.connect();        Action vIEwAction = Action.newAction(                Action.TYPE_VIEW,// Todo: choose an action type.                "StudentDashboard Page",// Todo: define a Title for the content shown.                // Todo: If you have web page content that matches this app activity's content,// make sure this auto-generated web page URL is correct.                // Otherwise,set the URL to null.                Uri.parse("http://host/path"),// Todo: Make sure this auto-generated app URL is correct.                Uri.parse("androID-app://com.technerdshub.vusocial.ActivitIEs/http/host/path")        );        AppIndex.AppIndexAPI.start(clIEnt,vIEwAction);    }    @OverrIDe    public voID onStop() {        super.onStop();        // ATTENTION: This was auto-generated to implement the App Indexing API.        // See https://g.co/AppIndexing/AndroIDStudio for more information.        Action vIEwAction = Action.newAction(                Action.TYPE_VIEW,// Todo: Make sure this auto-generated app URL is correct.                Uri.parse("androID-app://com.technerdshub.vusocial.ActivitIEs/http/host/path")        );        AppIndex.AppIndexAPI.end(clIEnt,vIEwAction);        clIEnt.disconnect();    }    class VIEwPagerAdapter extends FragmentPagerAdapter {        private final List<TaskFragment> mFragmentList = new ArrayList<>();        private final List<String> mFragmentTitleList = new ArrayList<>();        public VIEwPagerAdapter(FragmentManager manager) {            super(manager);        }        @OverrIDe        public TaskFragment getItem(int position) {            return mFragmentList.get(position);        }        @OverrIDe        public int getCount() {            return mFragmentList.size();        }        public voID addFragment(TaskFragment fragment,String Title) {            mFragmentList.add(fragment);            mFragmentTitleList.add(Title);        }        @OverrIDe        public CharSequence getPageTitle(int position) {            return mFragmentTitleList.get(position);        }    }}

解决方法 您应该在使用getSupportActionbar()之前设置Actionbar.

为此,请在布局中添加工具栏,然后找到它
工具栏t =(工具栏)findVIEwByID(R.ID.toolbar)
并调用方法setSupportActionbar(t);

然后它应该工作.

总结

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