android studio 尝试在 null 对象引用上调用虚拟方法 'void

Question

当我尝试运行我的代码时，它给我一个错误，说我正在尝试在空对象引用上尝试RecyckerView。安卓工作室检测到的问题就在这条线上 。有人知道这个问题吗？recyclerView.setHasFixedSize(true);这是活动：public class SalesHistoryActivity extends AppCompatActivity {    @BindView(R.id.sales_recycler)    RecyclerView recyclerView;    SalesAdapter adapter;    List<Sales> salesList;    Sales sales;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_sales_history);        salesList = new ArrayList<>();        recyclerView.setHasFixedSize(true);        recyclerView.setLayoutManager(new LinearLayoutManager(this));        adapter = new SalesAdapter(this, salesList);        recyclerView.setAdapter(adapter);        loadItem();    }这是适配器public class SalesAdapter extends RecyclerView.Adapter<SalesAdapter.ItemViewHolder> {    private Context mCtx;    private List<Sales> salesList;    public SalesAdapter(Context mCtx, List<Sales> salesList){        this.mCtx = mCtx;        this.salesList = salesList;    }    @NonNull    @Override    public ItemViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {        View rootView = LayoutInflater.from(mCtx).inflate(R.layout.list_sales, null, false);         RecyclerView.LayoutParams layoutParams = new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);        rootView.setLayoutParams(layoutParams);        return new ItemViewHolder(rootView);    }

Answer 1

这仅仅是因为您在使用ButterKnife库时忘记绑定视图。因此，您需要通过添加以下内容来更改代码：ButterKnife.bind(this)

public class SalesHistoryActivity extends AppCompatActivity {

    @BindView(R.id.sales_recycler)

    RecyclerView recyclerView;

    ..

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_sales_history);

        ButterKnife.bind(this);

        // now your views are binded. You can access them

        ...

        recyclerView.setHasFixedSize(true);

        ...

    }

}