Question

您好我是Android的新手，我目前正在尝试从我的MainActivity导航到TabbedActivity。我使用Android Studio中Gallery中的默认Tabbed Activity模板创建了一个新的TabbedActivity。

然而，在创建文件之后，我检查了代码，发现在创建Activity时使用的许多代码行都被删除了，这意味着它们已被弃用。

我尝试运行该应用程序，直到我尝试导航到此Tabbed Activity然后它崩溃并在我的Android监视器中抛出错误。

FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.xxxxxx.eventmanager/com.xxxxxx.eventmanager.EventDetailsActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
        at android.app.ActivityThread.access$600(ActivityThread.java:130)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4745)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
        at dalvik.system.NativeStart.main(Native Method)
        Caused by: java.lang.NullPointerException
        at com.xxxxxx.eventmanager.EventDetailsActivity.onCreate(EventDetailsActivity.java:42)
        at android.app.Activity.performCreate(Activity.java:5008)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
        at android.app.ActivityThread.access$600(ActivityThread.java:130) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:137) 
        at android.app.ActivityThread.main(ActivityThread.java:4745) 
        at java.lang.reflect.Method.invokeNative(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:511) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
        at dalvik.system.NativeStart.main(Native Method)

这是名为EventDetailsActivity的Tabbed Activity。我没有添加任何自定义代码或对create：

上生成的默认代码进行了任何更改

package com.xxxxxx.eventmanager;


import android.app.Activity;
import android.app.ActionBar;
import android.app.FragmentTransaction;

import android.app.Fragment;
import android.app.FragmentManager;
import android.support.v13.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;

import android.widget.TextView;

public class EventDetailsActivity extends Activity implements ActionBar.TabListener {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v13.app.FragmentStatePagerAdapter}.
     */
    private SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_event_details);
        getActionBar().setDisplayHomeAsUpEnabled(true);
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);


        // Set up the action bar.
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // When swiping between different sections, select the corresponding
        // tab. We can also use ActionBar.Tab#select() to do this if we have
        // a reference to the Tab.
        mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                actionBar.setSelectedNavigationItem(position);
            }
        });

        // For each of the sections in the app, add a tab to the action bar.
        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            // Create a tab with text corresponding to the page title defined by
            // the adapter. Also specify this Activity object, which implements
            // the TabListener interface, as the callback (listener) for when
            // this tab is selected.
            actionBar.addTab(
                    actionBar.newTab()
                            .setText(mSectionsPagerAdapter.getPageTitle(i))
                            .setTabListener(this));
        }

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_event_details, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        // When the given tab is selected, switch to the corresponding page in
        // the ViewPager.
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";

        public PlaceholderFragment() {
        }

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_event_details, container, false);
            TextView textView = (TextView) rootView.findViewById(R.id.section_label);
            textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER)));
            return rootView;
        }
    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            return PlaceholderFragment.newInstance(position + 1);
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "SECTION 1";
                case 1:
                    return "SECTION 2";
                case 2:
                    return "SECTION 3";
            }
            return null;
        }
    }
}

我是Java和Android的新手，对此感到非常困惑。

Answer 1

尝试更改此行

final ActionBar actionBar = getActionBar();

要

final ActionBar actionBar = getSupportActionBar();

Answer 2

使用extends AppCompatActivity并使用工具栏作为..

    private void setup_tabs(){
        pager = (CustomViewPager)findViewById(R.id.viewpager);
        pager.setOffscreenPageLimit(3);
        adapter = new Tabs_Pager_adapter(getSupportFragmentManager(),this);
        pager.setAdapter(adapter);

        pager.setPagingEnabled(true);
//      Title.setText(adapter.getPageTitle(pager.getCurrentItem()));

                tabLayout = (TabLayout) findViewById(R.id.tabs);

            //  tabLayout.addTab(tabLayout.newTab().setText("Home"));
                tabLayout.addTab(tabLayout.newTab().setText("Respondents"));
                tabLayout.addTab(tabLayout.newTab().setText("History"));
                tabLayout.setupWithViewPager(pager);

    }

    private void setupToolbar(){
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        if(toolbar != null)
            setSupportActionBar(toolbar);

        // Show menu icon
          final ActionBar ab = getSupportActionBar();
          ab.setDisplayHomeAsUpEnabled(true);

          ab.setDisplayShowTitleEnabled(false);

          toolbar.setNavigationIcon(R.drawable.ic_action_back);
          toolbar.setNavigationOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  finish();
              }
          });

    }

在你的布局中..

   <LinearLayout 
    android:layout_weight=".75"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:orientation="vertical"   >

     <android.support.design.widget.TabLayout
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabGravity="fill"
        app:tabMode="fixed"/>   

     <your.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />



 </LinearLayout>

这就是主题..

<style name="Base.Theme.your_theme" parent="Theme.AppCompat.Light.NoActionBar">

       <item name="colorPrimary">@color/my_primary</item>
        <item name="colorPrimaryDark">@color/my_primary_dark</item>
        <item name="colorAccent">@color/my_accent</item>
    </style>

使用带有android支持库的app compat库...以及设计支持库..它很棒。使用layout.xml中的工具栏布局并使用没有操作栏的appcompat设置主题。

Answer 3

我如何对待我的应用中的工具栏（例如）。

在我的活动布局xml文件中：

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

<include layout="@layout/toolbar" />
<!-- more xml code below -->

toolbat.xml的内容：

<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/toolbar_actionbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:minHeight="?android:attr/actionBarSize"
    app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
    app:popupTheme="@style/PopupMenuStyle"
    android:background="@color/bar_color">

    <!-- arbitrary components are in toolbar -->

</android.support.v7.widget.Toolbar>

现在我准备在我的应用程序中设置工具栏。在onCreate方法中，我必须使用此代码之前使用工具栏：

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

从那时起我可以使用getSupportActionBar()，它会返回正确的结果。

在您的情况下，您可以在布局文件中使用setActionBar(toolbar)代替setSupportActionBar(toolbar)和<android.widget.Toolbar代替<android.support.v7.widget.Toolbar。

选项卡活动不起作用，导致应用程序崩溃

3 个答案: