java - getAdapterPosition recyclerview上的空对象引用

Question

This question already has answers here:

What is a NullPointerException, and how do I fix it?

（12个答案）


7天前关闭。





我在YouTube上关注了一些教程，其中显示了Firebase的数据列表，然后我尝试使列表中的每个项目都可单击，但是应用程序崩溃并发出了 NullReferenceException错误。

每当我尝试点击时，它都会崩溃，并显示以下异常：

java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.logindemo.AdapterClass$OnCardListener.onCardClick(android.view.View, int)' on a null object reference
        at com.example.logindemo.AdapterClass$MyViewHolder.onClick(AdapterClass.java:67)
        at android.view.View.performClick(View.java:7339)

我不知道从哪里开始对问题进行故障排除，我尝试的每个更改都会给出相同的null错误。

AdapterClass.java：

public class AdapterClass extends RecyclerView.Adapter<AdapterClass.MyViewHolder>{

    static ArrayList<LocationDetail> list;
    private OnCardListener OnCardListener;

    public AdapterClass(ArrayList<LocationDetail> list)
    {
        this.list = list;

    }

    public AdapterClass(ArrayList<LocationDetail> list, ValueEventListener valueEventListener, OnCardListener onCardListener) {
        this.OnCardListener = onCardListener;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.card_holder, parent, false);
        return new MyViewHolder(view, OnCardListener);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        holder.nameloc.setText(list.get(position).getLocationName());
        holder.descriptionloc.setText(list.get(position).getLocationDescription());
        holder.categoryloc.setText(list.get(position).getLocationCategory());
    }

    @Override
    public int getItemCount() {
        return list.size();
    }

    class MyViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {

        TextView nameloc, descriptionloc, categoryloc;
        OnCardListener onCardListener;

        public MyViewHolder(@NonNull View itemView, OnCardListener OnCardListener) {
            super(itemView);
            nameloc = itemView.findViewById(R.id.nameloc);
            descriptionloc = itemView.findViewById(R.id.descriptionloc);
            categoryloc = itemView.findViewById(R.id.categoryloc);

            itemView.setOnClickListener(this);
            this.onCardListener = OnCardListener;

        }

        @Override
        public void onClick(View v) {

            onCardListener.onCardClick(v, getAdapterPosition());
        }
    }

    public interface OnCardListener {
        void onCardClick (View view, int position);
    }
}

SecondActivityTest.java：

public class SecondActivityTest extends AppCompatActivity implements AdapterClass.OnCardListener {

    private static final String TAG = "SecondActivityTest";
    private FirebaseAuth firebaseAuth;
    DatabaseReference ref;
    ArrayList<LocationDetail> list;
    RecyclerView recyclerView;
    SearchView searchView;
    Integer count;

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

        firebaseAuth = FirebaseAuth.getInstance();
        ref = FirebaseDatabase.getInstance().getReference().child("Location ");
        recyclerView = findViewById(R.id.rv);
        searchView = findViewById(R.id.searchView);

    }

    @Override
    protected void onStart() {
        super.onStart();
        if(ref != null){
            ref.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    if(dataSnapshot.exists())
                    {
                        list = new ArrayList<>();
                        for(DataSnapshot ds : dataSnapshot.getChildren())
                        {
                           list.add(ds.getValue(LocationDetail.class));
                        }
                        AdapterClass adapterClass = new AdapterClass(list);
                        recyclerView.setLayoutManager(new LinearLayoutManager(SecondActivityTest.this));
                        recyclerView.setAdapter(adapterClass);
                    }
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {
                    Toast.makeText(SecondActivityTest.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();
                }
            });
        }
        if(searchView != null)
        {
            searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
                @Override
                public boolean onQueryTextSubmit(String query) {
                    return false;
                }

                @Override
                public boolean onQueryTextChange(String newText) {
                    search(newText);
                    return true;
                }
            });
        }
    }

    private void search(String str) {
        ArrayList<LocationDetail> myList = new ArrayList<>();
        for(LocationDetail object : list)
        {
            if (object.getLocationName().toLowerCase().contains(str.toLowerCase()) || object.getLocationCategory().toLowerCase().contains(str.toLowerCase()) )
            {
                myList.add(object);
            }
        }
        AdapterClass adapterClass = new AdapterClass(myList);
        recyclerView.setAdapter(adapterClass);

    }


    private void Logout(){
        firebaseAuth.signOut();
        finish();
        startActivity(new Intent(SecondActivityTest.this, MainActivity.class));
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch(item.getItemId()) {
            case R.id.logoutMenu: {
                Logout();
                break;
            }
            case R.id.profileMenu: {
                startActivity(new Intent(SecondActivityTest.this, ProfileActivity.class));
                break;
            }
            case R.id.WeatherMenu: {
                startActivity(new Intent(SecondActivityTest.this, WeatherActivity.class));
                break;
            }
            case R.id.CurrentLocationMenu: {
                startActivity(new Intent(SecondActivityTest.this, GoogleMapsActivity.class));
                break;
            }

        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onCardClick(View view, int position) {
        Log.d(TAG, "Clicked");

        Toast.makeText(SecondActivityTest.this, "Click", Toast.LENGTH_SHORT).show();
        /*
        list.get(position);
        Intent intent = new Intent(this, LocationActivity.java);
        intent.putExtra("Som obj", "Some Ob");
        startActivity(intent);*/
    }
}

最佳答案

您没有将OnCardListener传递给适配器。它为null，因此引发异常。

修改SecondActivityTest.java中的以下行

AdapterClass adapterClass = new AdapterClass(list);

至

AdapterClass adapterClass = new AdapterClass(list,null,this);

而且您还必须修改AdapterClass构造函数：

 public AdapterClass(ArrayList<LocationDetail> list, ValueEventListener valueEventListener, OnCardListener onCardListener) {
        this.list = list;
        this.OnCardListener = onCardListener;
    }

关于java - getAdapterPosition recyclerview上的空对象引用，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/62356465/

上一篇：java - 以编程方式在 viewpager 内的 SupportMapFragment 上的按钮

下一篇：java - 使用 XSD、目录解析器和用于 XSLT 的 JAXP DOM 验证 XML

Answer 1

您没有将OnCardListener传递给适配器。它为null，因此引发异常。

修改SecondActivityTest.java中的以下行

AdapterClass adapterClass = new AdapterClass(list);

至

AdapterClass adapterClass = new AdapterClass(list,null,this);

而且您还必须修改AdapterClass构造函数：

 public AdapterClass(ArrayList<LocationDetail> list, ValueEventListener valueEventListener, OnCardListener onCardListener) {
        this.list = list;
        this.OnCardListener = onCardListener;
    }