Why is my list empty, java error when judging that the list is empty?

%;What is this?!

The expression should be written like this<%=

To add an equal sign (=) without a semicolon (; ）

Premise: You've got the data!

There is also a list that is taken from 0.

You take get(1) and hope you have at least 2 pieces of data!

In the servlet, the setattribute needs to be outside of the while loop.

And rs and rs2 need to be next together, so you also need to judge if ( in while

Then proceed with the following operations.

Should be. %=;

;Right? Gotta while (right.)

Shouldn't you use two RSs, how do you make sure that the users taken out by rs2 are the users of news?

"newslist", list);

It should be placed outside the loop.

Then confirm that your database has data.

First, listselectitem = null;

Directly afterward. Don't you think there's something missing, what's the difference?

It should be listselectitem = new arraylist<>(

Your second question is very simple, have you not figured out whether the selectitem wants to store an appointmentitem or a list??? According to your question 2, the list is stored as a listselectitem in the definition, and the other is add instead of addall. If it is indeed an appointmentitem, then you are writing correctly.

Problem 2: When selectitem is defined, the object must be created, otherwise the null pointer will be reported.

"That is, when I directly enter the address of the jsp page, I can print "hehe"."?

No, if you type it directly, you'll definitely throw an exception.

searchlist").The tostring() phrase means that you get the"searchlist"Object.

But just"object"If you want to use it, you have to change the type first<% list list = (list)"searchlist");

if(%>

If you enter the address directly, there is no one in the request"searchlist"object, which must be a null

If you call the tostring() method again, an exception will be thrown.

To determine the size of the list, you should use size().

list list = (list) "searchlist")if(

searchlist").tostring().length() is equivalent to the value of the string object@hashcode, and his length is greater than 0, strange, it is strange to open jsp directly without nullpointerexception??

If you judge whether there is data in the collection, if you don't judge, there will be a nullpointexception

arraylist al；If this is a record that your database queries up, you first if(

In this, write iterations, go through the journey, and you're going to see the specifics, and if you want to know, you have to look at jdkapi's isempty method, isempty is to determine whether a set is empty or not.

You**should first judge whether rs is null to determine whether the demolition spring is empty when the page is displayed**To be robust。

Make a judgment before output, undefined will output an empty string.

You didn't get it in the right way, even though the value stack is there, but you wrote it wrong on the jsp page. It is recommended to use a set put press-in. Especially when dealing with list collections.

listdepartment=;

department:"+department);I can print out the data.

You can definitely print out the data when you update the list. But preferably; See if there's any value.

AJAX polling the request will do the trick.

Who can read if you post it like this, let everyone guess Post your **, save the tips and other information.

Does your DAO layer take any value from the database?

We say that the essence of manipulation of data is that space is allocated to it in memory before data can be processed.

The meaning of null is empty. That is, no space is allocated.

So at this time if you give it an operation. assignment, the method will definitely be incorrect.

new, memory will allocate an address to your new object. That is, this object points to an address. Then act on it. Leave it to the computer.

A little deeper. Object assignment means that the address in the stack points to the change of data in the heap. Generally, the default value in the heap will be given when new.

I think: frm=null; It's just that the object is a null reference, but the object still exists, for example you can frm=new form2(); Yes, too; So frm=null, it doesn't have to be garbage**; As for when**, there should be a framework to decide, of course you can show the dispose; I think there's an easy way to test it:

You call directly:; Then see if frm has a value (frm is not null after dispose. ）