java - BaseActivity 首先扩展 Activity，但另一个 Activity 出现 nullpointerException

Question

我想创建基本 Activity 以供每个新 Activity 使用，有一个 fragment ，它创建 5 个新 Intent 。 FourOne 是第一个 Intent ，这个工作完美，但其他人遇到了空指针异常。我该如何解决这个问题？

基础 Activity

public class BaseActivity extends ActionBarActivity {
     private DrawerLayout mDrawerLayout;
     private ListView mDrawerList;
     private ActionBarDrawerToggle mDrawerToggle;
     protected RelativeLayout _completeLayout, _activityLayout;
     // nav drawer title
     private CharSequence mDrawerTitle;

      // used to store app title
     private CharSequence mTitle;

      private ArrayList<NavDrawerItem> navDrawerItems;
     private NavDrawerListAdapter adapter;

      @Override
     protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.base_activity);
       //if (savedInstanceState == null) {
       //on first time display view for first nav item
      //displayView(0);
      // }
     }

      public void set(String[] menutitles,TypedArray menuIcons) {
      mTitle = mDrawerTitle = getTitle();

       mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
       mDrawerList = (ListView) findViewById(R.id.slider_list);

       navDrawerItems = new ArrayList<NavDrawerItem>();

       // adding nav drawer items
      if(menuIcons==null){
      for(int i=0;i<menutitles.length;i++){
       navDrawerItems.add(new NavDrawerItem(menutitles[i])); 
      }}else{
       for(int i=0;i<menutitles.length;i++){
        navDrawerItems.add(new NavDrawerItem(menutitles[i],menuIcons.getResourceId(i, -1))); 
       }
      }

       mDrawerList.setOnItemClickListener(new SlideMenuClickListener());

       // setting the nav drawer list adapter
      adapter = new NavDrawerListAdapter(getApplicationContext(),
        navDrawerItems);
      mDrawerList.setAdapter(adapter);

       // enabling action bar app icon and behaving it as toggle button
      getSupportActionBar().setDisplayHomeAsUpEnabled(true);
      getSupportActionBar().setHomeButtonEnabled(true);
      // getSupportActionBar().setIcon(R.drawable.ic_drawer);

       mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
        R.drawable.ic_drawer, // nav menu toggle icon
        R.string.app_name, // nav drawer open - description for
             // accessibility
        R.string.app_name // nav drawer close - description for
             // accessibility
      ) {
       public void onDrawerClosed(View view) {
        getSupportActionBar().setTitle(mTitle);
        // calling onPrepareOptionsMenu() to show action bar icons
        supportInvalidateOptionsMenu();
       }

        public void onDrawerOpened(View drawerView) {
        getSupportActionBar().setTitle(mDrawerTitle);
        // calling onPrepareOptionsMenu() to hide action bar icons
        supportInvalidateOptionsMenu();
       }
      };
      mDrawerLayout.setDrawerListener(mDrawerToggle);

     }


      private class SlideMenuClickListener implements
       ListView.OnItemClickListener {
      @Override
      public void onItemClick(AdapterView<?> parent, View view, int position,
        long id) {



       // display view for selected nav drawer item
       displayView(position);
      }
     }

     @Override
     public boolean onCreateOptionsMenu(Menu menu) {
      //getSupportMenuInflater().inflate(R.menu.main, menu);
      return true;
     }

     @Override
     public boolean onOptionsItemSelected(MenuItem item) {

      if (item.getItemId() == android.R.id.home) {
       if (mDrawerLayout.isDrawerOpen(mDrawerList)) {
        mDrawerLayout.closeDrawer(mDrawerList);
       } else {
        mDrawerLayout.openDrawer(mDrawerList);
       }
      }

      return super.onOptionsItemSelected(item);
     }

     /***
      * Called when invalidateOptionsMenu() is triggered
      */
     @Override
     public boolean onPrepareOptionsMenu(Menu menu) {
      // if nav drawer is opened, hide the action items
      // boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
      // menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
      return super.onPrepareOptionsMenu(menu);
     }

     /**
      * Diplaying fragment view for selected nav drawer list item
      * */
     private void displayView(int position) {
      // update the main content by replacing fragments



         switch (position) {


         case 0:

               Intent intent = new Intent(this, FourOne.class);
               startActivity(intent);
               finish();
               break;
              case 1:
               Intent intent1 = new Intent(this, FourTwo.class);
               startActivity(intent1);
               finish();
               break;
              case 2:
               Intent intent2 = new Intent(this, FourThree.class);
               startActivity(intent2);
               finish();
               break;
              case 3:
               Intent intent3 = new Intent(this, FourFour.class);
               startActivity(intent3);
               finish();
               break;
              case 4:
               Intent intent4 = new Intent(this, FourFive.class);
               startActivity(intent4);
               finish();
               break;     
              default:
                         break;
      }

      // update selected item and title, then close the drawer
      mDrawerList.setItemChecked(position, true);
      mDrawerList.setSelection(position);
      mDrawerLayout.closeDrawer(mDrawerList);
     }

     @Override
     public void setTitle(CharSequence title) {
      mTitle = title;
      getActionBar().setTitle(mTitle);
     }

     /**
      * When using the ActionBarDrawerToggle, you must call it during
      * onPostCreate() and onConfigurationChanged()...
      */

     @Override
     protected void onPostCreate(Bundle savedInstanceState) {
      super.onPostCreate(savedInstanceState);
      // Sync the toggle state after onRestoreInstanceState has occurred.
      mDrawerToggle.syncState();
     }

     @Override
     public void onConfigurationChanged(Configuration newConfig) {
      super.onConfigurationChanged(newConfig);
      // Pass any configuration change to the drawer toggls
      mDrawerToggle.onConfigurationChanged(newConfig);
     }
    }

第一个 Activity ，这个有效

    public class FourOne extends BaseActivity {


    private String[] menutitles;
    private TypedArray menuIcons;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.four_one);



        menutitles = getResources().getStringArray(R.array.titlestwo); // load titles from strings.xml

        menuIcons = getResources()
                        .obtainTypedArray(R.array.iconstwo);//load icons from strings.xml



        set(menutitles, menuIcons);

    }



}

第二个 Activity

    public class FourTwo extends BaseActivity {


    private String[] menutitles;
    private TypedArray menuIcons;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.four_two);



        menutitles = getResources().getStringArray(R.array.titlestwo); // load titles from strings.xml

        menuIcons = getResources()
                        .obtainTypedArray(R.array.iconstwo);//load icons from strings.xml



        set(menutitles, menuIcons);

    }



}

logcat错误

07-15 07:36:17.647: E/AndroidRuntime(1703): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sggsoftware.airsoftg/com.sggsoftware.airsoftg.FourTwo}: java.lang.NullPointerException
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.os.Handler.dispatchMessage(Handler.java:102)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.os.Looper.loop(Looper.java:136)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at java.lang.reflect.Method.invokeNative(Native Method)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at java.lang.reflect.Method.invoke(Method.java:515)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at dalvik.system.NativeStart.main(Native Method)
07-15 07:36:17.647: E/AndroidRuntime(1703): Caused by: java.lang.NullPointerException
07-15 07:36:17.647: E/AndroidRuntime(1703):     at com.sggsoftware.airsoftg.BaseActivity.set(BaseActivity.java:65)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at com.sggsoftware.airsoftg.FourTwo.onCreate(FourTwo.java:28)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.Activity.performCreate(Activity.java:5231)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
07-15 07:36:17.647: E/AndroidRuntime(1703):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
07-15 07:36:17.647: E/AndroidRuntime(1703):     ... 11 more

Answer 1

当在扩展 BaseActivity 的 Activity 之一中调用 onCreate 时，您正在调用 super.onCreate，即 BaseActivity 的 onCreate。现在的问题是您在 BaseActivity 的 onCreate 中调用 setContentView ，然后在实际 Activity 中再次调用 setContentView 。这意味着您的base_activity.xml 分别被four_one.xml、four_two.xml 等替换。这意味着，除非 R.id.drawer_layout 等 View 也存在于 four_one.xml 等中，否则您将无法访问这些 View 。因此出现 NullPointerException。

有两种解决方案:

1. 删除 Activity 扩展，取而代之的是让 FourOne、FourTwo 扩展 Fragment，将容器布局放入 base_activity.xml 中，以及使用 Fragment 还需要做什么。

2. 从 BaseActivity 中删除 onCreate，然后将您需要的 base_activity.xml 中的所有 View 分别放入不同的布局 four_one.xml、four_two.xml...中。使用 include-tag，这样您就不必重复 XML 代码。