Question

我在android studio中有一组标签。他们都工作正常，但他们没有显示任何内容，即使他们确实有内容。所以我在这个网站上做了一些挖掘，我找到了这个Problems that users encounter with fragments

所以我按照用户给出的建议。但似乎我的应用程序因为空指针异常而崩溃。请有人帮我发现错误：
的 MainActivity.java

public class MainActivity extends AppCompatActivity implements Photos.OnFragmentInteractionListener, About.OnFragmentInteractionListener, Watch.OnFragmentInteractionListener{
    private TabLayout myTabLayOut;
    private  ViewPager viewPager;
    private  TabsPagerAdapter tabsPagerAdapter;
    public MainActivity()
    {

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

        myTabLayOut = findViewById(R.id.tabLayout);
        myTabLayOut.addTab(myTabLayOut.newTab().setText("Photos"));
        myTabLayOut.addTab(myTabLayOut.newTab().setText("About"));
        myTabLayOut.addTab(myTabLayOut.newTab().setText("Watch"));
        myTabLayOut.setTabGravity(TabLayout.GRAVITY_FILL);


        myTabLayOut.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                viewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });

        viewPager = findViewById(R.id.pager);
        //Item 0 being the photos fragment. Just for testing for the time being. WHERE The error happens
        tabsPagerAdapter = new TabsPagerAdapter(tabsPagerAdapter.getItem(0).getTargetFragment(), myTabLayOut.getTabCount());
        viewPager.setAdapter(tabsPagerAdapter);
        viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(myTabLayOut));
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.my_menu,menu);
        return true;
    }

    @Override
    public void onFragmentInteraction(Uri uri) {

    }
}

TabsPagerAdapter.java

public class TabsPagerAdapter extends FragmentPagerAdapter {
    //A field to hold the number of tabs
    private int numberOfTabs;
    public TabsPagerAdapter(Fragment fragmentManager, int numberOfTabs)
    {

        super(fragmentManager.getChildFragmentManager());
        this.numberOfTabs = numberOfTabs;
    }
    //Which item to return depending on the position
    @Override
    public Fragment getItem(int position) {
        switch (position)
        {
            case 0:
                return new Photos();
            case 1:
                return new About();
            case 2:
                return new Watch();
                default:
                    return null;
        }
    }

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

Answer 1

你的问题在于这一行：

tabsPagerAdapter = new TabsPagerAdapter(tabsPagerAdapter.getItem(0)...

尝试分配tabsPagerAdapter的值时，您尝试使用此对象的值NULL。

Answer 2

FragmentViewPagers getItem方法名称具有误导性 - 它不会返回寻呼机中显示的实际片段，它只用于创建一次。如果您尝试调用此方法，您将始终获得一个新项目。

为什么我的代码为存在的片段返回nullpointer？

2 个答案: