Question

在我的程序中，我有一个listview，它显示来自SQL数据库的各种客户信息，以及一个清除数据库中所有客户条目的按钮。我的问题是：

当数据库为空时，应用程序将崩溃。
当有新客户输入时，列表视图不会更新。一世尝试过notifyDataSetChanged（）和notifyDataSetInvalidated（），但列表视图仍未更新。

我的主要计划：

package attendance.emily.example.com.attendance;


import android.os.AsyncTask;
import android.os.StrictMode;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;

import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import org.json.JSONArray;

import org.json.JSONObject;

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;



public class MainActivity extends AppCompatActivity {

    Button cleardata;
    TextView textview3, displayUser, displayTimeout, displayTimein, textview7;
    ListView lv;
    ArrayAdapter<String> adapter;

    String address = "http://192.168.0.100:80/gps/get_logout.php";


    InputStream is = null;

    String line = null;

    String result = null;

    String data[];






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


        lv = (ListView) findViewById(R.id.listView1);


        cleardata = (Button) findViewById(R.id.button);
        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();

        StrictMode.setThreadPolicy(policy);

        StrictMode.setThreadPolicy((new StrictMode.ThreadPolicy.Builder().permitNetwork().build()));
        getdata3();


        adapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,data);
        lv.setAdapter(adapter);





        cleardata.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                String link = "http://192.168.0.100:80/gps/delete.php"; //using this IP for Genymotion emulator
                new updateData().execute(link);
                Toast.makeText(MainActivity.this, "Executed", Toast.LENGTH_LONG).show();
            }
        });




    }

    public class updateData extends AsyncTask<String, String, String> {

        @Override
        protected String doInBackground(String... params) {
            HttpURLConnection conn = null;

            try {
                URL url;
                url = new URL(params[0]);
                conn = (HttpURLConnection) url.openConnection();
                if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {
                    InputStream is = conn.getInputStream();
                } else {
                    InputStream err = conn.getErrorStream();
                }
                return "Done";
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                if (conn != null) {
                    conn.disconnect();
                }
            }
            return null;
        }

    }



private void getdata3()
{
    try{
        URL url = new URL(address);
        HttpURLConnection con = (HttpURLConnection)url.openConnection();
        con.setRequestMethod("GET");
        is = new BufferedInputStream(con.getInputStream());



    } catch(Exception e){
        e.printStackTrace();
    }

    try{
        BufferedReader br = new BufferedReader(new InputStreamReader(is));
        StringBuilder sb = new StringBuilder();
        while ((line=br.readLine()) != null){
            sb.append(line+"\n");

        }

        is.close();
        result = sb.toString();





    } catch(Exception e){
        e.printStackTrace();
    }

    try {
        String a="";
        String p="";
        String z="";
        JSONArray ja = new JSONArray(result);
        JSONObject jo = null;




        data=new String[ja.length()];


        for (int i=0; i<ja.length(); i++ ){
            jo = ja.getJSONObject(i);
           a =jo.getString("user");
            p = jo.getString("MIN(timein)");
            z = jo.getString("MAX(timeout)");


            data[i] = "User: "+ a + "\n" +"Time In: " + p + "\n" + "Time Out: " + z+"\n"   ;



        }

    }
    catch(Exception e){
        e.printStackTrace();
    }

}










}

错误：

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: attendance.emily.example.com.attendance, PID: 11833
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{attendance.emily.example.com.attendance/attendance.emily.example.com.attendance.MainActivity}: java.lang.NullPointerException: storage == null
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2596)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2653)
                      at android.app.ActivityThread.access$800(ActivityThread.java:156)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1355)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:157)
                      at android.app.ActivityThread.main(ActivityThread.java:5872)
                      at java.lang.reflect.Method.invokeNative(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:515)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:674)
                      at dalvik.system.NativeStart.main(Native Method)
                   Caused by: java.lang.NullPointerException: storage == null
                      at java.util.Arrays$ArrayList.<init>(Arrays.java:38)
                      at java.util.Arrays.asList(Arrays.java:155)
                      at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:128)
                      at attendance.emily.example.com.attendance.MainActivity.onCreate(MainActivity.java:73)
                      at android.app.Activity.performCreate(Activity.java:5312)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1111)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2552)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2653) 
                      at android.app.ActivityThread.access$800(ActivityThread.java:156) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1355) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:157) 
                      at android.app.ActivityThread.main(ActivityThread.java:5872) 
                      at java.lang.reflect.Method.invokeNative(Native Method) 
                      at java.lang.reflect.Method.invoke(Method.java:515) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:674) 
                      at dalvik.system.NativeStart.main(Native Method)

关于如何解决我的问题的任何建议？

Answer 1

在致电adapter.notifyDataSetChanged();之前，必须更新adapter（或其拥有的数据），如下所示：

String s = "new item";
adapter.add(s);

然后

adapter.notifyDataSetChanged();

然而，更好的方法：

由于您使用的是String[]，因此无法向其中添加项目。我建议使用ArrayList<String> list = new ArrayList<String>();，然后像现在一样使用它填充适配器。

adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, list);

然后，当您想要添加新项目时，请执行以下操作：

list.add(s);
adapter.notifyDataSetChanged();

因此，如果要显示新的列表项，请先更新列表，然后调用notifyDataSetChanged()。旁注：您也可以以相同的方式更新现有行，但您必须获得该位置，通常从onClick方法传入。

Here is another great questions on this with good answers

Listview未在notifyDataSetChanged上更新

1 个答案: