how null is checked in java?

Question

I was working on some project and got a condition when I have to check the object is null or not from a list and all variables of the object are null.

So can someone explain to me how an object is checked for null i.e. variable wise or some other way.

how an object is checked for null internally in java don't want the code. want the concept

Please in a little detail.

My Question: How Java internally checks if object contains a null value?

This is a google question, not really a programming question. — 9Deuce, Jul 24 '15 at 18:08
no no nothing like that.. i tried a lot on google but couldn't find anything so asked here..!!! and izZ acondition in my software..!!! — tushar garg, Jul 24 '15 at 18:14
Please be more specific in your question then, because as it's worded, it seems you're asking for a method to check an object for null. Edit your question. — 9Deuce, Jul 24 '15 at 18:16
sure i'll do it..but can sumbody explain to me how any object is checked for null concept.. — tushar garg, Jul 24 '15 at 18:18

score 2 · Answer 1 · answered Jul 24 '15 at 19:11

Apparently, you are actually asking how null checks are implemented under the hood.

The answer is implementation specific. It could be different for different JVMs and / or execution platforms. (If you want to research the specific implementation on a specific JVM, I suggest you checkout the JVM source code and/or get the JIT compiler to dump out the compiled native code for you to examine.)

Basically, there are two approaches:

An explicit x == null test will typically compile to an instruction sequence that compares the value of x against the value that represents a null. That is usually a 32-bit or 64-bit zero.
The implicit null check in x.toString() could be done the same way. Alternatively, it could be done by simply treating x as a machine address and attempting to fetch a value at that address. Assuming that the zero page has not been mapped, this will trigger a hardware "segmentation fault" exception. Java uses native code mechanisms to trap that exception, and turn it into a NullPointerException.

Is it then possible to print a corresponding int value of null? — joey rohan, Jul 24 '15 at 19:22
Not in regular Java. But you don't need to. See my suggestions about how to research the question. — Stephen C, Jul 24 '15 at 19:24
A very good explanation ! The question isn't so bad for down-votes, but not represented correctly. — joey rohan, Jul 24 '15 at 19:29

score 1 · Answer 2 · answered Jul 24 '15 at 18:10

1

If you're looking at a single item:

if(object == null)
{
   (...)
}

You mentioned a list. Let's pretend it's an ArrayList of Objects:

for(Object o : array_list)
{
   if(o == null)
   {
      (...)
   }
}

You'd also want to check to see if your list is null before you start looping through it.

answered Jul 24 '15 at 18:10

Lee Presswood

210
2
15

you got me wrong!! i wanted to ask how java checks for null i.e. by checking if all variables of that are null or some other way? – tushar garg Jul 24 '15 at 18:11
@tushargarg unclear what are you asking .you should clarify more – Madhawa Priyashantha Jul 24 '15 at 18:14
izZ just that i want to know how an object is checked for null internally in java don't want the code. want the concept. – tushar garg Jul 24 '15 at 18:17

score 1 · Answer 3 · answered Jul 24 '15 at 18:11

Basically any can be easily checked for null value. Every internal details and implementations of null and comparison with object are totally managed by java so all we need is to have a compare of the object with null as :-

Object obj = null; // Object can be replaced with any class 
if(obj == null){
// do your logics
}

As far as any List or Collection is considered, to see if object stored in it are null or not :-

List <String> list = new ArrayList<String>();
list.add("hi");
list.add(null);
for(String s : list){
 if(s == null){
// do your logics here
   }
}

score 0 · Answer 4 · answered Jul 24 '15 at 19:11

0

Java does not check if an object is a "null".

You cannot have a null object as null does not extend the Object class.

What you can do in java is have a variable assigned to null, meaning it references "nothing". (In reality, it is referencing the bytes that null is defined as)

That is what the other answers are doing, they are checking if a reference variable is actually pointing to nothing (null). An object itself, however, is never null.

answered Jul 24 '15 at 19:11

SwiftySwift

477
4
13

This is not helpful. You are correct that the JLS says that `null` is not an object. However, in reality Java professionals are happy to use the phrase "check an object for null" because we all know what it means ... to us. – Stephen C Jul 24 '15 at 19:31
@StephenC This is true. However, I was reading through some of OP's comments and in a few of them he asked if an object is null if "all variables are null .... [and] how an object is checked for null internally" I interpreted this as wanting to know exact details, one of them being than object itself is not null, and that the variable is made to reference `null`, which is how we check if "an object is null" – SwiftySwift Jul 24 '15 at 19:40
I put that down to poor English skills ... – Stephen C Jul 24 '15 at 20:15
Fair enough. Guess we won't know what exactly OP wanted. – SwiftySwift Jul 24 '15 at 20:18
@Marcin you are correct i wanted to ask the same thing and very thanks but by the time you answered i found it. – tushar garg Apr 06 '16 at 07:18

how null is checked in java?

4 Answers4