RecyclerView 没有显示来自 json 数组的数据

Question

我已经创建了Recyclerview，但我想从数据库检索数据到android recyclerview。我通过php从mysqlserver检索数据到JSON格式。但是当我尝试在recyclerview检索数据时，数据没有显示在recyclerview。

ActivityMain.java:(其代码如下):

package com.example.chatapp;

import android.app.DownloadManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.widget.Toast;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    RecyclerView recyclerView;

    List<PoliceStations> policeStations;


    private static  final String Police_station_URL="url of my php scrip";
    Adapter adapter;


    RecyclerView.LayoutManager layoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        recyclerView = findViewById(R.id.recyclerview);
        recyclerView.setHasFixedSize(true);
        layoutManager = new LinearLayoutManager(this);
       recyclerView.setLayoutManager(layoutManager);
            policeStations=new ArrayList<>();

         Load_Police_Stations();

    }

    private void Load_Police_Stations()
    {
        StringRequest stringRequest=new StringRequest(Request.Method.GET, Police_station_URL, new Response.Listener<String>()
        {
            @Override
            public void onResponse(String response)
            {
                try
                {
                    JSONArray jsonArray=new JSONArray(response);
                    for(int i=0;i<jsonArray.length();i++)
                    {
                        JSONObject jsonObject=jsonArray.getJSONObject(i);

                     int id=jsonObject.getInt("id");
                     String policeStation=jsonObject.getString("Police_Station");
                     PoliceStations policeStations1=new PoliceStations(id,policeStation);
                     policeStations.add(policeStations1);
                    }
                    adapter=new Adapter(MainActivity.this,policeStations);
                    recyclerView.setAdapter(adapter);

                }
                catch (JSONException e)
                {
                    e.printStackTrace();
                }
            }
        },
                new Response.ErrorListener()
                {
            @Override
            public void onErrorResponse(VolleyError error)
            {
                Toast.makeText(MainActivity.this,error.getMessage(),Toast.LENGTH_LONG).show();
            }
        });
        RequestQueue requestQueue = Volley.newRequestQueue(this);
        requestQueue.add(stringRequest);



    }
}




    enter code here

PoliceStations.java（此活动用于设置值） 包 com.example.chatapp;

public class PoliceStations
{
    private  int id;
     private String policestations;

    public PoliceStations(int id, String policestations)
    {
        this.id = id;
        this.policestations = policestations;
    }

    public int getId()
    {
        return id;
    }

    public String getPolicestations()
    {
        return policestations;
    }
}

    enter code here

(适配器.java) 此活动用于采用从数据源获取的值（在我的例子中它是 JSON 数组文件）：

package com.example.chatapp;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.List;

public class Adapter extends RecyclerView.Adapter<Adapter.ViewHolder>
{
    private Context context;
    List<PoliceStations> stations;


    public Adapter(Context context, List<PoliceStations> stations)
    {
        this.context = context;
        this.stations = stations;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i)
    {
        LayoutInflater inflater=LayoutInflater.from(context);
        View view=inflater.inflate(R.layout.layout_for_police_stations,viewGroup,false);
        ViewHolder viewHolder=new ViewHolder(view);
        return viewHolder;
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder viewHolder, int i)
    {
        PoliceStations policeStations=stations.get(i);
        viewHolder.police.setText(policeStations.getPolicestations());

    }

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

    public class ViewHolder extends RecyclerView.ViewHolder {

        TextView police;

        public ViewHolder(@NonNull View itemView)
        {
            super(itemView);
            police=itemView.findViewById(R.id.police);
        }
    }
}










enter code here
card.xml:
this is layout activity of my project that is used to design layout for individual item in recyclerview.


<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="?android:attr/listPreferredItemHeight">



    <TextView
        android:layout_marginTop="20dp"
        android:id="@+id/police"

        android:layout_width="match_parent"
        android:layout_height="wrap_content" />



</RelativeLayout>











enter code here

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"

tools:context=".MainActivity">

<android.support.v7.widget.RecyclerView
    android:id="@+id/recyclerview"
    android:scrollbars="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

</android.support.v7.widget.RecyclerView>

Answer 1

我没有看到布局管理器被设置。请尝试在 xml 或 Activity 中为您的 RecyclerView 设置 LinearLayoutManager