viewPager.setAdapter（new MyAdapter（getSupportFragmentManager（）））错误

Question

我正在尝试通过viewpager创建一个tabview。在输入“viewPager.setAdapter（new MyAdapter（getSupportFragmentManager（）））;”之前一切似乎都很好。 但是当我输入它时，我面对“不幸的是，......已停止”错误。虽然我添加了v4库，但我面临着错误。

您可以在下面找到以下代码：

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;`

MainActivity

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {

    ViewPager viewPager;
    ActionBar actionBar;

    @Override
    protected void onCreate(Bundle arg0) {
        super.onCreate(arg0);
        setContentView(R.layout.activity_main);

        viewPager = (ViewPager) findViewById(R.id.viewpager);
        viewPager.setAdapter(new MyAdapter(getSupportFragmentManager()));



        actionBar=getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);



        ActionBar.Tab tab1= actionBar.newTab();
        tab1.setIcon(R.drawable.ic_first);
        tab1.setTabListener(this);

        ActionBar.Tab tab2=actionBar.newTab();
        tab2.setIcon(R.drawable.ic_second);
        tab2.setTabListener(this);

        ActionBar.Tab tab3=actionBar.newTab();
        tab3.setIcon(R.drawable.ic_third);
        tab3.setTabListener(this);


        actionBar.addTab(tab1);
        actionBar.addTab(tab2);
        actionBar.addTab(tab3);


    }

    @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) {

    }
}

MyAdapter

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


public class MyAdapter extends FragmentStatePagerAdapter {
    public MyAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int arg0) {
        Fragment fragment=null;
        if(arg0==0){
            fragment = new Fragment1();
        }
        if(arg0==1){
            fragment = new Fragment_2();
        }
        if(arg0==2){
            fragment = new Fragment3();
        }

        return fragment;
    }

    @Override
    public int getCount() {
        return 3;
    }
}

viewpager

<android.support.v4.view.ViewPager
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/viewpager"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"
 />

并且logcat是：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.aras.yoop/com.example.aras.yoop.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.aras.yoop.MainActivity.onCreate(MainActivity.java:22)
            at android.app.Activity.performCreate(Activity.java:5133)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)

提前致谢。