Question

我正在从我的数据库中检索和显示数据我正在使用android asynctask和php web服务但当我执行它时，我得到一个例外，这里是我的代码：

AsynctaskListViewLoader类：

package com.hassen.testannonceasyntask;

import android.app.Application;
import android.app.ProgressDialog;
import android.os.AsyncTask;

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

import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

class AsyncListViewLoader extends AsyncTask<String, Void, List<Contact>> {
  //  private  ProgressDialog dialog = new ProgressDialog();
    SimpleAdapter adpt;

    AsyncListViewLoader() {
    }

    @Override
    protected void onPostExecute(List<Contact> result) {
        super.onPostExecute(result);
       // dialog.dismiss();
        adpt.setItemList(result);
        adpt.notifyDataSetChanged();
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
       // dialog.setMessage("Downloading contacts...");
       // dialog.show();
    }

    @Override
    protected List<Contact> doInBackground(String... params) {
        List<Contact> result = new ArrayList<Contact>();

        try {
            URL u = new URL(params[0]);

            HttpURLConnection conn = (HttpURLConnection) u.openConnection();
            conn.setRequestMethod("GET");

            conn.connect();
            InputStream is = conn.getInputStream();

// Read the stream
            byte[] b = new byte[1024];
            ByteArrayOutputStream baos = new ByteArrayOutputStream();

            while ( is.read(b) != -1)
                baos.write(b);

            String JSONResp = new String(baos.toByteArray());

            JSONArray arr = new JSONArray(JSONResp);
            for (int i=0; i < arr.length(); i++) {
                result.add(convertContact(arr.getJSONObject(i)));
            }

            return result;
        }
        catch(Throwable t) {
            t.printStackTrace();
        }
        return null;
    }

    private Contact convertContact(JSONObject obj) throws JSONException {
        String name = obj.getString("name");
        String surname = obj.getString("surname");
        String email = obj.getString("email");
        String phoneNum = obj.getString("phoneNum");

        return new Contact(name, surname, email, phoneNum);
    }

}

SimpleAdapter类：

package com.hassen.testannonceasyntask;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;

import java.util.List;

class SimpleAdapter extends ArrayAdapter<Contact> {

    private List<Contact> itemList;
    private Context context;

    public SimpleAdapter(List<Contact> itemList, Context ctx) {
        super(ctx, android.R.layout.simple_list_item_1, itemList);
        this.itemList = itemList;
        this.context = ctx;
    }

    public int getCount() {
        if (itemList != null)
            return itemList.size();
        return 0;
    }

    public Contact getItem(int position) {
        if (itemList != null)
            return itemList.get(position);
        return null;
    }

    public long getItemId(int position) {
        if (itemList != null)
            return itemList.get(position).hashCode();
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View v = convertView;
        if (v == null) {
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = inflater.inflate(R.layout.list_item, null);
        }

        Contact c = itemList.get(position);
        TextView text = (TextView) v.findViewById(R.id.name);
        text.setText(c.getName());

        TextView text1 = (TextView) v.findViewById(R.id.surname);
        text1.setText(c.getSurname());

        TextView text2 = (TextView) v.findViewById(R.id.email);
        text2.setText(c.getEmail());

        TextView text3 = (TextView) v.findViewById(R.id.phone);
        text3.setText(c.getPhoneNum());

        return v;

    }

    public List<Contact> getItemList() {
        return itemList;
    }

    public void setItemList(List<Contact> itemList) {
        this.itemList = itemList;
    }

}

我的联系人课程：

package com.hassen.testannonceasyntask;

import java.io.Serializable;

public class Contact implements Serializable {

    private String name;
    private String surname;
    private String email;
    private String phoneNum;

    public Contact(String name, String surname, String email, String phoneNum) {
        super();
        this.name = name;
        this.surname = surname;
        this.email = email;
        this.phoneNum = phoneNum;
    }

// get and set methods


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSurname() {
        return surname;
    }

    public void setSurname(String surname) {
        this.surname = surname;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPhoneNum() {
        return phoneNum;
    }

    public void setPhoneNum(String phoneNum) {
        this.phoneNum = phoneNum;
    }
}

MainActivity类：

package com.hassen.testannonceasyntask;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ListView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    SimpleAdapter adpt;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        adpt = new SimpleAdapter(new ArrayList(), this);
        ListView lView = (ListView) findViewById(R.id.myList);

        lView.setAdapter(adpt);

// Exec async load task
        (new AsyncListViewLoader()).execute("http://192.168.83.2/AndroidPhpDemo/user.php");
    }
}

list_item布局：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="390dp"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Name"
        android:id="@+id/name"
        android:layout_gravity="center_horizontal" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="SurName"
        android:id="@+id/surname" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Email"
        android:id="@+id/email" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Phone"
        android:id="@+id/phone" />

</LinearLayout>

activity_main layout：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/myList"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />
</RelativeLayout>

我收到了这个错误：

09-07 13:02:19.400 1629-1629/com.hassen.testannonceasyntask E/OpenGLRenderer: Getting MAX_TEXTURE_SIZE from GradienCache
09-07 13:02:19.412 1629-1629/com.hassen.testannonceasyntask E/OpenGLRenderer: Getting MAX_TEXTURE_SIZE from Caches::initConstraints()
09-07 13:02:20.872 1629-1629/com.hassen.testannonceasyntask E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              java.lang.NullPointerException
                                                                                  at com.hassen.testannonceasyntask.AsyncListViewLoader.onPostExecute(AsyncListViewLoader.java:29)
                                                                                  at com.hassen.testannonceasyntask.AsyncListViewLoader.onPostExecute(AsyncListViewLoader.java:18)
                                                                                  at android.os.AsyncTask.finish(AsyncTask.java:631)
                                                                                  at android.os.AsyncTask.access$600(AsyncTask.java:177)
                                                                                  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                                  at android.os.Looper.loop(Looper.java:137)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5103)
                                                                                  at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                  at java.lang.reflect.Method.invoke(Method.java:525)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
                                                                                  at dalvik.system.NativeStart.main(Native Method)

任何帮助请!!!!

Answer 1

您需要初始化document.body.style.backgroundColor = 'red'，或者通过adpt constructor传递它，或者只在那里初始化它并将其设置在listView中。

基本上你的适配器引用为null，因此产生空指针异常。

使用Asynctask和php Web服务从数据库中检索数据

1 个答案: