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What is a NullReferenceException, and how do I fix it?

由 我们两清 提交于 2020-01-16 20:08:08
问题 This question's answers are a community effort . Edit existing answers to improve this post. It is not currently accepting new answers or interactions. I have some code and when it executes, it throws a NullReferenceException , saying: Object reference not set to an instance of an object. What does this mean, and what can I do to fix this error? 回答1: What is the cause? Bottom Line You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null , or

对象-Map 相互转换

由 痞子三分冷 提交于 2020-01-16 17:09:20
package org.scivf.common.map; import java.beans.PropertyDescriptor; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.lang.reflect.Modifier; import java.sql.SQLException; import java.sql.Timestamp; import java.util.HashMap; import java.util.Map; import org.springframework.beans.BeanUtils; public class MapUtils { /** * 把指定的复杂对象属性，按照指定的内容，封装到新的map中 * * @param source * 目标对象 * @param ps * 需要封装到map中的属性 * @return */ public static Map<String, Object> obj2map(Object source, String[] ps) { Map<String, Object> map = new

Last Non-Null Value in Redshift by Group

由 守給你的承諾、 提交于 2020-01-16 01:47:08
问题 I am using Redshift and want to receive the last non-Null value by userid. Here is an example dataset: Date UserID Value 4-18-2018 abc 1 4-19-2018 abc NULL 4-20-2018 abc NULL 4-21-2018 abc 8 4-19-2018 def 9 4-20-2018 def 10 4-21-2018 def NULL 4-22-2018 tey NULL 4-23-2018 tey 2 If the new user starts out with a NULL then replace with 0. I want my final dataset to look like this: Date UserID Value 4-18-2018 abc 1 4-19-2018 abc 1 4-20-2018 abc 1 4-21-2018 abc 8 4-19-2018 def 9 4-20-2018 def 10 4

What does a variable set to “&NULL” means in PHP?

由 不羁岁月 提交于 2020-01-15 20:30:31
问题 I'm working on a tree structure in PHP. When looping through the nodes, an exception is sometime thrown because some node that should not be null is null, more precisely it's set to "&NULL": array(13) { // ... // some data... // ... ["segments"]=> NULL ["leaf"]=> bool(false) ["children"]=> &NULL } Since it's not inside quotes, I assume it's some kind of special value, but what does it mean? 回答1: It just means that it is a reference to a value NULL $a = array(); $n = null; $a[1] =& $n; var

Hello Google Maps Example NULL Pointer Exception

由 醉酒当歌 提交于 2020-01-15 18:50:33
问题 I am creating a google maps application for android and I am following the guide at http://developer.android.com/resources/tutorials/views/hello-mapview.html to get started. I can run the app just fine in my emulator, but when I click on the overlay item I get a NULL Pointer Exception. The trace is below. I'm new to Android and have no idea how to debug this. So any help would be greatly appreciated. 12-20 02:57:05.898: W/dalvikvm(473): threadid=1: thread exiting with uncaught exception

Initializing variable at address zero in C

由 ▼魔方 西西 提交于 2020-01-15 12:20:08
问题 This may be a pretty basic question. I understand that there is a C convention to set the value of null pointers to zero. Is it possible that you can ever allocate space for a new variable in Windows, and the address of that allocated space happens to be zero? If not, what usually occupies that address region? 回答1: On MS-DOS the null pointer is a fairly valid pointer and due to the OS running in real mode it was actually possible to overwrite the 0x0 address with garbage and corrupt the

Unique results for SQL command with GROUP, MIN and NULL values

由 北城余情 提交于 2020-01-15 12:15:47
问题 I have a table inquiry with columns job , gender ( TRUE for women, FALSE for men) and salary . None of the columns is unique, salary may contain NULL . How to find the minimum salary per job for women ( TRUE )? If e.g. there are entries [pilot ; TRUE ; 100] , [pilot ; FALSE ; 100] , [pilot ; TRUE ; NULL] and [pilot ; FALSE ; 120] , the code below returns [pilot ; 100] twice instead of once. SELECT TOP (100) PERCENT T.JOB, T.SALARY FROM INQUIRY AS T INNER JOIN (SELECT JOB, MIN(SALARY) AS SL

Why Mongo query for null filters in FETCH after performing IXSCAN

由 Deadly 提交于 2020-01-15 11:53:37
问题 According to Mongo Documentation, The { item : null } query matches documents that either contain the item field whose value is null or that do not contain the item field. I can't find documentation for this, but as far as I can tell, both cases (value is null or field is missing) are stored in the index as null . So if I do db.orders.createIndex({item: 1}) and then db.orders.find({item: null}) , I would expect an IXSCAN to find all documents that either contain the item field whose value is

Why Mongo query for null filters in FETCH after performing IXSCAN

由 南楼画角 提交于 2020-01-15 11:52:59
问题 According to Mongo Documentation, The { item : null } query matches documents that either contain the item field whose value is null or that do not contain the item field. I can't find documentation for this, but as far as I can tell, both cases (value is null or field is missing) are stored in the index as null . So if I do db.orders.createIndex({item: 1}) and then db.orders.find({item: null}) , I would expect an IXSCAN to find all documents that either contain the item field whose value is
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