`NotifyDatasetchanged`不起作用。在点击按钮后应用程序崩溃。

huangapple
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英文:

NotifyDatasetchanged not working. AppCrashes after ButtonOnClick

问题

以下是翻译好的部分：

我一直在尝试创建一个按钮，当点击按钮时，可以将新值添加到我的ArrayList（countries）中，但实际上应用程序崩溃了，而不是添加我的项目（Card）。我还在使用Fragments。以下是我的代码。

这是我的Fragment页面类：

  1. package com.example.barryblack;
  3. import android.os.Bundle;
  4. import android.view.LayoutInflater;
  5. import android.view.View;
  6. import android.view.ViewGroup;
  7. import android.widget.Button;
  9. import androidx.annotation.NonNull;
  10. import androidx.annotation.Nullable;
  11. import androidx.fragment.app.Fragment;
  12. import androidx.recyclerview.widget.LinearLayoutManager;
  13. import androidx.recyclerview.widget.RecyclerView;
  15. import java.util.ArrayList;
  17. public class FragmentPage extends Fragment {
  19.     private RecyclerView.Adapter mAdapter;
  20.     private ArrayList<String> countries;
  21.     Button btn;
  23.     @Nullable
  24.     @Override
  25.     public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
  26.         CreateExampleList();
  28.         View view = inflater.inflate(R.layout.page_fragment, container, false);
  30.         RecyclerView recyclerView = view.findViewById(R.id.listRecyclerView);
  32.         btn = view.findViewById(R.id.button_insert);
  34.         btn.setOnClickListener(new View.OnClickListener() {
  35.             @Override
  36.             public void onClick(View v) {
  37.                 InsertItem();
  38.             }
  39.         });
  41.         recyclerView.setHasFixedSize(true);
  42.         RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
  43.         recyclerView.setLayoutManager(layoutManager);
  44.         RecyclerView.Adapter adapter = new MyAdapter(countries);
  45.         recyclerView.setAdapter(adapter);
  47.         return view;
  48.     }
  50.     public void InsertItem() {
  51.         countries.add("Smile");
  52.         mAdapter.notifyDataSetChanged();
  53.     }
  55.     private void CreateExampleList() {
  56.         countries = new ArrayList<>();
  57.         countries.add("Eat");
  58.         countries.add("Kill");
  59.     }
  60. }

这是我的自定义适配器类：

  1. public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
  2.     private ArrayList<String> cards;
  4.     public MyAdapter(ArrayList<String> countries) {
  5.         this.cards = countries;
  6.     }
  8.     @Override
  9.     public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
  10.         View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, false);
  11.         return new ViewHolder(view);
  12.     }
  14.     @Override
  15.     public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {
  16.         viewHolder.tv_country.setText(cards.get(i));
  17.     }
  19.     @Override
  20.     public int getItemCount() {
  21.         return cards.size();
  22.     }
  24.     public class ViewHolder extends RecyclerView.ViewHolder {
  25.         private TextView tv_country;
  27.         public ViewHolder(View view) {
  28.             super(view);
  29.             tv_country = (TextView) view.findViewById(R.id.tv_country);
  30.         }
  31.     }
  32. }

这是您提供的Logcat日志：

java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView$Adapter.notifyDataSetChanged()' on a null object reference
at com.example.barryblack.FragmentPage.InsertItem(FragmentPage.java:60)
at com.example.barryblack.FragmentPage$1.onClick(FragmentPage.java:43)

英文:

I've been trying to create a button that adds a new Value to my Arraylist (contries) when clicked, but it instead of adding my item(Card) the app simply crashes. I am also using Fragments. Here's my code.

That one is my Class for my FragmentPage

  1. package com.example.barryblack;
  3. import android.os.Bundle;
  4. import android.view.LayoutInflater;
  5. import android.view.View;
  6. import android.view.ViewGroup;
  7. import android.widget.Button;
  9. import androidx.annotation.NonNull;
  10. import androidx.annotation.Nullable;
  11. import androidx.fragment.app.Fragment;
  12. import androidx.recyclerview.widget.LinearLayoutManager;
  13. import androidx.recyclerview.widget.RecyclerView;
  15. import java.util.ArrayList;
  17. public class FragmentPage extends Fragment {
  19. private RecyclerView.Adapter mAdapter;
  20. private ArrayList countries;
  21. Button btn;
  28. @Nullable
  29. @Override
  30. public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable 
  31. Bundle savedInstanceState) {
  33.     CreateExampleList();
  35.     View view = inflater.inflate(R.layout.page_fragment, container, false);
  37.     RecyclerView recyclerView = view.findViewById(R.id.listRecyclerView);
  39.     btn = view.findViewById(R.id.button_insert);
  41.     btn.setOnClickListener(new View.OnClickListener() {
  42.         @Override
  43.         public void onClick(View v) {
  44.             InsertItem();
  46.         }
  47.     });
  49.     recyclerView.setHasFixedSize(true);
  50.     RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
  51.     recyclerView.setLayoutManager(layoutManager);
  52.     RecyclerView.Adapter adapter = new MyAdapter(countries);
  53.     recyclerView.setAdapter(adapter);
  56.     return view;
  57. }
  59. public void InsertItem() {
  60.     countries.add(&quot;Smile&quot;);
  61.     mAdapter.notifyDataSetChanged();
  64. }
  66. private void CreateExampleList() {
  67.     countries = new ArrayList&lt;String&gt;();
  68.     countries.add(&quot;Eat&quot;);
  69.     countries.add(&quot;Kill&quot;);
  71. }
  77. }

This is my CustomAdapter

  1. public class MyAdapter extends RecyclerView.Adapter&lt;MyAdapter.ViewHolder&gt; {
  2. private ArrayList&lt;String&gt; cards;
  4. public MyAdapter(ArrayList&lt;String&gt; countries) {
  5.     this.cards = countries;
  6. }
  8. @Override
  9. public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
  10.     View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, 
  11. false);
  12.     return new ViewHolder(view);
  13. }
  15. @Override
  16. public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {
  18.     viewHolder.tv_country.setText(cards.get(i));
  19. }
  21. @Override
  22. public int getItemCount() {
  23.     return cards.size();
  24. }
  26. public class ViewHolder extends RecyclerView.ViewHolder{
  27.     private TextView tv_country;
  28.     private Button tv_button;
  29.     public ViewHolder(View view) {
  30.         super(view);
  32.         tv_country = (TextView)view.findViewById(R.id.tv_country);
  34.     }
  35. }
  37. }public class MyAdapter extends RecyclerView.Adapter&lt;MyAdapter.ViewHolder&gt; {
  38. private ArrayList&lt;String&gt; cards;
  40. public MyAdapter(ArrayList&lt;String&gt; countries) {
  41.     this.cards = countries;
  42. }
  44. @Override
  45. public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
  46.     View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, 
  47. false);
  48.     return new ViewHolder(view);
  49. }
  51. @Override
  52. public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {
  54.     viewHolder.tv_country.setText(cards.get(i));
  55. }
  57. @Override
  58. public int getItemCount() {
  59.     return cards.size();
  60. }
  62. public class ViewHolder extends RecyclerView.ViewHolder{
  63.     private TextView tv_country;
  64.     private Button tv_button;
  65.     public ViewHolder(View view) {
  66.         super(view);
  68.         tv_country = (TextView)view.findViewById(R.id.tv_country);
  70.     }
  71. }
  73. }

And my LogcatText

java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView$Adapter.notifyDataSetChanged()' on a null object reference
at com.example.barryblack.FragmentPage.InsertItem(FragmentPage.java:60)
at com.example.barryblack.FragmentPage$1.onClick(FragmentPage.java:43)

答案1

得分: 0

这段代码：

  1. private RecyclerView.Adapter mAdapter;

需要修改为：

  1. private MyAdapter mAdapter;

以及将这段代码：

  1. RecyclerView.Adapter adapter = new MyAdapter(countries);

修改为：

  1. MyAdapter mAdapter = new MyAdapter(countries);
英文:

this code private RecyclerView.Adapter mAdapter;

must be private MyAdapter mAdapter;

and this RecyclerView.Adapter adapter = new MyAdapter(countries);

to this MyAdapter mAdapter = new MyAdapter(countries);

huangapple
  • 本文由 发表于 2020年4月6日 01:22:01
  • 转载请务必保留本文链接：https://java.coder-hub.com/61046534.html
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