what is null pointer exception

RT, by the way, we can see that a blog has such a sentence. if there is a default value, it cannot be blank. Otherwise, there are conflicts, after the data is modified, it is still its default value. what I want to know is that if I set the default value and do not set the v

Let's start with this simple statement:String x = null;What does this line of code do?Recall what is called a variable (variable) and what is called a value. We usually liken a variable to a box. We can use boxes to load things, a

What if SQL fuzzy query encounters a null value ?, SQL fuzzy query is blank Author: iamlaosong The SQL query statement uses % for fuzzy query. in the program, users are generally required to enter part of the information and perform fuzzy query based on the information. For example, if you enter 340104, the following statement queries all emails starting with 340

Let's start with the following simple statement: String x = null; What does this line of code do? Let's first think about what is variable and value ). We usually compare a variable to a box. We can use a box to hold things. Similarly, we can use variables to change the storage value. When defining a variable,

Arrays are either created in a static store (such as a global array) or created on a stack. Pointers can point to any type of memory block at any time.(1) Changes in the content of the differences Char a[] = "Hello"; They have their own memory space;A[0] = ' X ';Char *p = "World"; Note P points to the constant stringP[0] = ' X '; The compiler cannot find the error, run-time error (2) Use the operator sizeof to calculate the capacity (in bytes) of the array. sizeof (p), p

Who determines the pointer size? What is it?It should be said that this is a very basic problem. in textbooks, the pointer size is the same as the machine Character length, that is, the

name) { This. Name =name; } Public intGetage () {returnAge ; } Public voidSetage (intAge ) { This. Age =Age ; } PublicString getcity () {returnCity ; } Public voidsetcity (String city) { This. City =City ; } PublicString GetInfo () {return"The person was id=" + ID + ", name=" + name + ", age=" + Age + ", city=" +City ; } }Results:How do you use NULL parameter construction methods in Java to automatically generate object

The returned result of json data parsing is null. what is the problem? This is the case: Website A provides A connection interface. After I verify that the login information is passed, A json data

The returned result of json data parsing is null. what is the problem? This is the case: Website A provides A connection interface. After I verify that the logon information is passed, A json data

What is the value of the null and non-null settings when MySQL creates a table?

Data The start node refers to the first node in the list, and it has no direct predecessor. The head pointer is a pointer to the start node (in the case of No head node). A single linked list can be uniquely determined by its head pointer, and a single linked list is typica

PublicListgetAll1 (Stringname) throwsException {ListresultnewArrayList (); ConnectionconDatabaseConnection. getConnection (); Stringnanull; Stringsql1 quot; select * fromguanzhuwhereguanzhu? Quot; PreparedStatementps1con. prepareS java database mysql computer jsp Public ListgetAll1 (String name) throws Exception {Listresult = new ArrayList ();Connection con = DatabaseConnection. getConnection ();String na =

Pre {Line-Height: 1; color: #9f1d66; Background-color: # e1e1e1; font-size: 16px ;}. sysfunc {color: # 5d57ff; font-style: italic; font-weight: bold ;}. selffuc {color: #8e0ed3 ;}. bool {color: #008000 ;}. condition {color: #008000; font-weight: bold ;}. key {color: #440080 ;}. vaR {color: #008000; font-style: italic ;}. digit {color: #000080; font-weight: bold ;}. includepre {color: # 661d9f ;}. operator? {Color: # fd1a53; font-weight: bold ;} ???? The second-level

three types of connections: Male to female, male to male. Mother to mother. Each of the three connectors has crossover lines and straight lines, so there is a total of 6 connections in the same way.Question 2: How to identify the cross-serial line and direct-connect serial line?Check it with a multimeter. Suppose 2 and 2 pass. The general 3 and 3 are straight serial lines. Assume that the 2 and 3 pass, 3 and 2 general are cross-serial lines.Someone c

C ++ starts from scratch (5) -- what is a pointer?Original Source: Network This article describes the key to the duplication in C ++-pointer type and two meaningful concepts-static and dynamic. Array As mentioned above, the memory is accessed through variables in C ++, but a

What is the difference between return and return null in Java?The biggest difference:ReturnThe return value of the method must be void!return null;The return value of the method must not be the original data type (the wrapper class has been removed) and the void!Return is th

In C #, there is an application method called delegation. Its application methods and implementation functions are quite similar to the C ++ function pointers we have previously introduced. In this article, we will give a brief introduction to these two methods to help you differentiate and understand them. Both the delegate and function pointer describe the method/function signature and call different impl

In C #, there is an application method called delegation. Its application methods and implementation functions are quite similar to the C ++ function pointers we have previously introduced. In this articleArticle. Both the delegate and function pointer describe the method/function signature and call different implementations through a unified interface. However, the two have obvious differences. Simply pu