如何解決活動和片段通信錯誤？

Question

教程在線顯示此功能適用於其他人。我的代碼顯示了錯誤的位置：

java.lang.NullPointerException: Attempt to invoke virtual method 'void pl.michalz.hideonscrollexample.fragment.PartThreeFragment.volleyJson()' on a null object reference 
      at pl.michalz.hideonscrollexample.activity.partthree.PartThreeActivity$1.onQueryTextSubmit(PartThreeActivity.java:59) 
      at android.support.v7.widget.SearchView.onSubmitQuery(SearchView.java:1156) 
      at android.support.v7.widget.SearchView.access$900(SearchView.java:101) 
      at android.support.v7.widget.SearchView$9.onEditorAction(SearchView.java:1134) 
      at android.widget.TextView.onEditorAction(TextView.java:4770) 
      at com.android.internal.widget.EditableInputConnection.performEditorAction(EditableInputConnection.java:139) 
      at com.android.internal.view.IInputConnectionWrapper.executeMessage(IInputConnectionWrapper.java:304) 
      at com.android.internal.view.IInputConnectionWrapper$MyHandler.handleMessage(IInputConnectionWrapper.java:78) 
      at android.os.Handler.dispatchMessage(Handler.java:102) 
      at android.os.Looper.loop(Looper.java:148) 
      at android.app.ActivityThread.main(ActivityThread.java:5417) 
      at java.lang.reflect.Method.invoke(Native Method) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

搜索查看被監聽的搜索查詢MainActivity類別，我想在mainActivity與ViewPager創建片段使用搜索查詢recyclerView：

public class PartThreeActivity extends AppCompatActivity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     setTheme(R.style.AppThemeBlue); 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_part_three); 
     Log.w("myApp", "onCreate -started- "); 
     initToolbar(); 
     initViewPagerAndTabs(); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 

     Log.w("myApp", "onCreateOptionsMenu -started- "); 

     SearchManager searchManager = (SearchManager) getSystemService(Context.SEARCH_SERVICE); 
     MenuItem menuItem = menu.findItem(R.id.action_search); 
     SearchView searchView = (SearchView) MenuItemCompat.getActionView(menuItem); 

     searchView.setSearchableInfo(searchManager.getSearchableInfo(getComponentName())); 
     searchView.setQueryHint(getResources().getString(R.string.hint)); 

     searchView.setOnQueryTextListener(
       new SearchView.OnQueryTextListener() { 
        @Override 
        public boolean onQueryTextSubmit(String query) { 
         Log.w("myApp", "onQueryTextSubmit query = " + query); 

         PartThreeFragment fragment = (PartThreeFragment) getSupportFragmentManager().findFragmentById(R.id.recyclerView); 
         fragment.volleyJson(); 
         return false; 
        } 

        @Override 
        public boolean onQueryTextChange(String newText) { 
         Log.w("myApp", "onQueryTextChange "); 
         return false; 
        } 
       }); 
     return true; 
    } 


    private void initToolbar() { 
     Log.w("myApp", "initToolbar -started- "); 
     Toolbar mToolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(mToolbar); 
     setTitle(getString(R.string.app_name)); 
     mToolbar.setTitleTextColor(getResources().getColor(android.R.color.white)); 
    } 

    private void initViewPagerAndTabs() { 
     ViewPager viewPager = (ViewPager) findViewById(R.id.viewPager); 
     PagerAdapter pagerAdapter = new PagerAdapter(getSupportFragmentManager()); 
     pagerAdapter.addFragment(PartThreeFragment.createInstance(), getString(R.string.tab_1)); 
     pagerAdapter.addFragment(BlankFragment.createInstance("F word"), getString(R.string.tab_2)); 
     viewPager.setAdapter(pagerAdapter); 
     TabLayout tabLayout = (TabLayout) findViewById(R.id.tabLayout); 
     tabLayout.setupWithViewPager(viewPager); 
    } 

    static class PagerAdapter extends FragmentPagerAdapter { 

     private final List<Fragment> fragmentList = new ArrayList<>(); 
     private final List<String> fragmentTitleList = new ArrayList<>(); 

     public PagerAdapter(FragmentManager fragmentManager) { 
      super(fragmentManager); 
     } 

     public void addFragment(Fragment fragment, String title) { 
      fragmentList.add(fragment); 
      fragmentTitleList.add(title); 
     } 

     @Override 
     public Fragment getItem(int position) { 
      return fragmentList.get(position); 
     } 

     @Override 
     public int getCount() { 
      return fragmentList.size(); 
     } 
     @Override 
     public CharSequence getPageTitle(int position) { 
      return fragmentTitleList.get(position); 
     } 
    } 


    //kai paskaudzia toolbaro iconas 
    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     switch (item.getItemId()) { 
      case R.id.action_settings: 
       // User chose the "Settings" item, show the app settings UI... 
       return true; 

      case R.id.action_search: 
       // User chose the "Favorite" action, mark the current item 
       // as a favorite... 
       return true; 

      default: 
       // If we got here, the user's action was not recognized. 
       // Invoke the superclass to handle it. 
       return super.onOptionsItemSelected(item); 

     } 
    } 

} 

解決方案編輯1：

public class PartThreeActivity extends AppCompatActivity { 
    PagerAdapter pagerAdapter; 
... 

    public boolean onQueryTextSubmit(String query) { 
     Log.w("myApp", "onQueryTextSubmit query = " + query); 
     PartThreeFragment fragment = (PartThreeFragment) pagerAdapter.fragmentList.get(0); 
     fragment.volleyJson(); 
     return false; 
    } 

Answer 1

這條線：

PartThreeFragment fragment = (PartThreeFragment) getSupportFragmentManager().findFragmentById(R.id.recyclerView); 

返回null。仔細檢查你是否在尋找正確的ID。