错误创建标签 [英] Error creating tabs

问题描述

这是什么尝试调用虚拟方法无效android.app.ActionBar.setNavigationMode（INT）的空对象引用是什么意思？我试过很多解决方案，但没有人能解决我的问题。

What does Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' on a null object reference meant? I tried many solution but no one can solve my problem.

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)

而在我Mainfest.xml，编译器说

And in my Mainfest.xml, the compiler said

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

是不能分配给android.app ....

is not assignable to android.app....

推荐答案

尝试替换此行：

actionBar = getActionBar();

使用：

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

和添加此有关的onCreate的顶部（）

And add this on top of onCreate()

Activity mActivity = (Activity) this;

和这个活动的启动：

 <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>

这是监听器：

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();

        }

    }

请参阅本：<一href=\"http://stackoverflow.com/questions/15318518/android-activity-vs-fragmentactivity\">FragmentActivity VS活动

还记得使用活动，如果你使用的是android.app.Fragment;如果您使用的是android.support.v4.app.Fragment使用FragmentActivity。千万不要附加android.support.v4.app.Fragment到android.app.Activity，因为这将导致一个异常被抛出。

Also remember to use Activity if you are using android.app.Fragment; use FragmentActivity if you are using android.support.v4.app.Fragment. Never attach a android.support.v4.app.Fragment to an android.app.Activity, as this will cause an exception to be thrown.

这篇关于错误创建标签的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！