如何用Java为RecyclerView填充符合条件的Firebase数据（仅男性候选人）

我的RecyclerView目前加载了Firebase数据库中的所有数据，但我希望仅填充男性候选人的数据，尝试添加if语句筛选后页面显示空白。

当前显示效果截图：

原代码 #

ListViewActivity 代码 #

public class ListViewActivity extends AppCompatActivity {
    RecyclerView recyclerView;
    DatabaseReference databaseReference;
    MyAdapter myAdapter;
    ArrayList<EmployerInfo> list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list_view);

        recyclerView = findViewById(R.id.recyclerViewList);
        databaseReference = FirebaseDatabase.getInstance().getReference("EmployerInfo");
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        list = new ArrayList<>();
        myAdapter = new MyAdapter(this, list);
        recyclerView.setAdapter(myAdapter);

        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                for (DataSnapshot dataSnapshot: snapshot.getChildren()){
                    EmployerInfo employerInfo = dataSnapshot.getValue(EmployerInfo.class);
                    list.add(employerInfo);
                }
                myAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }
}

MyAdapter 类代码 #

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {

    Context context;
    ArrayList<EmployerInfo> list;

    public MyAdapter(Context context, ArrayList<EmployerInfo> list) {
        this.context = context;
        this.list = list;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
       View v = LayoutInflater.from(context).inflate(R.layout.item, parent, false);
       return  new MyViewHolder(v);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        EmployerInfo employerInfo = list.get(position);
        holder.firstName.setText(employerInfo.getName());
        holder.lastName.setText(employerInfo.getSurname());
        holder.gender.setText(employerInfo.getGender());
    }

    @Override
    public int getItemCount() {
        return list.size();
    }

    public  static  class  MyViewHolder extends  RecyclerView.ViewHolder{

        TextView firstName, lastName, gender;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);

            firstName =itemView.findViewById(R.id.textViewFirstName);
            lastName = itemView.findViewById(R.id.textViewLastName);
            gender = itemView.findViewById(R.id.textViewGender);
        }
    }
}

尝试修改后但无效的代码 #

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list_view);

    recyclerView = findViewById(R.id.recyclerViewList);
    databaseReference = FirebaseDatabase.getInstance().getReference("EmployeeProfile");
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));

    list = new ArrayList<>();
    myAdapter = new MyAdapter(this, list);
    recyclerView.setAdapter(myAdapter);

    databaseReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
            for (DataSnapshot dataSnapshot: snapshot.getChildren()){
                EmployerInfo employerInfo = dataSnapshot.getValue(EmployerInfo.class);

                if(employerInfo.getGender() == "Male"){
                    list.add(employerInfo);
                }
            }
            myAdapter.notifyDataSetChanged();
        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {

        }
    });
}

问题分析与解决方案 #

核心错误点 #

1. 字符串比较方式错误：Java中==比较的是对象引用地址，而非字符串内容，应该用equals()或equalsIgnoreCase()进行内容比较。
2. 数据库路径不一致：修改后的代码将原节点EmployerInfo改为EmployeeProfile，如果数据库中实际数据节点是EmployerInfo，会导致无法读取数据，页面空白。

方案一：客户端修正筛选逻辑 #

修改onDataChange方法，修正字符串比较逻辑，同时每次数据更新时清空旧数据避免重复加载：

databaseReference.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        list.clear(); // 清空旧数据，避免重复添加
        for (DataSnapshot dataSnapshot: snapshot.getChildren()){
            EmployerInfo employerInfo = dataSnapshot.getValue(EmployerInfo.class);
            // 非空判断+正确的字符串比较
            if(employerInfo != null && "Male".equals(employerInfo.getGender())){
                list.add(employerInfo);
            }
        }
        myAdapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        Toast.makeText(ListViewActivity.this, "数据加载失败", Toast.LENGTH_SHORT).show();
    }
});

方案二：服务端筛选（更高效） #

直接通过Firebase查询仅获取性别为Male的数据，减少带宽消耗，比客户端筛选更高效：

// 构建服务端查询：按gender字段筛选值为Male的数据
Query maleQuery = databaseReference.orderByChild("gender").equalTo("Male");

maleQuery.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        list.clear();
        for (DataSnapshot dataSnapshot: snapshot.getChildren()){
            EmployerInfo employerInfo = dataSnapshot.getValue(EmployerInfo.class);
            if(employerInfo != null){
                list.add(employerInfo);
            }
        }
        myAdapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        Toast.makeText(ListViewActivity.this, "数据加载失败", Toast.LENGTH_SHORT).show();
    }
});

注意：使用服务端查询时，需在Firebase控制台的数据库规则中为gender字段创建索引，避免性能警告，规则示例：

{
  "rules": {
    "EmployerInfo": {
      ".indexOn": ["gender"]
    }
  }
}

内容的提问来源于stack exchange，提问作者Arizen Bolt