Question

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，但它会在 getSupportActionBar（）上不断向我显示此错误，请任何人指导我。

FATAL EXCEPTION: main
Process: com.technerdshub.vusocial, PID: 20633
java.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.access$900(ActivityThread.java:177)
  at android.app.ActivityThread$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: 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.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);
        }


    }

}

Answer 1

在使用getSupportActionBar()之前，您应该设置一个ActionBar。

为此，请在布局中添加工具栏，然后找到它 Toolbar t = (Toolbar) findViewById(R.id.toolbar) 并调用方法setSupportActionBar(t);

然后它应该工作。

Answer 2

您的styles.xml中可能有NoActionBar主题应用于此活动。

将styles.xml中的AppTheme更改为：

<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light">
    <!-- Customize your theme here. -->
</style>

确保您的活动继承此主题。

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

2 个答案: