回收器查看 in Android Studio 错误 - "null object reference"

Question

我是安卓新手，我刚刚搞砸了一些东西。 我使用了回收视图模板，它工作正常，但我想将对象的类型从字符串更改为包含 "标签管理器对象">

private String tagName;
private String gender;
private String mAgeMin;
private String mAgeMax;
private String mDistance;

从那以后，我就遇到了这个错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.tinderapp, PID: 10921
java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.tinderapp.Tags.TagsManagerAdapter$ItemClickListener.onDeleteClick(int)' on a null object reference
at com.example.tinderapp.Tags.TagsManagerAdapter$ViewHolder$1.onClick(TagsManagerAdapter.java:78)
at android.view.View.performClick(View.java:6614)
at android.view.View.performClickInternal(View.java:6587)
at android.view.View.access$3100(View.java:787)
at android.view.View$PerformClick.run(View.java:26122)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:201)
at android.app.ActivityThread.main(ActivityThread.java:6831)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:927)

我的适配器看起来像：

public class TagsManagerAdapter extends RecyclerView.Adapter<TagsManagerAdapter.ViewHolder>{
private LayoutInflater mInflater;
private ImageView mDeleteImage;
private List<TagsManagerObject> mTagsManagerObject;
private ItemClickListener mItemClickListener;
// data is passed into the constructor
public TagsManagerAdapter(Context context,List<TagsManagerObject> TagsManagerObject) {
this.mInflater = LayoutInflater.from(context);
this.mTagsManagerObject = TagsManagerObject;
}
// inflates the row layout from xml when needed
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = mInflater.inflate(R.layout.item_tags_manager, parent, false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
//  String tag = TagsManagerObject.get(position);
holder.tagName.setText("#"+mTagsManagerObject.get(position).getTagName());
holder.gender.setText(mTagsManagerObject.get(position).getGender());
holder.distance.setText(mTagsManagerObject.get(position).getmDistance());
holder.tagAge.setText(mTagsManagerObject.get(position).getmAgeMin() + "-" + mTagsManagerObject.get(position).getmAgeMax());
}
// binds the data to the TextView in each row
// total number of rows
@Override
public int getItemCount() {
return mTagsManagerObject.size();
}
public void setClickListener(ItemClickListener mItemClickListener) {
this.mItemClickListener = mItemClickListener;
}

// stores and recycles views as they are scrolled off screen
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
private TextView tagName,gender,tagAge,distance;
ViewHolder(View itemView) {
super(itemView);
tagName= itemView.findViewById(R.id.tag);
gender = itemView.findViewById(R.id.tag_gender);
distance = itemView.findViewById(R.id.tag_distance);
tagAge = itemView.findViewById(R.id.tag_age);
mDeleteImage = itemView.findViewById(R.id.tag_delete);
itemView.setOnClickListener(this);

mDeleteImage.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position = getAdapterPosition();
if(position!=RecyclerView.NO_POSITION){
mItemClickListener.onDeleteClick(position); <<--- HERE IS THE ERROR
}
}
});
}
@Override
public void onClick(View view) {
if (mItemClickListener != null) mItemClickListener.onItemClick(view, getAdapterPosition());
}

}
// convenience method for getting data at click position
String getItem(int id) {
return mTagsManagerObject.get(id).toString();
}
// allows clicks events to be caught
// parent activity will implement this method to respond to click events
public interface ItemClickListener {
void onItemClick(View view, int position);
void onDeleteClick(int position);
}
}

因此，当我单击删除按钮时，它会崩溃。有谁知道为什么？ 谢谢。

Answer 1

在对null 对象调用onDeleteClick(position)时出现此异常，因此在调用此方法之前，必须将值设置为mItemClickListeneronDeleteClick(position)

在代码中，必须在使用此适配器的活动/片段中调用适配器setClickListener()。

Answer 2

你需要做两件事。

首先，您需要检查 mItemClickListener 是否为 null。这样做将防止空指针异常。

mDeleteImage.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int position = getAdapterPosition();
if(position!=RecyclerView.NO_POSITION){
if(mItemClickListener !=null){
mItemClickListener.onDeleteClick(position); <<--- HERE IS THE ERROR
}
}
}
}
);

第二件事是你应该使用RecyclerviewAdapter的实例为活动/片段设置/调用setClickListener。

这样做将使 Activity/Fragment 实现您的ItemClickListener和onDeleteClick((方法。

adapter.setClickListener(this);

或者，您也可以在此处使用Anonymous类。

Answer 3

谢谢大家的帮助。问题是，在我的意识中，我有：

1.创建带有空标签列表的适配器

2.设置点击监听器

adapter.setOnItemClickListener(new TagsManagerAdapter.OnItemClickListener() {
@Override
public void onItemClick(int position) {
}
@Override
public void onDeleteClick(int position) {
System.out.println("works?");
removeItem(position);
}
});

3. 然后添加项目。

相反

adapter.notifyDataSetChanged();

我使用过：

adapter = new TagsManagerAdapter(myTagsList);

这导致了问题，它找不到我的听众