條形碼片段「調用虛擬方法」無效android.widget.Button.setOnClickListener（android.view.View $ OnClickListener）'空對象參考「

Question

我想使用ZXing庫進行條碼掃描片段。 我已經成功導入庫，但是當我從APP選擇這個片段的應用程序崩潰與下面的回溯：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
         at com.fastnetserv.barcodeapp.Barcode.onResume(Barcode.java:86) 
         at android.support.v4.app.Fragment.performResume(Fragment.java:2020) 
         at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1107) 
         at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1252) 
         at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:742) 
         at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1617) 
         at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517) 
         at android.os.Handler.handleCallback(Handler.java:739) 
         at android.os.Handler.dispatchMessage(Handler.java:95) 
         at android.os.Looper.loop(Looper.java:135) 
         at android.app.ActivityThread.main(ActivityThread.java:5254) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at java.lang.reflect.Method.invoke(Method.java:372) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 

而且這裏的片段代碼

package com.fastnetserv.barcodeapp; 

import com.google.zxing.integration.android.IntentIntegrator; 
import com.google.zxing.integration.android.IntentResult; 
import android.content.Context; 
import android.net.Uri; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.app.Activity; 
import android.content.Intent; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.TextView; 
import android.widget.Toast;  

/** 
* A simple {@link Fragment} subclass. 
* Activities that contain this fragment must implement the 
* {@link Barcode.OnFragmentInteractionListener} interface 
* to handle interaction events. 
* Use the {@link Barcode#newInstance} factory method to 
* create an instance of this fragment. 
*/ 
public class Barcode extends Fragment implements OnClickListener { 
    // TODO: Rename parameter arguments, choose names that match 
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER 
    private static final String ARG_PARAM1 = "param1"; 
    private static final String ARG_PARAM2 = "param2"; 

    // TODO: Rename and change types of parameters 
    private String mParam1; 
    private String mParam2; 

    private OnFragmentInteractionListener mListener; 

    public Barcode() { 
     // Required empty public constructor 
    } 

    /** 
    * Use this factory method to create a new instance of 
    * this fragment using the provided parameters. 
    * 
    * @param param1 Parameter 1. 
    * @param param2 Parameter 2. 
    * @return A new instance of fragment Barcode. 
    */ 
    // TODO: Rename and change types and number of parameters 
    public static Barcode newInstance(String param1, String param2) { 
     Barcode fragment = new Barcode(); 
     Bundle args = new Bundle(); 
     args.putString(ARG_PARAM1, param1); 
     args.putString(ARG_PARAM2, param2); 
     fragment.setArguments(args); 
     return fragment; 
    } 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     if (getArguments() != null) { 
      mParam1 = getArguments().getString(ARG_PARAM1); 
      mParam2 = getArguments().getString(ARG_PARAM2); 
     }  
    } 

    private Button scanBtn; 
    private TextView formatTxt, contentTxt; 

    @Override 
    public void onResume() { 
     super.onResume(); 

     //instantiate button, text, content 

     scanBtn = (Button) getActivity().findViewById(R.id.scan_button); 
     formatTxt = (TextView) getActivity().findViewById(R.id.scan_format); 
     contentTxt = (TextView) getActivity().findViewById(R.id.scan_content); 

     scanBtn.setOnClickListener(this); 
    } 

    // TODO: Rename method, update argument and hook method into UI event 
    public void onButtonPressed(Uri uri) { 
     if (mListener != null) { 
      mListener.onFragmentInteraction(uri); 
     } 
    } 

    @Override 
    public void onAttach(Context context) { 
     super.onAttach(context); 
     try { 
      mListener = (OnFragmentInteractionListener) context; 
     } catch (ClassCastException e) { 
      throw new ClassCastException(context.toString() 
        + " must implement OnFragmentInteractionListener"); 
     } 
    } 

    @Override 
    public void onDetach() { 
     super.onDetach(); 
     mListener = null; 
    } 

    public void onActivityResult(int requestCode, int resultCode, Intent intent) { 
     //retrieve scan result 
     IntentResult scanningResult = IntentIntegrator.parseActivityResult(requestCode, resultCode, intent); 
     if (scanningResult != null) { 
      //we have a result 
      String scanContent = scanningResult.getContents(); 
      String scanFormat = scanningResult.getFormatName(); 
      formatTxt.setText("FORMAT: " + scanFormat); 
      contentTxt.setText("CONTENT: " + scanContent); 
     }else{ 
      Toast toast = Toast.makeText(getActivity(), 
        "No scan data received!", Toast.LENGTH_SHORT); 
      toast.show(); 
     } 
    }  

    @Override 
    public void onClick(View v) { 
     //respond to clicks 
     if(v.getId()==R.id.scan_button){ 
     //scan 
     IntentIntegrator scanIntegrator = new IntentIntegrator(getActivity()); 
     scanIntegrator.initiateScan(); 
     } 
    }  
} 

什麼我丟失或什麼我做錯了？

Answer 1

您沒有定義的任何視圖您的片段，所以它不能因此找到相關的ID產生在Null Pointer Exception。 覆蓋在你的片段onCreateView()方法，並在提供的視圖，然後寫你findViewById代碼在這個方法中這樣

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
View view = inflater.inflate(R.layout.layout, container, false); 
btn_name= (Button) view.findViewById(R.id.your_button_id); 
return view; 
} 

Answer 2

你不能在Fragment中使用getActivity（）實例化視圖。

你必須做像下面和刪除的onResume實例（）：

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.your_fragment_layout, container, false); 
    scanBtn = (Button) view.findViewById(R.id.scan_button); 
    formatTxt = (TextView) view.findViewById(R.id.scan_format); 
    contentTxt = (TextView) view.findViewById(R.id.scan_content); 
    scanBtn.setOnClickListener(this); 
    return view; 
}