为什么我得到一个空对象引用错误时，试图使一个按钮在长按一个回收器视图项目后可见?

Question

我试图使一个按钮可见，当用户长点击一行在回收器视图。然而，我得到一个空对象引用错误，不知道为什么。Toast工作了，所以我知道长按每一行都在工作。

编辑-崩溃日志:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.it_stock, PID: 13959
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setVisibility(int)' on a null object reference
at com.example.it_stock.StockAdapter$ViewHolder.onLongClick(StockAdapter.java:85)
at android.view.View.performLongClickInternal(View.java:5714)
at android.view.View.performLongClick(View.java:5672)
at android.view.View.performLongClick(View.java:5690)
at android.view.View$CheckForLongPress.run(View.java:22402)
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:6121)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)

主要活动:

public class MainActivity extends AppCompatActivity {
private DBHandler db;
Button btnAddItem;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
RecyclerView stockView = findViewById(R.id.rvStock);
LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
stockView.setLayoutManager(linearLayoutManager);
stockView.setHasFixedSize(true);
db = new DBHandler(this);
ArrayList<Stock> allStock = db.getAllStock();
if (allStock.size() > 0) {
stockView.setVisibility(View.VISIBLE);
StockAdapter stockAdapter = new StockAdapter(this, allStock);
stockView.setAdapter(stockAdapter);
}
else {
stockView.setVisibility(View.GONE);
Toast.makeText(this, "No Items!", Toast.LENGTH_LONG).show();
}
btnAddItem = findViewById(R.id.btnAdd);
btnAddItem.setOnClickListener(v -> {
addNewItem();
});
}
protected void addNewItem() {
Intent i = new Intent(MainActivity.this, AddNewItem.class);
startActivity(i);
}
}

适配器:

public class StockAdapter extends RecyclerView.Adapter<StockAdapter.ViewHolder> {
private Context context;
private ArrayList<Stock> listStock;
private ArrayList<Stock> mArrayList;
private DBHandler db;
private MainActivity mainActivity;
StockAdapter(Context context, ArrayList<Stock> listStock) {
this.context = context;
this.listStock = listStock;
this.mArrayList = listStock;
db = new DBHandler(context);
mainActivity = new MainActivity();
}
@NonNull
@Override
public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
LayoutInflater inflater = LayoutInflater.from(parent.getContext());
View stockView = inflater.inflate(R.layout.stock_row, parent, false);
return new ViewHolder(stockView);
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
final Stock stock = listStock.get(position);
holder.tvItem.setText(stock.getItem());
holder.tvCondition.setText(stock.getCondition());
holder.tvQuantity.setText(String.valueOf(stock.getQuantity()));
holder.tvLocation.setText(stock.getLocation());
holder.tvCategory.setText(stock.getCategory());
}
@Override
public int getItemCount() {
return listStock.size();
}
class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener, View.OnLongClickListener {
final TextView tvItem;
final TextView tvCondition;
final TextView tvQuantity;
final TextView tvLocation;
final TextView tvCategory;
Button btnRemove;
View stockView;
ViewHolder(View stockView) {
super(stockView);
this.stockView = stockView;
stockView.setOnClickListener(this);
stockView.setOnLongClickListener(this);
tvItem = stockView.findViewById(R.id.tvItem);
tvCondition = stockView.findViewById(R.id.tvCondition);
tvQuantity = stockView.findViewById(R.id.tvQuantity);
tvLocation = stockView.findViewById(R.id.tvLocation);
tvCategory = stockView.findViewById(R.id.tvCategory);
btnRemove = stockView.findViewById(R.id.btnRemove);
}
@Override
public void onClick(View stockView) {
}
@Override
public boolean onLongClick(View stockView) {
btnRemove.setVisibility(View.VISIBLE);
Toast.makeText(context, "long pressed", Toast.LENGTH_SHORT).show();
return false;
}
}
}

按钮应该出现在activity_main中的回收器视图下方，我已经在我的xml文件中设置了这一点，并将按钮设置为"不可见"。请有人解释为什么我得到错误和如何解决它?

Answer 1

如果btnRemove为null，则表示stockView.findViewById(R.id.btnRemove)返回null，意味着在R.layout.stock_row

中没有id为btnRemove

的视图。检查R.layout.stock_row中是否有这一行:

android:id="@+id/btnRemove"

Answer 2

像这样修改代码，在按钮之前初始化侦听器。

ViewHolder(View stockView) {
super(stockView);
this.stockView = stockView;
tvItem = stockView.findViewById(R.id.tvItem);
tvCondition = stockView.findViewById(R.id.tvCondition);
tvQuantity = stockView.findViewById(R.id.tvQuantity);
tvLocation = stockView.findViewById(R.id.tvLocation);
tvCategory = stockView.findViewById(R.id.tvCategory);
btnRemove = stockView.findViewById(R.id.btnRemove);
stockView.setOnClickListener(this);
stockView.setOnLongClickListener(this);
}