创建选项卡时出错

Question

Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' on a null object reference 是什么意思？我尝试了很多解决方案，但没有人能解决我的问题。

Tab.java

package com.example.project.project;
     import android.os.Bundle;
    import android.app.ActionBar;
    import android.app.FragmentTransaction;
    import android.support.v4.app.FragmentActivity;
    import android.support.v4.view.ViewPager;


public class Tab extends FragmentActivity {
         ViewPager Tab;
         TabPagerAdapter TabAdapter;
         ActionBar actionBar;

         @Override
         protected void onCreate(Bundle savedInstanceState) {
             super.onCreate(savedInstanceState);
             setContentView(R.layout.tab1);

             TabAdapter = new TabPagerAdapter(getSupportFragmentManager());


             Tab = (ViewPager)findViewById(R.id.pager);

             Tab.setOnPageChangeListener(
                     new ViewPager.SimpleOnPageChangeListener() {
                         @Override
                         public void onPageSelected(int position) {

                             actionBar = getActionBar();
                             actionBar.setSelectedNavigationItem(position);                    }
                     });

             Tab.setAdapter(TabAdapter);

             actionBar = getActionBar();

             //Enable Tabs on Action Bar
             actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
             ActionBar.TabListener tabListener = new ActionBar.TabListener(){

                 public void onTabReselected(android.app.ActionBar.Tab tab,
                                             FragmentTransaction ft) {
                     // TODO Auto-generated method stub
                     //Toast.makeText(getApplicationContext(), "Tab selected", 2000).show();

               }

                 @Override
                 public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {

                     Tab.setCurrentItem(tab.getPosition());
                 }

                 @Override
                 public void onTabUnselected(android.app.ActionBar.Tab tab,
                                             FragmentTransaction ft) {
                     // TODO Auto-generated method stub

                 }};
             //Add New Tabs
             actionBar.addTab(actionBar.newTab().setText("Information").setTabListener(tabListener));
             actionBar.addTab(actionBar.newTab().setText("Work Force").setTabListener(tabListener));
             actionBar.addTab(actionBar.newTab().setText("Work Details").setTabListener(tabListener));

         }




     }

TabPagerAdapter.java

package com.example.project.project;

        import android.support.v4.app.Fragment;
        import android.support.v4.app.FragmentManager;
        import android.support.v4.app.FragmentStatePagerAdapter;

public class TabPagerAdapter extends FragmentStatePagerAdapter {
    public TabPagerAdapter(FragmentManager fm) {
        super(fm);
        // TODO Auto-generated constructor stub
    }

   @Override
    public Fragment getItem(int i) {

        switch (i) {
            case 0:

                return new Information();

            case 1:

                return new WorkDetailsTable();
        }
        return null ;

    }

    @Override
    public int getCount() {
        // TODO Auto-generated method stub
        return 3; //No of Tabs you can give your number of tabs
    }


}

Tab1.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

AndroidMainfest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.project.project" >

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

        </activity>
        <activity
            android:name=".Tab">

        </activity>
        <activity android:name=".WorkDetailsTable" android:screenOrientation="landscape"  />
        <activity android:name="Information"/>



    </application>

</manifest>

错误

10-04 11:11:07.492    2146-2146/com.example.project.project E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.project.project, PID: 2146
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.project.project/com.example.project.project.Tab}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' 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)

在我的 Mainfest.xml 中，编译器说

<activity android:name=".WorkDetailsTable" android:screenOrientation="landscape"  />
        <activity android:name="Information"/>

不能分配给 android.app....

Answer 1

尝试替换

 actionBar = getActionBar();

与

 actionBar = ((AppCompatActivity) activity).getSupportActionBar();

在哪里

Activity activity = (Activity) this;

Answer 2

尝试替换此行：

actionBar = getActionBar();

与：

actionBar = ((AppCompatActivity) mActivity).getSupportActionBar();

并将其添加到 onCreate() 之上

Activity mActivity = (Activity) this;

并以此开始活动：

 <activity
        android:name=".YourActivityClassName"
        android:label="YourActivity"
        android:screenOrientation="landscape">
        <intent-filter>
            <action android:name="package.name.YOURACTIVITYCLASSNAME" />
            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
 </activity>

这是听众：

ActionBar.TabListener tabListener = new ActionBar.TabListener() {

        @Override
        public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {

        }

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

        }

        @Override
        public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {
            // TODO Auto-generated method stub
            //Toast.makeText(getApplicationContext(), "Tab selected", 2000).show();

        }

    }

参考这个：FragmentActivity vs Activity

如果你使用的是android.app.Fragment，还要记得使用Activity；如果您使用的是 android.support.v4.app.Fragment，请使用 FragmentActivity。永远不要将 android.support.v4.app.Fragment 附加到 android.app.Activity，因为这会导致抛出异常。

Answer 3

Actionbar has to be replaced with toolbar now, all you need to do is include a toolbar in your xml files. I used the appcompact library toolbar, for this you may have to add support library in your gradle file
    compile 'com.android.support:support-v4:22.0.0'
    compile 'com.android.support:appcompat-v7:22.0.0'

Add follwing toolbar to xml file 
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:theme="@style/Base.ThemeOverlay.AppCompat.Dark">

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


Don't forget to inherit your activity from Appcompactactivity, you may use your own toolbar, finally access the toolbaar in activity
  Toolbar toolbar = (Toolbar) this.findViewById(R.id.toolbar);
   setSupportActionBar(toolbar);