NotifyDatasetchanged 不起作用。 ButtonOnClick 后 AppCrashes [重复]

Question

我一直在尝试创建一个按钮，在单击时向我的 Arraylist (contries) 添加一个新值，但它没有添加我的项目 (Card)，而是应用程序崩溃了。我也在使用片段。这是我的代码。

那是我的 FragmentPage 类

package com.example.barryblack;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ArrayList;

public class FragmentPage extends Fragment {

private RecyclerView.Adapter mAdapter;
private ArrayList countries;
Button btn;






@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable 
Bundle savedInstanceState) {

    CreateExampleList();

    View view = inflater.inflate(R.layout.page_fragment, container, false);

    RecyclerView recyclerView = view.findViewById(R.id.listRecyclerView);

    btn = view.findViewById(R.id.button_insert);

    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            InsertItem();

        }
    });

    recyclerView.setHasFixedSize(true);
    RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
    recyclerView.setLayoutManager(layoutManager);
    RecyclerView.Adapter adapter = new MyAdapter(countries);
    recyclerView.setAdapter(adapter);


    return view;
}

public void InsertItem() {
    countries.add("Smile");
    mAdapter.notifyDataSetChanged();


}

private void CreateExampleList() {
    countries = new ArrayList<String>();
    countries.add("Eat");
    countries.add("Kill");

}





}

这是我的自定义适配器

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
private ArrayList<String> cards;

public MyAdapter(ArrayList<String> countries) {
    this.cards = countries;
}

@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
    View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, 
false);
    return new ViewHolder(view);
}

@Override
public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {

    viewHolder.tv_country.setText(cards.get(i));
}

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

public class ViewHolder extends RecyclerView.ViewHolder{
    private TextView tv_country;
    private Button tv_button;
    public ViewHolder(View view) {
        super(view);

        tv_country = (TextView)view.findViewById(R.id.tv_country);

    }
}

}public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
private ArrayList<String> cards;

public MyAdapter(ArrayList<String> countries) {
    this.cards = countries;
}

@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
    View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, 
false);
    return new ViewHolder(view);
}

@Override
public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {

    viewHolder.tv_country.setText(cards.get(i));
}

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

public class ViewHolder extends RecyclerView.ViewHolder{
    private TextView tv_country;
    private Button tv_button;
    public ViewHolder(View view) {
        super(view);

        tv_country = (TextView)view.findViewById(R.id.tv_country);

    }
}

}

还有我的 LogcatText

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“void androidx.recyclerview.widget.RecyclerView$Adapter.notifyDataSetChanged()” 在 com.example.barryblack.FragmentPage.InsertItem(FragmentPage.java:60) 在 com.example.barryblack.FragmentPage$1.onClick(FragmentPage.java:43)

Answer 1

此代码private RecyclerView.Adapter mAdapter;

必须是private MyAdapter mAdapter;

还有这个RecyclerView.Adapter adapter = new MyAdapter(countries);

到这个MyAdapter mAdapter = new MyAdapter(countries);