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。
答案 0 :(得分：0)
尝试更改此行
final ActionBar actionBar = getActionBar();
要
final ActionBar actionBar = getSupportActionBar();
答案 1 :(得分：0)
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。
答案 2 :(得分：0)
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。