使用 hashmap 的多列列表中的 nullPointerException [英] nullPointerException in multi column list using hashmap
本文介绍了使用 hashmap 的多列列表中的 nullPointerException的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我已经在 SO 上尝试过类似问题的参考资料，但没有找到合适的解决方案.
我正在尝试从网页中获取数据并以包含 4 列的行的格式显示它.
I'm trying to fetch the data from a webpage and display it in format consisting of rows having 4 columns.
网页上的数据:
SBIN ;1916.00;1886.85;1.54@LT ;1315.50;1310.30;0.40@TCS ;1180.00;1178.00;0.17@AXISBANK;1031.30.5PN901.53.590.53.59090.30.530.30.30.30.30.30.30.30.30.30.30.40916.35;1.70@GAIL;400.00;398.45;0.39@
我想以表格形式显示
SBIN.........1916.00.....1886.85.....1.54
LT...........1315.50.....1310.30.....0.40 and so on.
请注意，我不想要点，我希望每个值都是一行中的一个单独的列.
Note that I don't want dots, I want each value to be a separate column within a row.
我的数据由 7 行组成.我使用了 http://www.technotalkative.com/android-multi- 中的代码列列表视图/
My Data consists of 7 rows. I have used the code from http://www.technotalkative.com/android-multi-column-listview/
当我运行下面的代码时，我得到 nullPointerException.
When I run the below code, I get nullPointerException.
请注意，当我尝试数组 arr[] 和 subarr[] 的值时，它们会显示正确的内容.
MultiCol.java
package com.multicol;
import static com.multicol.Constant.FIRST_COLUMN;
import static com.multicol.Constant.FOURTH_COLUMN;
import static com.multicol.Constant.SECOND_COLUMN;
import static com.multicol.Constant.THIRD_COLUMN;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListView;
public class MultiCol extends Activity {
String subarr[] = new String[28];// 7 rows with 4 columns each
private ArrayList<HashMap<String, String>> list;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
ListView lview = (ListView) findViewById(R.id.listview);
DownloadWebPageTask task = new DownloadWebPageTask();
task.execute(new String[] { "http://ipad.idealake.com/default.aspx?id=G" });
}
private class DownloadWebPageTask extends AsyncTask<String, Void, String> {
@Override
protected String doInBackground(String... urls) {
String response = "";
for (String url : urls) {
DefaultHttpClient client = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(url);
try {
HttpResponse execute = client.execute(httpGet);
InputStream content = execute.getEntity().getContent();
BufferedReader buffer = new BufferedReader(
new InputStreamReader(content));
String s = "";
while ((s = buffer.readLine()) != null) {
response += s;
}
} catch (Exception e) {
e.printStackTrace();
}
}
return response;
}
@Override
protected void onPostExecute(String result) {
int sub = result.lastIndexOf('@', result.length() - 1);
String s1 = result.substring(0, sub + 2);
String temp[];
String arr1[] = s1.split("@");
String arr[] = new String[7];
//To remove 8th element which is null
for (int j = 0; j < arr.length; j++) {
arr[j] = arr1[j];
}
for (int i = 0; i < arr.length; i++) {
temp = arr[i].split(";");
subarr[(4 * i)] = temp[0];
subarr[(4 * i) + 1] = temp[1];
subarr[(4 * i) + 2] = temp[2];
subarr[(4 * i) + 3] = temp[3];
}
list = new ArrayList<HashMap<String, String>>();
for (int i = 0; i < ((subarr.length) / 4); i++) {
HashMap<String, String> addList = new HashMap<String, String>();
addList.put(FIRST_COLUMN, subarr[(4 * i)]);
addList.put(SECOND_COLUMN, subarr[((4 * i) + 1)]);
addList.put(THIRD_COLUMN, subarr[((4 * i) + 2)]);
addList.put(FOURTH_COLUMN, subarr[((4 * i) + 3)]);
list.add(addList);
}
//I was writing these two statements before populating list. So they were giving me nullPointerException.
listviewAdapter adapter = new listviewAdapter(this, list);
lview.setAdapter(adapter);
}
}
}
main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ListView
android:id="@+id/listview"
android:layout_height="wrap_content"
android:layout_width="match_parent">
</ListView>
</LinearLayout>
listviewAdapter.java
package com.multicol;
import static com.multicol.Constant.FIRST_COLUMN;
import static com.multicol.Constant.FOURTH_COLUMN;
import static com.multicol.Constant.SECOND_COLUMN;
import static com.multicol.Constant.THIRD_COLUMN;
import java.util.ArrayList;
import java.util.HashMap;
import android.app.Activity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;
public class listviewAdapter extends BaseAdapter {
public ArrayList<HashMap<String, String>> list;
Activity activity;
public listviewAdapter(Activity activity,
ArrayList<HashMap<String, String>> list) {
super();
this.activity = activity;
this.list = list;
}
@Override
public int getCount() {
return list.size();
}
@Override
public Object getItem(int position) {
return list.get(position);
}
@Override
public long getItemId(int position) {
return 0;
}
private class ViewHolder {
TextView txtFirst;
TextView txtSecond;
TextView txtThird;
TextView txtFourth;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder;
LayoutInflater inflater = activity.getLayoutInflater();
if (convertView == null) {
convertView = inflater.inflate(R.layout.listview_row, null);
holder = new ViewHolder();
holder.txtFirst = (TextView) convertView
.findViewById(R.id.FirstText);
holder.txtSecond = (TextView) convertView
.findViewById(R.id.SecondText);
holder.txtThird = (TextView) convertView
.findViewById(R.id.ThirdText);
holder.txtFourth = (TextView) convertView
.findViewById(R.id.FourthText);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
HashMap<String, String> map = list.get(position);
holder.txtFirst.setText(map.get(FIRST_COLUMN));
holder.txtSecond.setText(map.get(SECOND_COLUMN));
holder.txtThird.setText(map.get(THIRD_COLUMN));
holder.txtFourth.setText(map.get(FOURTH_COLUMN));
return convertView;
}
}
Constant.java
package com.multicol;
public class Constant {
public static final String FIRST_COLUMN = "First";
public static final String SECOND_COLUMN = "Second";
public static final String THIRD_COLUMN = "Third";
public static final String FOURTH_COLUMN = "Fourth";
}
listview_row.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:id="@+id/relativeLayout1"
android:layout_height="fill_parent"
android:layout_width="fill_parent"
xmlns:android="http://schemas.android.com/apk/res/android">
<TextView
android:id="@+id/FirstText"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="First"
android:layout_weight="1">
</TextView>
<TextView
android:id="@+id/SecondText"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="Second"
android:layout_weight="2">
</TextView>
<TextView
android:id="@+id/ThirdText"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="Third"
android:layout_weight="1">
</TextView>
<TextView
android:id="@+id/FourthText"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="Fourth"
android:layout_weight="1">
</TextView>
</LinearLayout>
LOGCAT
12-07 11:40:19.767: E/AndroidRuntime(2283): FATAL EXCEPTION: main
12-07 11:40:19.767: E/AndroidRuntime(2283): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.multicol/com.multicol.MultiCol}: java.lang.NullPointerException
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.ActivityThread.access$600(ActivityThread.java:122)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.os.Handler.dispatchMessage(Handler.java:99)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.os.Looper.loop(Looper.java:137)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.ActivityThread.main(ActivityThread.java:4340)
12-07 11:40:19.767: E/AndroidRuntime(2283): at java.lang.reflect.Method.invokeNative(Native Method)
12-07 11:40:19.767: E/AndroidRuntime(2283): at java.lang.reflect.Method.invoke(Method.java:511)
12-07 11:40:19.767: E/AndroidRuntime(2283): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
12-07 11:40:19.767: E/AndroidRuntime(2283): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
12-07 11:40:19.767: E/AndroidRuntime(2283): at dalvik.system.NativeStart.main(Native Method)
12-07 11:40:19.767: E/AndroidRuntime(2283): Caused by: java.lang.NullPointerException
12-07 11:40:19.767: E/AndroidRuntime(2283): at com.multicol.listviewAdapter.getCount(listviewAdapter.java:31)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.widget.ListView.setAdapter(ListView.java:460)
12-07 11:40:19.767: E/AndroidRuntime(2283): at com.multicol.MultiCol.onCreate(MultiCol.java:38)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.Activity.performCreate(Activity.java:4465)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
12-07 11:40:19.767: E/AndroidRuntime(2283): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
12-07 11:40:19.767: E/AndroidRuntime(2283): ... 11 more
任何帮助
推荐答案
您正在 MultiCol 的 onCreate 中创建 listViewAdapter 并传入对 的引用列表.但是，此时 list 还没有被实例化，因为在您的 AsyncTask 完成之前不会发生这种情况.换句话说:你传入了 null，因此 LogCat 中的 nullpointer 异常.
You are creating the listViewAdapter in the onCreate of MultiCol and passing in a reference to list. However, at that point list has not been instantiated yet, since that does not happen until your AsyncTask finishes. In other words: you're passing in null, hence the nullpointer exception in LogCat.
要更正此问题，您可能需要将以下两行代码移至 DownloadWebPageTask 的 onPostExecute() 末尾:
To correct this, you'll probably want to move the following two lines of code to the end of the onPostExecute() of your DownloadWebPageTask:
listviewAdapter adapter = new listviewAdapter(this, list);
lview.setAdapter(adapter);
这样你就会为适配器提供一个实例化的(非空)列表对象.
That way you will supply the adapter with an instantiated (non-null) list object.
这篇关于使用 hashmap 的多列列表中的 nullPointerException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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