寻找空指针异常的原因 [英] Looking for Cause of Null Pointer Exception
本文介绍了寻找空指针异常的原因的处理方法,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
我敢肯定有东西,我得到的错误空蓬异常错误背后的简单,但是,我没有看到它。下面是相关的code:
包com.example.gameexpensetracker;进口android.app.Activity;
进口android.database.Cursor;
进口android.os.Bundle;
进口android.widget.Toast;公共类Chart_screen_activity延伸活动{ 双strategyTotalDB;
DBAdapter DB =新DBAdapter(本); 保护无效的onCreate(捆绑savedInstanceState){
super.onCreate(savedInstanceState);
的setContentView(R.layout.chart_screen_activity);
db.open();
pieGraphData();
} 公共无效pieGraphData(){
光标cursor17 = db.rawQuery(SELECT(strategy_total)FROM strategyTotal WHERE _id =(SELECT MAX(_id)FROM strategyTotal),
空值);
如果(cursor17.moveToFirst()){
strategyTotalDB = cursor17.getDouble(0);
cursor17.close();
Toast.makeText(+ strategyTotalDB +!补充说:这一点,新条目,Toast.LENGTH_LONG).show();
db.close();
}}}
在LogCat中说,23线和32引起的问题:
10月12日至29日:46:39.979:D / AndroidRuntime(1754年):关闭VM
十月12日至29日:46:39.979:W / dalvikvm(1754年):主题ID = 1:螺纹未捕获的异常(组= 0x40015560)退出
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):致命异常:主要
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):了java.lang.RuntimeException:无法启动活动ComponentInfo {com.example.gameexpensetracker / com.example.gameexpensetracker.Chart_screen_activity}:显示java.lang.NullPointerException
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.ActivityThread.access $ 1500(ActivityThread.java:117)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.ActivityThread $ H.handleMessage(ActivityThread.java:931)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.os.Handler.dispatchMessage(Handler.java:99)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.os.Looper.loop(Looper.java:123)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.ActivityThread.main(ActivityThread.java:3683)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在java.lang.reflect.Method.invokeNative(本机方法)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在java.lang.reflect.Method.invoke(Method.java:507)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run(ZygoteInit.java:839)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在dalvik.system.NativeStart.main(本机方法)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):由:显示java.lang.NullPointerException
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在com.example.gameexpensetracker.Chart_screen_activity.pieGraphData(Chart_screen_activity.java:32)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在com.example.gameexpensetracker.Chart_screen_activity.onCreate(Chart_screen_activity.java:23)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
十月12日至29日:46:39.989:E / AndroidRuntime(1754年):在android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
十月12日至29日:46:39.989:E / AndroidRuntime(1754):11 ...更多
下面是我的DBAdapter:
进口android.content.ContentValues;
进口android.content.Context;
进口android.database.Cursor;
进口android.database.SQLException;
进口android.database.sqlite.SQLiteDatabase;
进口android.database.sqlite.SQLiteOpenHelper;公共类DBAdapter {
静态最后弦乐KEY_ROWID =_id;
静态最后弦乐KEY_TITLE =称号;
静态最后弦乐KEY_PLATFORM =平台;
静态最后弦乐KEY_GENRE =流派;
静态最后弦乐KEY_DATE =日期;
静态最后弦乐KEY_PRICE =价格;
静态最后弦乐KEY_TOTAL =总;
静态最后弦乐KEY_WHEREBOUGHT =wherebought;
静态最后弦乐TAG =DBAdapter; 静态最后弦乐DATABASE_NAME =gamesDB;
静态最后弦乐DATABASE_TABLE =购买;
静态最终诠释DATABASE_VERSION = 1; 静态最后弦乐DATABASE_CREATE =CREATE TABLE IF NOT EXISTS购买(_id整数主键自动增量
+标题文本,文本平台,流派文本,文本日期,价格双,wherebought文本); 静态最后弦乐DATABASE_CREATE2 =CREATE TABLE IF NOT EXISTS grandTotal(_id整数主键自动增量
+总双);; 静态最后弦乐DATABASE_CREATE3 =CREATE TABLE IF NOT EXISTS strategyTotal(_id整数主键自动增量
+strategy_prices双,双strategy_total);; 静态最后弦乐DATABASE_CREATE4 =CREATE TABLE IF NOT EXISTS adventureTotal(_id整数主键自动增量
+adventure_prices双,双adventure_total);; 静态最后弦乐DATABASE_CREATE5 =CREATE TABLE IF NOT EXISTS actionTotal(_id整数主键自动增量
+action_prices双,双action_total);; 静态最后弦乐DATABASE_CREATE6 =CREATE TABLE IF NOT EXISTS MMOTotal(_id整数主键自动增量
+mmo_prices双,双mmo_total);; 静态最后弦乐DATABASE_CREATE7 =CREATE TABLE IF NOT EXISTS racingTotal(_id整数主键自动增量
+racing_prices双,双racing_total);; 静态最后弦乐DATABASE_CREATE8 =CREATE TABLE IF NOT EXISTS rpgTotal(_id整数主键自动增量
+rpg_prices双,双rpg_total);; 静态最后弦乐DATABASE_CREATE9 =CREATE TABLE IF NOT EXISTS simulationTotal(_id整数主键自动增量
+simulation_prices双,双simulation_total);; 静态最后弦乐DATABASE_CREATE10 =CREATE TABLE IF NOT EXISTS sportsTotal(_id整数主键自动增量
+sports_prices双,双sports_total);; 静态最后弦乐DATABASE_CREATE11 =CREATE TABLE IF NOT EXISTS otherTotal(_id整数主键自动增量
+other_prices双,双other_total);; 静态最后弦乐DATABASE_CREATE12 =CREATE TABLE IF NOT EXISTS puzzleTotal(_id整数主键自动增量
+puzzle_prices双,双puzzle_total);;
双columntotal,strategyPriceInput,actionPriceInput,adventurePriceInput,MMOPriceInput,racingPriceInput,rpgPriceInput,simulationPriceInput,
sportsPriceInput,puzzlePriceInput,otherPriceInput; 最后上下文的背景下; DatabaseHelper DBHelper;
SQLiteDatabase分贝; 公共DBAdapter(上下文CTX){
this.context = CTX;
DBHelper =新DatabaseHelper(背景);
} 私有静态类DatabaseHelper扩展SQLiteOpenHelper {
DatabaseHelper(上下文的背景下){
超(背景下,DATABASE_NAME,空,DATABASE_VERSION);
} @覆盖
公共无效的onCreate(SQLiteDatabase DB){
尝试{
db.execSQL(DATABASE_CREATE);
db.execSQL(DATABASE_CREATE2);
db.execSQL(DATABASE_CREATE3);
db.execSQL(DATABASE_CREATE4);
db.execSQL(DATABASE_CREATE5);
db.execSQL(DATABASE_CREATE6);
db.execSQL(DATABASE_CREATE7);
db.execSQL(DATABASE_CREATE8);
db.execSQL(DATABASE_CREATE9);
db.execSQL(DATABASE_CREATE10);
db.execSQL(DATABASE_CREATE11);
db.execSQL(DATABASE_CREATE12);
}赶上(的SQLException E){
e.printStackTrace();
}
} @覆盖
公共无效onUpgrade(SQLiteDatabase分贝,INT oldVersion,诠释静态网页){
db.execSQL(DATABASE_CREATE2);
}
} // ---打开数据库---
公共DBAdapter的open()抛出的SQLException {
DB = DBHelper.getWritableDatabase();
返回此;
} // ---关闭数据库---
公共无效的close(){
DBHelper.close();
} // ---插入一条记录到数据库---
众长insertRecord(标题字符串,字符串平台,字符串类型,
字串的日期,双价,弦乐wherebought){
ContentValues initialValues =新ContentValues();
initialValues.put(KEY_TITLE,职称);
initialValues.put(KEY_PLATFORM,平台);
initialValues.put(KEY_GENRE,体裁);
initialValues.put(KEY_DATE,日期);
initialValues.put(KEY_PRICE,价格);
initialValues.put(KEY_WHEREBOUGHT,wherebought);
返回db.insert(DATABASE_TABLE,空,initialValues); } //删除所有记录 公共无效deleteAll(){
context.deleteDatabase(DATABASE_NAME); } // ---删除一个特定的记录---
公共布尔deleteRecord(长ROWID){
返回db.delete(DATABASE_TABLE,KEY_ROWID +=+ ROWID,NULL)> 0;
} // ---检索的所有记录---
公共光标getAllRecords(){
返回db.query(DATABASE_TABLE,新的String [] {KEY_ROWID,KEY_TITLE,
KEY_GENRE,KEY_DATE,KEY_PRICE,KEY_WHEREBOUGHT},NULL,NULL,
NULL,NULL,NULL);
} // ---检索特定的记录---
公共光标getRecord(长ROWID)抛出的SQLException {
光标mCursor = db.query(真,DATABASE_TABLE,新的String [] {
KEY_ROWID,KEY_TITLE,KEY_PLATFORM,KEY_GENRE,KEY_DATE,
KEY_PRICE,KEY_WHEREBOUGHT},KEY_ROWID +=+ ROWID,空,
NULL,NULL,NULL,NULL);
如果(mCursor!= NULL){
mCursor.moveToFirst();
}
返回mCursor;
} // ---更新的纪录---
公共布尔updateRecord(长ROWID,标题字符串,字符串的平台,
字符串类型,字符串日期,双价,弦乐wherebought){
ContentValues ARGS =新ContentValues();
args.put(KEY_TITLE,职称);
args.put(KEY_PLATFORM,平台);
args.put(KEY_GENRE,体裁);
args.put(KEY_DATE,日期);
args.put(KEY_PRICE,价格);
args.put(KEY_WHEREBOUGHT,wherebought);
返回db.update(DATABASE_TABLE,ARGS,KEY_ROWID +=+ ROWID,NULL)> 0;
} 公共光标rawQuery(串串,Object对象){
// TODO自动生成方法存根
返回null;
}
}
解决方案
您 DBAdapter.rawQuery()
未实现,它只是返回一个存根无效
。试图调用 moveToFirst()
在此空
引用导致NPE。
也就是说,变化
公共光标rawQuery(串串,Object对象){
// TODO自动生成方法存根
返回null;
}
要像
公共光标rawQuery(SQL字符串,字串[] args){
返回db.rawQuery(SQL,参数);
}
I'm sure there's something simple behind the error Null Pont Exception error I am getting, but, I don't see it. Here's the relevant code:
package com.example.gameexpensetracker;
import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.widget.Toast;
public class Chart_screen_activity extends Activity {
double strategyTotalDB;
DBAdapter db = new DBAdapter(this);
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.chart_screen_activity);
db.open();
pieGraphData();
}
public void pieGraphData() {
Cursor cursor17 = db.rawQuery("SELECT (strategy_total) FROM strategyTotal WHERE _id=(SELECT max(_id) FROM strategyTotal)",
null);
if (cursor17.moveToFirst()){
strategyTotalDB = cursor17.getDouble(0);
cursor17.close();
Toast.makeText(this, "New entry"+strategyTotalDB+ "added!", Toast.LENGTH_LONG).show();
db.close();
}}}
The LogCat says lines 23 and 32 are causing the problem:
12-29 10:46:39.979: D/AndroidRuntime(1754): Shutting down VM
12-29 10:46:39.979: W/dalvikvm(1754): threadid=1: thread exiting with uncaught exception (group=0x40015560)
12-29 10:46:39.989: E/AndroidRuntime(1754): FATAL EXCEPTION: main
12-29 10:46:39.989: E/AndroidRuntime(1754): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.gameexpensetracker/com.example.gameexpensetracker.Chart_screen_activity}: java.lang.NullPointerException
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.os.Handler.dispatchMessage(Handler.java:99)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.os.Looper.loop(Looper.java:123)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.ActivityThread.main(ActivityThread.java:3683)
12-29 10:46:39.989: E/AndroidRuntime(1754): at java.lang.reflect.Method.invokeNative(Native Method)
12-29 10:46:39.989: E/AndroidRuntime(1754): at java.lang.reflect.Method.invoke(Method.java:507)
12-29 10:46:39.989: E/AndroidRuntime(1754): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
12-29 10:46:39.989: E/AndroidRuntime(1754): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
12-29 10:46:39.989: E/AndroidRuntime(1754): at dalvik.system.NativeStart.main(Native Method)
12-29 10:46:39.989: E/AndroidRuntime(1754): Caused by: java.lang.NullPointerException
12-29 10:46:39.989: E/AndroidRuntime(1754): at com.example.gameexpensetracker.Chart_screen_activity.pieGraphData(Chart_screen_activity.java:32)
12-29 10:46:39.989: E/AndroidRuntime(1754): at com.example.gameexpensetracker.Chart_screen_activity.onCreate(Chart_screen_activity.java:23)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
12-29 10:46:39.989: E/AndroidRuntime(1754): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
12-29 10:46:39.989: E/AndroidRuntime(1754): ... 11 more
Here's my DBAdapter:
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DBAdapter {
static final String KEY_ROWID = "_id";
static final String KEY_TITLE = "title";
static final String KEY_PLATFORM = "platform";
static final String KEY_GENRE = "genre";
static final String KEY_DATE = "date";
static final String KEY_PRICE = "price";
static final String KEY_TOTAL = "total";
static final String KEY_WHEREBOUGHT = "wherebought";
static final String TAG = "DBAdapter";
static final String DATABASE_NAME = "gamesDB";
static final String DATABASE_TABLE = "purchases";
static final int DATABASE_VERSION = 1;
static final String DATABASE_CREATE = "CREATE TABLE IF NOT EXISTS purchases(_id integer primary key autoincrement, "
+ "title text , platform text, genre text , date text , price double , wherebought text );";
static final String DATABASE_CREATE2 = "CREATE TABLE IF NOT EXISTS grandTotal(_id integer primary key autoincrement, "
+ "total double);";
static final String DATABASE_CREATE3 = "CREATE TABLE IF NOT EXISTS strategyTotal(_id integer primary key autoincrement, "
+ "strategy_prices double, strategy_total double);";
static final String DATABASE_CREATE4 = "CREATE TABLE IF NOT EXISTS adventureTotal(_id integer primary key autoincrement, "
+ "adventure_prices double, adventure_total double);";
static final String DATABASE_CREATE5 = "CREATE TABLE IF NOT EXISTS actionTotal(_id integer primary key autoincrement, "
+ "action_prices double, action_total double);";
static final String DATABASE_CREATE6 = "CREATE TABLE IF NOT EXISTS MMOTotal(_id integer primary key autoincrement, "
+ "mmo_prices double, mmo_total double);";
static final String DATABASE_CREATE7 = "CREATE TABLE IF NOT EXISTS racingTotal(_id integer primary key autoincrement, "
+ "racing_prices double, racing_total double);";
static final String DATABASE_CREATE8 = "CREATE TABLE IF NOT EXISTS rpgTotal(_id integer primary key autoincrement, "
+ "rpg_prices double, rpg_total double);";
static final String DATABASE_CREATE9 = "CREATE TABLE IF NOT EXISTS simulationTotal(_id integer primary key autoincrement, "
+ "simulation_prices double, simulation_total double);";
static final String DATABASE_CREATE10 = "CREATE TABLE IF NOT EXISTS sportsTotal(_id integer primary key autoincrement, "
+ "sports_prices double, sports_total double);";
static final String DATABASE_CREATE11 = "CREATE TABLE IF NOT EXISTS otherTotal(_id integer primary key autoincrement, "
+ "other_prices double, other_total double);";
static final String DATABASE_CREATE12 = "CREATE TABLE IF NOT EXISTS puzzleTotal(_id integer primary key autoincrement, "
+ "puzzle_prices double, puzzle_total double);";
double columntotal, strategyPriceInput, actionPriceInput, adventurePriceInput, MMOPriceInput, racingPriceInput, rpgPriceInput, simulationPriceInput,
sportsPriceInput, puzzlePriceInput, otherPriceInput;
final Context context;
DatabaseHelper DBHelper;
SQLiteDatabase db;
public DBAdapter(Context ctx) {
this.context = ctx;
DBHelper = new DatabaseHelper(context);
}
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
try {
db.execSQL(DATABASE_CREATE);
db.execSQL(DATABASE_CREATE2);
db.execSQL(DATABASE_CREATE3);
db.execSQL(DATABASE_CREATE4);
db.execSQL(DATABASE_CREATE5);
db.execSQL(DATABASE_CREATE6);
db.execSQL(DATABASE_CREATE7);
db.execSQL(DATABASE_CREATE8);
db.execSQL(DATABASE_CREATE9);
db.execSQL(DATABASE_CREATE10);
db.execSQL(DATABASE_CREATE11);
db.execSQL(DATABASE_CREATE12);
} catch (SQLException e) {
e.printStackTrace();
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL(DATABASE_CREATE2);
}
}
// ---opens the database---
public DBAdapter open() throws SQLException {
db = DBHelper.getWritableDatabase();
return this;
}
// ---closes the database---
public void close() {
DBHelper.close();
}
// ---insert a record into the database---
public long insertRecord(String title, String platform, String genre,
String date, double price, String wherebought) {
ContentValues initialValues = new ContentValues();
initialValues.put(KEY_TITLE, title);
initialValues.put(KEY_PLATFORM, platform);
initialValues.put(KEY_GENRE, genre);
initialValues.put(KEY_DATE, date);
initialValues.put(KEY_PRICE, price);
initialValues.put(KEY_WHEREBOUGHT, wherebought);
return db.insert(DATABASE_TABLE, null, initialValues);
}
// delete all records
public void deleteAll() {
context.deleteDatabase(DATABASE_NAME);
}
// ---deletes a particular record---
public boolean deleteRecord(long rowId) {
return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}
// ---retrieves all the records---
public Cursor getAllRecords() {
return db.query(DATABASE_TABLE, new String[] { KEY_ROWID, KEY_TITLE,
KEY_GENRE, KEY_DATE, KEY_PRICE, KEY_WHEREBOUGHT }, null, null,
null, null, null);
}
// ---retrieves a particular record---
public Cursor getRecord(long rowId) throws SQLException {
Cursor mCursor = db.query(true, DATABASE_TABLE, new String[] {
KEY_ROWID, KEY_TITLE, KEY_PLATFORM, KEY_GENRE, KEY_DATE,
KEY_PRICE, KEY_WHEREBOUGHT }, KEY_ROWID + "=" + rowId, null,
null, null, null, null);
if (mCursor != null) {
mCursor.moveToFirst();
}
return mCursor;
}
// ---updates a record---
public boolean updateRecord(long rowId, String title, String platform,
String genre, String date, double price, String wherebought) {
ContentValues args = new ContentValues();
args.put(KEY_TITLE, title);
args.put(KEY_PLATFORM, platform);
args.put(KEY_GENRE, genre);
args.put(KEY_DATE, date);
args.put(KEY_PRICE, price);
args.put(KEY_WHEREBOUGHT, wherebought);
return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}
public Cursor rawQuery(String string, Object object) {
// TODO Auto-generated method stub
return null;
}
}
解决方案
Your DBAdapter.rawQuery()
is not implemented, it's just a stub that returns null
. Attempting to call moveToFirst()
on this null
reference causes the NPE.
That is, change
public Cursor rawQuery(String string, Object object) {
// TODO Auto-generated method stub
return null;
}
to something like
public Cursor rawQuery(String sql, String[] args) {
return db.rawQuery(sql, args);
}
这篇关于寻找空指针异常的原因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!
查看全文