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

Question

我正在使用Android工作室。 当我尝试切换标签我的应用程序将停止，这让我这个错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference 
     at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394) 
     at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389) 
     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:103) 
     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1005) 
     at android.support.v4.view.ViewPager.populate(ViewPager.java:1219) 
     at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:665) 
     at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:627) 
     at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:608) 
     at android.support.design.widget.TabLayout$ViewPagerOnTabSelectedListener.onTabSelected(TabLayout.java:2169) 
     at android.support.design.widget.TabLayout.dispatchTabSelected(TabLayout.java:1164) 
     at android.support.design.widget.TabLayout.selectTab(TabLayout.java:1157) 
     at android.support.design.widget.TabLayout.selectTab(TabLayout.java:1127) 
     at android.support.design.widget.TabLayout$Tab.select(TabLayout.java:1426) 
     at android.support.design.widget.TabLayout$TabView.performClick(TabLayout.java:1536) 
     at android.view.View$PerformClick.run(View.java:21173) 
     at android.os.Handler.handleCallback(Handler.java:739) 
     at android.os.Handler.dispatchMessage(Handler.java:95) 
     at android.os.Looper.loop(Looper.java:145) 
     at android.app.ActivityThread.main(ActivityThread.java:6838) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:372) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199) 

我的代码存储的大部分信息上的标签和碎片

请帮我看看我的错误。

private SectionsPagerAdapter mSectionsPagerAdapter; 

private ViewPager mViewPager; 

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

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager()); 


    mViewPager = (ViewPager) findViewById(R.id.container); 
    mViewPager.setAdapter(mSectionsPagerAdapter); 

    TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs); 
    tabLayout.setupWithViewPager(mViewPager); 

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
    fab.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View view) { 
      Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
        .setAction("Action", null).show(); 
     } 
    }); 

} 


@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_vault_screen, menu); 
    return true; 
} 

@Override 
public boolean onOptionsItemSelected(MenuItem item) { 
    // Handle action bar item clicks here. The action bar will 
    // automatically handle clicks on the Home/Up button, so long 
    // as you specify a parent activity in AndroidManifest.xml. 
    int id = item.getItemId(); 

    //noinspection SimplifiableIfStatement 
    if (id == R.id.action_settings) { 
     return true; 
    } 

    return super.onOptionsItemSelected(item); 
} 
//deleted PlaceholderFragment class from here 

/** 
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to 
* one of the sections/tabs/pages. 
*/ 
public class SectionsPagerAdapter extends FragmentPagerAdapter { 

    public SectionsPagerAdapter(FragmentManager fm) { 
     super(fm); 
    } 

    @Override 
    public Fragment getItem(int position) { 

     switch (position) { 
      case 0: 
       Tab1Contacts tab1 = new Tab1Contacts(); 
       return tab1; 
      case 1: 
       Tab2Chat tab2 = new Tab2Chat(); 
       return tab2; 
      case 3: 
       Tab3Online tab3 = new Tab3Online(); 
       return tab3; 
      default: 
     return null; 
    } 
    } 

    @Override 
    public int getCount() { 
     // Show 3 total pages. 
     return 3; 
    } 

    @Override 
    public CharSequence getPageTitle(int position) { 
     switch (position) { 
      case 0: 
       return "Home"; 
      case 1: 
       return "Private Browser"; 
      case 2: 
       return "Notes"; 
     } 
     return null; 
    } 

} 

Answer 1

我有相同的错误，因为我试图通过TAG或ID获取碎片。但是我必须通过创建新的对象来获取碎片，至少如果没有的话。

Fragment fragment = new BlankFragment();