为什么项目 select 在普通列表的 recyclerview 中不起作用，但在搜索列表中起作用？

Question

我已经为存储在 firebase 数据库中的项目创建了数组列表。我能够在 recyclerview 中列出项目。甚至搜索也能正常工作。但是当涉及到单击 recyclerview 列表中的项目时，单击在搜索列表中有效，但在正常列表中无效。代码如下

public class MainActivity extends AppCompatActivity implements Adapter.SelectedProject {

    DatabaseReference mDatabase;
    ArrayList<projects> list;
    RecyclerView PList;

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

        PList = findViewById(R.id.ProjList);
        PList.setHasFixedSize(true);
        mDatabase = FirebaseDatabase.getInstance().getReference().child("Projects");
    }

    @Override
    protected void onStart() {
        super.onStart();

        if (mDatabase != null) {

            mDatabase.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(projects.class));
                        }
                        Adapter adapter = new Adapter(list, null);
                        PList.setAdapter(adapter);
                    }
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                    Toast.makeText(MainActivity.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();

                }
            });
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu_home, menu);
        MenuItem item = menu.findItem(R.id.search);
        SearchView searchView = (SearchView) item.getActionView();

        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;
                }
            });
        }

        return super.onCreateOptionsMenu(menu);
    }

    private void search(String str) {

        ArrayList<projects> sList = new ArrayList<>();

        for (projects object : list) {

            if (object.getPname().toLowerCase().contains(str.toLowerCase())) {

                sList.add(object);
            }

        }
        Adapter adapter = new Adapter(sList, this);
        PList.setAdapter(adapter);

    }

    @Override
    public void selectedProject(projects projects) {
        startActivity(new Intent(MainActivity.this, InfoActivity.class));
    }
}

适配器Class

public class Adapter extends RecyclerView.Adapter < Adapter.ProjViewHolder > {

    private ArrayList < projects > list;
    private SelectedProject selectedProject;

    public Adapter(ArrayList < projects > list, SelectedProject selectedProject) {

        this.list = list;
        this.selectedProject = selectedProject;
    }

    @NonNull
    @Override
    public ProjViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.proj_list, parent, false);
        return new ProjViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ProjViewHolder holder, int position) {

        holder.prname.setText(list.get(position).getPname());
        holder.prlocation.setText(list.get(position).getPlocation());

    }

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

    public interface SelectedProject {
        void selectedProject(projects projects);
    }

    class ProjViewHolder extends RecyclerView.ViewHolder {

        TextView prname, prlocation;

        public ProjViewHolder(@NonNull View itemView) {
            super(itemView);

            prname = itemView.findViewById(R.id.FPName);
            prlocation = itemView.findViewById(R.id.FPLocation);

            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    selectedProject.selectedProject(list.get(getAdapterPosition()));
                }
            });
        }
    }
}

选择在搜索适配器中被接受。

Adapter adapter = new Adapter(sList,this);

但普通适配器显示以下代码错误。

Adapter adapter = new Adapter(list,this);

但是如果我如下将其更改为 null，错误就会消失。

Adapter adapter = new Adapter(list,null);

但是当我点击时，应用程序关闭了。

请告诉我如何更正错误？

stackrace 详细信息

 java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.volq.Adapter$SelectedProject.selectedProject(com.example.volq.projects)' on a null object reference
    at com.example.volq.Adapter$ProjViewHolder.onClick(Adapter.java:68)
    at android.view.View.performClick(View.java:5610)
    at android.view.View$PerformClick.run(View.java:22265)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6077)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

Answer 1

请注意您为搜索设置适配器与正常设置适配器的方式之间的区别

Adapter adapter = new Adapter(sList, this); // `this`as the second parameter/listener is ok

但对于另一个：

Adapter adapter = new Adapter(list, null);

这里监听器为null，这个错误是有道理的：

java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.volq.Adapter$SelectedProject.selectedProject(com.example.volq.projects)' on a null object reference

而不是 null，只需在 Activity 的 onStart() 回调中传递 MainActivity.this：

Adapter adapter = new Adapter(list, MainActivity.this);

But normal adapter shows error for the below code.

Adapter adapter = new Adapter(list,this);

是因为this在onDataChange()回调里面没有引用MainActivity，而是引用了ValueEventListener

为什么项目 select 在普通列表的 recyclerview 中不起作用，但在搜索列表中起作用？

Why item select doesn't work in recyclerview on normal list but works in search list?

android

adapter

android-recyclerview