我正在尝试从sqlite数据库中搜索并检索列表视图中的结果。当我尝试搜索时，该应用程序关闭。
Logcat
04-20 11:50:56.550 2297-2297/com.example.asus.myapplication3.app
E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.asus.myapplication3.app, PID: 2297
java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase
com.example.asus.myapplication3.app.dbtry$DatabaseHelper.getReadableDatabase()'
on a null object reference
at com.example.asus.myapplication3.app.searchActivity$1.onClick(searchActivity.java:62)
at android.view.View.performClick(View.java:4756)
at android.view.View$PerformClick.run(View.java:19749)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
代码
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import java.net.ContentHandler;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
public class ListAdapter extends ArrayAdapter {
List list= new ArrayList();
static class layoutHandler
{
TextView nameText;
}
public ListAdapter(Context context, int resource)
{
super(context, resource);
}
@Override
public void add (Object object){
super.add(object);
list.add(object);
}
@Override
public int getCount() {
return list.size();
}
@Override
public Object getItem(int position)
{
return list.get(position);
}
@Override
// retern each row of data
public View getView(int position, View convertView, ViewGroup parent) {
View row = convertView;
layoutHandler layouthandler;
if (row==null){
LayoutInflater layoutInflater =(LayoutInflater)this.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
row=layoutInflater.inflate(R.layout.result,parent,false);
// row=layoutInflater.inflate(R.layout.listrow,parent,false);
layouthandler=new layoutHandler();
layouthandler.nameText=(TextView)row.findViewById(R.id.nameText);
row.setTag(layouthandler);
}
else
{
layouthandler =(layoutHandler) row.getTag();
}
// return each abject from the list
dataProvider provider=(dataProvider)this.getItem(position);
//assaien each row of data
layouthandler.nameText.setText(provider.getName());
return row;
}
}
public class dataProvider {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public dataProvider(String name) {
this.name = name;
}
}
import android.app.Activity;
import android.content.Intent;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.app.Activity;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Adapter;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.CursorAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
public class searchActivity extends Activity {
EditText rname;
TextView result;
Button search;
dbtry db;
ListView data;
ListAdapter listAdapter;
String search_name;
SQLiteDatabase sqLiteDatabase;
dbtry.DatabaseHelper dbhlper;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.search);
search = (Button) findViewById(R.id.search);
rname = (EditText) findViewById(R.id.rname);
data = (ListView) findViewById(R.id.data);
search.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// setContentView(R.layout.result);
search_name = rname.getText().toString();
//db = new dbtry(getBaseContext());
sqLiteDatabase= dbhlper.getReadableDatabase();
Log.e("the app search ","dbhlper");
db.open();
Cursor d = db.returndata(search_name, sqLiteDatabase);
Log.e("the app search ","yah!");
listAdapter = new ListAdapter(getApplicationContext(), R.layout.listrow);
data.setAdapter(listAdapter);
data.setVisibility(View.GONE);
if (d.moveToFirst()) {
do {
String result_search;
result_search = d.getString(0);
//adding data to adapter
dataProvider provider = new dataProvider(result_search);
listAdapter.add(provider);
Log.e("the app search ","good");
} while (d.moveToNext());
}
// startActivity( new Intent(searchActivity.this, resultActivity.class));
}
}); }}
package com.example.asus.myapplication3.app;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
import java.sql.SQLException;
public class dbtry {
public static int _id;
public static final String ecapp_col_name = "name";
public static final String ecapp_col_discrption = "discrption";
public static final String ecapp_col_ingredant = "ingredant";
// public static final String ecapp_image = "image";
public static final String ecapp_col_Optional_ingredant = "Optional_ingredant";
public static final String TABLE_NAME1 = "recipe";
public static final String TABLE_NAME2 = "ingredant";
public static final String DATABASE_NAME="ecapp";
public static final int DATABASE_VERSION=1;
public static final String DATABASE_create1 = "create"+ TABLE_NAME1+ "("+_id +"int not null auto_increment,"+ecapp_col_name+" text UNIQUE,"+ecapp_col_discrption +"text+ );";
public static final String DATABASE_create2 = "create "+TABLE_NAME2+ "("+ ecapp_col_ingredant+"text ,"+ecapp_col_Optional_ingredant+"text ,"+_id+ "number );";
DatabaseHelper dbh;
Context con;
SQLiteDatabase db2;
public dbtry(Context con)
{
this.con = con;
dbh = new DatabaseHelper(con);
}
public static class DatabaseHelper extends SQLiteOpenHelper {
public DatabaseHelper (Context con)
{
super(con,DATABASE_NAME,null,DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
try {
db.execSQL(DATABASE_create1);
db.execSQL(DATABASE_create2);
Log.d("db created ", "table2");
}
catch (Exception e) {
e.printStackTrace();
Log.e("db not created ", "bad");
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS ecapp");
onCreate(db);
}
}
public dbtry open()
{
db2= dbh.getWritableDatabase();
return this;
}
public void close()
{
dbh.close();
}
public void insertData(String name, String ingredant, String discrption) {
ContentValues content = new ContentValues();
content.put(ecapp_col_name, name);
Log.e("db created ", "good");
content.put(ecapp_col_ingredant, ingredant);
Log.e("db created ", "good");
content.put(ecapp_col_discrption, discrption);
Log.e("db created ", "good");
db2.insert("ecapp", null, content);
Log.e("db created ", "good");
}
//retrieve
public Cursor returndata(String sname, SQLiteDatabase sqldb)
{
sqldb = dbh.getReadableDatabase();
String selection, selectionArg;
Cursor query;
String[] projection = {ecapp_col_name};
String[] where = {ecapp_col_name};
query = sqldb.query(TABLE_NAME1, projection,sname,null, null, null, null);
// query = sqldb.query( "select"+ecapp_col_name+ "form" +TABLE_NAME1+"where "+ecapp_col_name+"="+sname+;)
Log.e( "get all data:", "query");
return query;
}
}
答案 0 :(得分：0)
在项目的searchActivity中，在初始化sqLiteDatabase= dbhlper.getReadableDatabase();之前在search.setOnClickListener中添加这些行db = new dbtry(searchActivity.this);以避免NullPointerException。
同时对此行sqLiteDatabase= dbhlper.getReadableDatabase();发表评论，因为您已在SQLiteDatabase db2;方法中使用open()作为
public dbtry open()
{
db2= dbh.getWritableDatabase();
return this;
}
所以不需要dbHelper＆amp; searchActivity类中的sqliteDatabase，只需调用db.open()即可。如果您想从数据库中读取，请创建另一个方法read()，类似于dbtry类中的open()方法
public dbtry read()
{
db2= dbh.getReadableDatabase();
return this;
}
用法：db.read（）;只有在初始化db。
之后才能在searchActivity中调用它
快乐编码.. !!