Question

I am having some trouble with this piece of code:

MyAdapter.java:

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


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

    }

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

    @Override
    public Fragment getItem(int position) {
        switch (position) {
        case 0: return new SpeedZonesFragment();
        case 1: return new DistanceFragment();
        case 2: return new FuelConsumptionFragment();
        default: return null;
        }
    }
}

MainActivity.java:

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

public class MainActivity extends FragmentActivity {

    private MyAdapter myAdapter;
    private ViewPager myViewPager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.view_pager);

        myAdapter = new MyAdapter(getSupportFragmentManager());

        myViewPager = (ViewPager) findViewById(R.id.pager);
        myViewPager.setAdapter(myAdapter);

view_pager.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <android.support.v4.view.ViewPager
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/view_pager" />

</LinearLayout>

My problem goes that every example i can find declares MyAdapter as a public static class however Eclipse raise the error: Illegal modifier for the class MyAdapter; only public, abstract & final are permitted.

If i try to run my code without the static it returns (from LogCat):

Caused by: java.lang.NullPointerException
02-02 23:13:26.076: E/AndroidRuntime(928):
at simcas.fartberegneren.MainActivity.onCreate(MainActivity.java:23)

EDIT: Line 23 is the line myViewPager.setAdapter(myAdapter);

Clearly i am doing something wrong but i just can’t locate it.

Answer 1

Firstly, no, static classes are only used for inner classes wherein the inner class’s instances should not be tied to the outer class’s instance. (That is, in an outer class, the static modified should not be used.)

Second, the null pointer is in the statement myViewPager.setAdapter(myAdapter);, as you deduced, meaning myViewPager is null. Therefore, the findViewById() is returning null (item not found):

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

Notice, in your XML: android:id="@+id/view_pager". The above line should then be:

myViewPager = (ViewPager) findViewById(R.id.view_pager);