FragmentPagerAdapter崩溃

Question

我尝试用几个片段构建viewPager。

但是，当我启动应用程序时，我会遇到短信崩溃。

尝试在空对象引用上调用虚拟方法'java.lang.Class java.lang.Object.getClass()‘

我的代码如下：

    public static class MyPagerAdapter extends FragmentPagerAdapter {
        private static int NUM_ITEMS = 4;

        MainFragment main;
        CartFragment cart;

        public MyPagerAdapter(FragmentManager fragmentManager) {
            super(fragmentManager);

            main = new MainFragment();
            cart = new CartFragment();
        }

        // Returns total number of pages
        @Override
        public int getCount() {
            return NUM_ITEMS;
        }

        @Override
        public Fragment getItem(int position) {
            switch (position) {
                case 0: 
                    return main;
                case 1: 
                    return null;
                case 2: 
                    return null;
                case 3:
                    return cart;
                default:
                    return null;
            }
        }
    }

Answer 1

使用下面的代码调用片段..。

  @Override
    public android.support.v4.app.Fragment getItem(int position) {
        Fragment f = new Fragment();
        switch (position) {
            case 0:
                f = new LogoFragment();
                break;
            case 1:
                f = new AboutUsFragment();
                break;
            case 2:
                f = new ServicesFragment();
                break;
        }
        return f;
    }

Answer 2

这是我使用它的方法：

public class ViewPagerAdapter extends FragmentPagerAdapter {

ArrayList<PagerFragments> mFragments;

public ViewPagerAdapter(FragmentManager fm) {
    super(fm);
    this.mFragments = new ArrayList<>();
}

public void addFragment(Fragment fragment,String title) {
    PagerFragments t = new PagerFragments();
    t.setTitle(title);
    t.setFragment(fragment);
    mFragments.add(t);
    notifyDataSetChanged();
}
public void addFragment(Fragment fragment) {
    PagerFragments t = new PagerFragments();
    t.setFragment(fragment);
    mFragments.add(t);
    notifyDataSetChanged();
}
//This method return the fragment for the every position in the View Pager
@Override
public Fragment getItem(int position) {
    return mFragments.get(position).getFragment();
}

// This method return the titles for the Tabs in the Tab Strip

@Override
public CharSequence getPageTitle(int position) {
    return mFragments.get(position).getTitle();
}

// This method return the Number of tabs for the tabs Strip

@Override
public int getCount() {
    return mFragments.size();
}

}

PagerFragment.java：

import android.support.v4.app.Fragment;


public class PagerFragments {
Fragment fragment;
String title;

public Fragment getFragment() {
    return fragment;
}

public void setFragment(Fragment fragment) {
    this.fragment = fragment;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

}

然后在片段或活动中：

  private ViewPagerAdapter viewPagerAdapter;
viewPagerAdapter = new ViewPagerAdapter(getChildFragmentManager());
                    viewPagerAdapter.addFragment(new RTopGainLossFragment(oneDay), "1D");
                    viewPagerAdapter.addFragment(new RTopGainLossFragment(oneWeek), "1W");
                    viewPagerAdapter.addFragment(new RTopGainLossFragment(oneMonth), "1M");
                    viewPagerAdapter.addFragment(new RTopGainLossFragment(oneYear), "YTD");
                    viewPagerAdapter.addFragment(new RTopGainLossFragment(ytd), "1Y");
                    viewPagerAdapter.addFragment(new RTopGainLossFragment(max), "MAX");
                    viewpager.setAdapter(viewPagerAdapter);
                    tab.setupWithViewPager(viewpager);