RecyclerView 使应用程序崩溃

Question

我正在尝试使用 Java 在 Android Studio 中练习使用 Recycler。 到目前为止，我已经取得了很大的进步。 当我运行应用程序时，它立即崩溃，我检查了日志，但出现以下错误： Attempt to invoke virtual method 'android.view.View androidx.recyclerview.widget.RecyclerView.findViewById(int)' on a null object reference 和 Attempt to invoke virtual method 'android.view.View androidx.recyclerview.widget.RecyclerView.findViewById(int)' on a null object reference

下面是我的MainActivity类：

package com.example.recyleview;

import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.os.Bundle;
import android.widget.LinearLayout;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    private RecyclerView recView;

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

        recView.findViewById(R.id.recView);

        recView.setLayoutManager(new LinearLayoutManager(this));
        setContentView(R.layout.activity_main);

        ArrayList<Contact> contacts = new ArrayList<>();
        contacts.add(new Contact("Jacob", "jacob@gmail.com", "later"));
        contacts.add((new Contact("Sara ", "sara@gmail.com", "later")));
        contacts.add(new Contact("Nino", "nino@gmail.com", "later"));

        ContactsRecViewAdapter adapter = new ContactsRecViewAdapter(this);
        adapter.setContacts(contacts);
        recView.setAdapter(adapter);
    }
}

这是另一个名为ContactsRecViewAdapter的类

package com.example.recyleview;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.RelativeLayout;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ArrayList;

public class ContactsRecViewAdapter extends RecyclerView.Adapter<ContactsRecViewAdapter.ViewHolder>{

    private ArrayList<Contact> contacts = new ArrayList<>();
    private final Context context;


    public ContactsRecViewAdapter(Context context) {
        this.context = context;
    }

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

    @Override
    public void onBindViewHolder(@NonNull ContactsRecViewAdapter.ViewHolder holder, int position) {
        holder.txtName.setText(contacts.get(position).getName());
        holder.parent.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(context, contacts.get(position).getName() + " Clicked", Toast.LENGTH_SHORT).show();
            }
        });
    }

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

    public void setContacts(ArrayList<Contact> contacts) {
        this.contacts = contacts;
        notifyDataSetChanged();
    }

    public static class ViewHolder extends RecyclerView.ViewHolder{

        private final TextView txtName;
        private final RelativeLayout parent;

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

            txtName = itemView.findViewById(R.id.txtName);
            parent = itemView.findViewById(R.id.parent);
        }
    }

}

Answer 1

改变

recView.findViewById(R.id.recView);

到

recView = findViewById(R.id.recView);

你也复制了：

 setContentView(R.layout.activity_main);

在尝试查找视图之前，您必须设置 contentView。