java - 使用数据库中的数据创建自定义 ListView

Question

我使用本教程作为引用: http://www.learn2crack.com/2013/10/android-custom-listview-images-text-example.html

我使用数据库中的值代替预定义值

Mainactivity.java

    package com.example.listview1;

import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;
import android.app.Activity;
public class MainActivity extends Activity {
  ListView list;
  datamanager dm;
  String[] web;

  int[] imageId;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    dm=new datamanager(this);
    list=(ListView)findViewById(R.id.list);
    web=dm.getNames("Books");
    imageId=getID();
    CustomList adapter = new
        CustomList(MainActivity.this, web, imageId);

       list.setAdapter(adapter);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view,
                                        int position, long id) {
                    Toast.makeText(MainActivity.this, "You Clicked at " +web[+ position], Toast.LENGTH_SHORT).show();
                }
            });
  }
  public int[] getID()
  {
      int[] id=null;
      String[] str=dm.getID("Books");
      for(int i=0;i<str.length;i++)
      {
           id[i]=getResources().getIdentifier(str[i] , "drawable", getPackageName());
      }
      return id;
  }
}

数据管理器.java

package com.example.listview1;



import java.util.ArrayList;

import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class datamanager {
    SQLiteDatabase DB1;
    private final String Db_NAME = "dbshopkart.db";
    private final int DB_VERSION =3;
    Context c1;
    public datamanager(Context c) {
        c1=c;
        openingclass o=new openingclass(c);
        DB1=o.getWritableDatabase();

    }
    public class openingclass extends SQLiteOpenHelper
    {
        public openingclass(Context c) {
            super(c,Db_NAME, null, DB_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase arg0) {




        }

        @Override
        public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) {
            // TODO Auto-generated method stub

        }
}
    public String[] getNames(String category)
    {
        ArrayList<String> dataList = new ArrayList<String>();


        Cursor cu=DB1.rawQuery("SELECT NAME from products where CATEGORY ='"+category+"'",null);  
        cu.moveToFirst();


        if (!cu.isAfterLast())
        {
            do
            {


                dataList.add(cu.getString(0));
//              dataList.add(cu.getString(1));
//              dataList.add(cu.getString(2));
//              dataList.add(cu.getString(3));
//              dataList.add(cu.getString(4));
//              dataList.add(cu.getString(5));
//              dataList.add(cu.getString(6));
//              dataList.add(cu.getString(7));



            }

            while (cu.moveToNext());
            cu.close();
        }

        // return the ArrayList that holds the data collected from
        // the database.
        return dataList.toArray(new String[dataList.size()]);
    }
    public String[] getID(String category)
    {
ArrayList<String> dataList = new ArrayList<String>();


        Cursor cu=DB1.rawQuery("SELECT PID from products where CATEGORY ='"+category+"'",null);  
        cu.moveToFirst();


        if (!cu.isAfterLast())
        {
            do
            {


                dataList.add(cu.getString(0));
//              dataList.add(cu.getString(1));
//              dataList.add(cu.getString(2));
//              dataList.add(cu.getString(3));
//              dataList.add(cu.getString(4));
//              dataList.add(cu.getString(5));
//              dataList.add(cu.getString(6));
//              dataList.add(cu.getString(7));



            }

            while (cu.moveToNext());
            cu.close();
        }

        // return the ArrayList that holds the data collected from
        // the database.
        String[] str=dataList.toArray(new String[dataList.size()]);
        return str;

    }
}

自定义适配器

package com.example.listview1;

import android.app.Activity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;
public class CustomList extends ArrayAdapter<String>{
private final Activity context;
private final String[] web;
private final int[] imageId;
public CustomList(Activity context,
String[] web, int[] imageId) {
super(context, R.layout.list_single, web);
this.context = context;
this.web = web;
this.imageId = imageId;
}
@Override
public View getView(int position, View view, ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View rowView= inflater.inflate(R.layout.list_single, null, true);
TextView txtTitle = (TextView) rowView.findViewById(R.id.txt);
ImageView imageView = (ImageView) rowView.findViewById(R.id.img);
txtTitle.setText(web[position]);
imageView.setImageResource(imageId[position]);
return rowView;
}
}

我得到以下日志

   04-19 02:29:01.182: E/AndroidRuntime(9405): FATAL EXCEPTION: main  
    04-19 02:29:01.182: E/AndroidRuntime(9405): Process: com.example.listview1, PID: 9405  
    04-19 02:29:01.182: E/AndroidRuntime(9405): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.listview1/com.example.listview1.MainActivity}: java.lang.NullPointerException  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.ActivityThread.access$700(ActivityThread.java:135)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.os.Handler.dispatchMessage(Handler.java:102)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.os.Looper.loop(Looper.java:137)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.ActivityThread.main(ActivityThread.java:4998)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at java.lang.reflect.Method.invokeNative(Native Method)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at java.lang.reflect.Method.invoke(Method.java:515)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at dalvik.system.NativeStart.main(Native Method)  
    04-19 02:29:01.182: E/AndroidRuntime(9405): Caused by: java.lang.NullPointerException  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at com.example.listview1.MainActivity.getID(MainActivity.java:41)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at com.example.listview1.MainActivity.onCreate(MainActivity.java:22)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.Activity.performCreate(Activity.java:5243)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)  
    04-19 02:29:01.182: E/AndroidRuntime(9405):     ... 11 more  

救命!!!

Answer 1

您获得 NPE:

public int[] getID()
  {
      int[] id=null;   <--- you need initialize this
      String[] str=dm.getID("Books");
      for(int i=0;i<str.length;i++)
      {
           id[i]=getResources().getIdentifier(str[i] , "drawable", getPackageName());  <--- this line cause NPE
      }
      return id;
  }

您没有在代码中初始化id

如果您知道所需的数组大小:

id = new int[size];

如果您不知道，最好使用ArrayList而不是int[]

您的代码必须是:

 public int[] getID()
      {

          String[] str=dm.getID("Books");
          int[] id= new int[str.length]
          for(int i=0;i<str.length;i++)
          {
               id[i]=getResources().getIdentifier(str[i] , "drawable", getPackageName());  
          }
          return id;
      }