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我是新来的Java编程的Android编程。我正在创建一个活动，显示我从数据库创建的ArrayList。修复与SQLite数据库的NullPointerException
阵列完全在我的主要活动，但是当我尝试显示并调用该方法在另一个活动，它会显示以下信息：
了java.lang.RuntimeException：无法启动活动> ComponentInfo {com.example.carlos.application1/com.example.carlos.application1.ema> il}：java.lang.NullPointerException：试图调用虚拟方法>'java.util.ArrayList com.example.carlos.application1.DBHelper .getAllCotacts（）'on>空对象引用
这里发生了什么？为什么会有NullPointerException？我必须初始化？
活动：
import android.app.Activity;
import android.content.ActivityNotFoundException;
import android.content.Intent;
import android.os.Bundle;
import android.os.PersistableBundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;
import android.widget.ArrayAdapter;
import java.util.ArrayList;
/**
* Created by Carlos on 16/05/2016.
*/
public class email extends Activity {
private Button send;
DBHelper mydb1;
private ListView obj;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.email_display);
ArrayList array_list = mydb1.getAllCotacts();
ArrayAdapter arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, array_list);
//ArrayAdapter arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, array_list);
//obj = (ListView) findViewById(R.id.listView1);
//obj.setAdapter(arrayAdapter);
send = (Button) findViewById(R.id.send_button);
send.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
try {
Intent emailIntent = new Intent(Intent.ACTION_SEND);
emailIntent.putExtra(Intent.EXTRA_EMAIL, new String[] {
"[email protected]"
});
emailIntent.putExtra(Intent.EXTRA_SUBJECT, "Subject");
emailIntent.putExtra(Intent.EXTRA_TEXT, "HEY");
emailIntent.putExtra(Intent.EXTRA_CC, "[email protected]");
emailIntent.setType("message/rfc822");
startActivity(emailIntent);
} catch (ActivityNotFoundException anfe) {
Toast toast = Toast.makeText(email.this, "Sorry, no email client found", Toast.LENGTH_LONG);
toast.show();
}
}
});
}
}
数据库处理器
package com.example.carlos.application1;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Hashtable;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.DatabaseUtils;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteDatabase;
public class DBHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "MyDBName.db";
public static final String CONT_TABLE_NAME = "tablename";
public static final String CONT_COLUMN_ID = "id";
public static final String CONT_COLUMN_CONDUCTIVITY = "conductivity";
public static final String CONT_COLUMN_MOISTURE = "moisture";
public static final String CONT_COLUMN_OXYGEN = "oxygen";
public static final String CONT_COLUMN_PH = "ph";
private HashMap hp;
public DBHelper(Context context)
{
super(context, DATABASE_NAME , null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
db.execSQL(
"create table tablename " +
"(id integer primary key, conductivity text,ph text,oxygen text, moisture text)"
);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
db.execSQL("DROP TABLE IF EXISTS tablename");
onCreate(db);
}
public boolean insertContact (String conductivity, String ph, String oxygen, String moisture)
{
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put("conductivity", conductivity);
contentValues.put("ph", ph);
contentValues.put("oxygen", oxygen);
contentValues.put("moisture", moisture);
db.insert("tablename", null, contentValues);
return true;
}
public Cursor getData(int id){
SQLiteDatabase db = this.getReadableDatabase();
Cursor res = db.rawQuery("select * from tablename where id="+id+"", null);
return res;
}
public int numberOfRows(){
SQLiteDatabase db = this.getReadableDatabase();
int numRows = (int) DatabaseUtils.queryNumEntries(db, CONT_TABLE_NAME);
return numRows;
}
public boolean updateContact (Integer id, String conductivity, String ph, String oxygen, String moisture)
{
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put("conductivity", conductivity);
contentValues.put("ph", ph);
contentValues.put("oxygen", oxygen);
contentValues.put("moisture", moisture);
db.update("tablename", contentValues, "id = ? ", new String[] { Integer.toString(id) });
return true;
}
public Integer deleteContact (Integer id)
{
SQLiteDatabase db = this.getWritableDatabase();
return db.delete("tablename",
"id = ? ",
new String[] { Integer.toString(id) });
}
public ArrayList<String> getAllCotacts()
{
ArrayList<String> array_list = new ArrayList<String>();
//hp = new HashMap();
SQLiteDatabase db = this.getReadableDatabase();
Cursor res = db.rawQuery("select * from tablename", null);
res.moveToFirst();
while(res.isAfterLast() == false){
array_list.add(res.getString(res.getColumnIndex(CONT_COLUMN_CONDUCTIVITY)));
res.moveToNext();
}
return array_list;
}
}