为什么在长时间按下回收器视图项目后尝试使按钮可见时出现空对象引用错误？

Question

我试图在用户长按回收站视图中的一行时显示一个按钮。但是我收到空对象引用错误并且不确定原因。 Toast 有效，所以我知道长按对每一行有效。

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Activity:

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;
        }
    }
}

该按钮应该出现在 activity_main 中回收站视图的正下方，我已经在我的 xml 文件中进行了设置，并将按钮设置为 'invisible'。有人可以解释为什么我收到错误以及如何解决它吗？

Answer 1

如果 btnRemove 为空，则表示 stockView.findViewById(R.id.btnRemove) returns null 所以这意味着 R.layout.stock_row

中没有 ID 为 btnRemove 的视图

检查 R.layout.stock_row 中是否有此行：

android:id="@+id/btnRemove"

Answer 2

像这样修改你的代码，你在按钮之前初始化了监听器。

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);

        }

为什么在长时间按下回收器视图项目后尝试使按钮可见时出现空对象引用错误？

Why do I get a null object reference error when trying to make a button visible after long pressing on a recycler view item?

java

android

android-recyclerview