首页 文章

片段中的空指针异常

提问于
浏览
5

我从其他片段开始一个片段时得到NullPointerException . 我试图在动态启动片段后调用第二个片段中的方法 .

这是我的logcat:

05-20 09:58:31.907: E/AndroidRuntime(2585): FATAL EXCEPTION: main
05-20 09:58:31.907: E/AndroidRuntime(2585): java.lang.NullPointerException
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.exercise.FragmentTest.MyFragment3.setImage(MyFragment3.java:22)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.exercise.FragmentTest.FragmentTestActivity$1.onClick(FragmentTestActivity.java:48)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.view.View.performClick(View.java:4204)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.view.View$PerformClick.run(View.java:17355)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.os.Handler.handleCallback(Handler.java:725)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.os.Handler.dispatchMessage(Handler.java:92)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.os.Looper.loop(Looper.java:137)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at android.app.ActivityThread.main(ActivityThread.java:5041)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at java.lang.reflect.Method.invokeNative(Native Method)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at java.lang.reflect.Method.invoke(Method.java:511)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-20 09:58:31.907: E/AndroidRuntime(2585):     at dalvik.system.NativeStart.main(Native Method)

和我的主要活动代码:

public class FragmentTestActivity extends FragmentActivity{

    Fragment fragment;
    Button btnFragment1, btnFragment2, btnFragment3;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btnFragment1 = (Button)findViewById(R.id.displayfragment1);
        btnFragment2 = (Button)findViewById(R.id.displayfragment2);
        btnFragment3 = (Button)findViewById(R.id.displayfragment3);

       // get an instance of FragmentTransaction from your Activity 
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

        //add a fragment 
        MyFragment myFragment = new MyFragment();
        fragmentTransaction.add(R.id.myfragment, myFragment);
        fragmentTransaction.commit();

        btnFragment1.setOnClickListener(btnFragmentOnClickListener);
        btnFragment2.setOnClickListener(btnFragmentOnClickListener);
        btnFragment3.setOnClickListener(btnFragmentOnClickListener);
    }

    Button.OnClickListener btnFragmentOnClickListener
    = new Button.OnClickListener(){

        @Override
        public void onClick(View v) {


            if(v == btnFragment3){
                MyFragment3 newfragment = new MyFragment3();

                FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

                // Replace whatever is in the fragment_container view with this fragment,
                // and add the transaction to the back stack
                transaction.replace(R.id.myfragment, newfragment);
                transaction.addToBackStack(null);

                // Commit the transaction
                transaction.commit();
                            newfragment.setImage();
                return;

            }


            // TODO Auto-generated method stub
            Fragment newFragment = null;

            // Create new fragment
            if(v == btnFragment1){
                newFragment = new MyFragment();
            }
                else if(v == btnFragment2){
                    newFragment = new MyFragment2();
                }


            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

            // Replace whatever is in the fragment_container view with this fragment,
            // and add the transaction to the back stack
            transaction.replace(R.id.myfragment, newFragment);
            transaction.addToBackStack(null);

            // Commit the transaction
            transaction.commit();

            // Create new transaction

        }};

}

而我的Fragment3类是:

public class MyFragment3 extends Fragment {
    ImageView iv;
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
       Bundle savedInstanceState) {
      // TODO Auto-generated method stub
      View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
      iv = (ImageView)myFragmentView.findViewById(R.id.image); 

      return myFragmentView;
     }
     public void setImage(){
         iv.setImageResource(R.drawable.penguins);
     }
}

请帮我克服这个问题 .

5 回答

  • 4

    在Fragment中使用 getActivity.findViewById

    public class MyFragment3 extends Fragment {
        ImageView iv;
         @Override
         public View onCreateView(LayoutInflater inflater, ViewGroup container,
           Bundle savedInstanceState) {
          // TODO Auto-generated method stub
          View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
          iv = (ImageView)getActivity.findViewById(R.id.image); 
    
          return myFragmentView;
         }
         public void setImage(){
             iv.setImageResource(R.drawable.penguins);
         }
    }
    
  • 0

    在我的情况下问题是因为我试图使用 SwipeRefreshLayout 作为片段布局XML的根元素 . 一旦我将 SwipeRefreshLayout 添加为片段使用的 ConstraintLayout 的子节点,它就开始工作了 .

  • 0

    你不能这样做,因为尚未调用onCreateView所以你的 iv 为空 . 您可以将 iv.setImageResource(R.drawable.penguins); 放在 onCreateView() 中,也可以放在 onAttach() 中(我不确定第二个) .

  • 4

    你的错误发生在这里:

    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
    newfragment.setImage();
    

    它正在发生,因为变量 iv 仍然是 null .

    你必须等到片段显示(膨胀)后才能访问膨胀到其中的视图 .

    更多信息,请阅读Fragment lifecycle .

    onCreateView()当片段第一次绘制其用户界面时,系统会调用此方法 . 要为片段绘制UI,必须从此方法返回视图,该视图是片段布局的根 . 如果片段不提供UI,则可以返回null .

    创建此方法的一种非常好的方法是使用 setArgs 方法并传递 Bundle 以及有关图像的信息 .

    尝试使用此代码:

    public MyFragment3 extends Fragment {
    
        public static final IMAGE_RES = "IMAGE_RES";
        private int imageRes;
        private ImageView iv;
    
        public static MyFragment3 init(int imageRes) {
            MyFragment3 frag = new MyFragment3();
    
            Bundle args = new Bundle();
            args.putInt(IMAGE_RES, imageRes);
            frag.setArguments(args);
    
            return frag;
        }
    
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(SavedInstanceState);
    
            imageRes = (getArguments() != null)? getArguments().getInt(IMAGE_RES) : 0;
        }
    
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
           Bundle savedInstanceState) {
              View myFragmentView = inflater.inflate(R.layout.fragmentlayout3, container, false);
             iv = (ImageView)myFragmentView.findViewById(R.id.image); 
             iv.setImageResource(imageRes);
    
             return myFragmentView;
        }
    }
    
  • 0
    MyFragment3 newfragment = new MyFragment3();
    
    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
    newfragment.setImage();
    

    您正在尝试访问将在片段的onCreateView中创建的imageview . 当您调用 newfragment.setImage(); 时,您的视图尚未创建,因此您将获得NPE .

相关问题