java - 应用程序在 Android Studio 中停止，但它可以编译

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

如果之前有人问过这个问题，我很抱歉，但我似乎找不到该问题的解决方案。

我正在 Android Studio 中工作，并遵循一本有关开发应用程序和使用 RecyclerView 的书中的教程，并且该应用程序正在停止，即使它在 Android Studio 中编译。

该代码与书中的代码完全相同，因此我检查了 Logcat 给出了以下类型的错误:

FATAL EXCEPTION: main     Process: ndroid.bignerdranch.com, PID: 17436    
    java.lang.NullPointerException: Attempt to invoke virtual method
    'void android.widget.TextView.setText(java.lang.CharSequence)' on a
    null object reference         at
    ndroid.bignerdranch.com.CrimeListFragment$CrimeHolder.bind
    (CrimeListFragment.java:62)
        at ndroid.bignerdranch.com.CrimeListFragment$CrimeAdapter.onBindViewHolder** 
   (CrimeListFragment.java:91)**
        at ndroid.bignerdranch.com.CrimeListFragment$CrimeAdapter.onBindViewHolder** 
   (CrimeListFragment.java:74)**
        at androidx.recyclerview.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:7056)
        at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:7098)
        at 


 androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:6010)

这里是 CrimeListFragment 类代码，似乎有错误:

        package ndroid.bignerdranch.com;        import android.os.Bundle;    import android.view.LayoutInflater;    import android.view.View;    import android.view.ViewGroup;    import android.widget.TextView;    import android.widget.Toast;    import java.util.List;    import androidx.fragment.app.Fragment;    import androidx.recyclerview.widget.LinearLayoutManager;    import androidx.recyclerview.widget.RecyclerView;    import ndroid.bignerdranch.com.R;    public class CrimeListFragment extends Fragment {        private RecyclerView mCrimeRecyclerView;        private CrimeAdapter mAdapter;        @Override        public View onCreateView(LayoutInflater inflater, ViewGroup container,                                 Bundle savedInstanceState) {            View view = inflater.inflate(R.layout.fragment_crime_list, container, false);            mCrimeRecyclerView = (RecyclerView) view                    .findViewById(R.id.crime_recycler_view);            mCrimeRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));            updateUI();            return view;        }        private void updateUI() {            CrimeLab crimeLab = CrimeLab.get(getActivity());            List crimes = crimeLab.getCrimes();            mAdapter = new CrimeAdapter(crimes);            mCrimeRecyclerView.setAdapter(mAdapter);        }        private class CrimeHolder extends RecyclerView.ViewHolder                implements View.OnClickListener {            private Crime mCrime;            private TextView mTitleTextView;            private TextView mDateTextView;            public CrimeHolder(LayoutInflater inflater, ViewGroup parent) {                super(inflater.inflate(R.layout.list_item_crime, parent, false));                itemView.setOnClickListener(this);                mTitleTextView = (TextView) itemView.findViewById(R.id.crime_title);                mDateTextView = (TextView) itemView.findViewById(R.id.crime_date);            }            public void bind(Crime crime) {                mCrime = crime;                mTitleTextView.setText(mCrime.getTitle());                mDateTextView.setText(mCrime.getDate().toString());            }            @Override            public void onClick(View view) {                Toast.makeText(getActivity(),                        mCrime.getTitle() + " clicked!", Toast.LENGTH_SHORT)                        .show();            }        }        private class CrimeAdapter extends RecyclerView.Adapter {            private List mCrimes;            public CrimeAdapter(List crimes) {                mCrimes = crimes;            }            @Override            public CrimeHolder onCreateViewHolder(ViewGroup parent, int viewType) {                LayoutInflater layoutInflater = LayoutInflater.from(getActivity());                return new CrimeHolder(layoutInflater, parent);            }            @Override            public void onBindViewHolder(CrimeHolder holder, int position) {                Crime crime = mCrimes.get(position);                holder.bind(crime);            }            @Override            public int getItemCount() {                return mCrimes.size();            }        }    } 

Answer 1

您的mTitleTextView为null，并且您尝试调用null的方法。变量在上面的方法中被赋值:

mTitleTextView = (TextView) itemView.findViewById(R.id.crime_title);

当 findViewById 方法在调用它的 View (在您的情况下为 itemView)内找不到具有给定 id 的 View 时，它会返回 null。我在您的代码中找不到此 View 的来源，但您需要检查并确保 ID 为 crime_title 的 View 实际上是其中的一部分。