null and not null in java

[Premila Devi Dayalan]

Hi,
I've a simple object class as testObj.java with getter and setter methods.
In my code i need to check if the testObj object is null.

My code is :
testObj test = new testObj();
//invoke a method A which returns test as return type
if(test==null || test.equals(""))
{
System.out.println("The test object is null);
}

But i'm not getting the expected results. How to check if this test object is null?

One more question?
what is the different between null and not null in java?

From:Behalf of Premila Devi
Name: Yong Mei Ling

[Pramod P Deore]

To check Object is null or not use

if (test.equals(null))
		{
			System.out.println("The test object is null");
		}

[Jaikiran Pai]

Premila,

Since this question is not related to servlets, I'm moving the thread to a better forum.

Premila Devi Dayalan wrote:
From:Behalf of Premila Devi
Name: Yong Mei Ling

Hmm... what does that mean?

[Manish Singh]

to check whether an object is null use objRef == null

[Manish Singh]

if (test.equals(null))  
     {  
         System.out.println("The test object is null");  
     }

have you tried it???

[Campbell Ritchie]

You are confusing a null String and the empty String "". The two are different. You will of course only get true from that equals("") call if test is a String "".

You can always test whether an object is null with x == null or x != null and ought not to use the equals method, which will throw an Exception if called on a null reference.

[Campbell Ritchie]

Code tags for code, please Manish Singh, not quote tags.

[Rob Spoor]

test.equals(null) will (or at least should) always return false. And that makes sense - if test is null that code will throw a NullPointerException.

To compare against null, always use "== null" or "!= null". Everything else should be avoided.

[Mohnish Khiani]

a reference having a value null means that it doesn't point to any object.it is simply declaring a refernce variable.but "not null" means that the refernce is pointing to some object and stores "a memory address about how to get to the object in the memory"

[Premila Devi Dayalan]

Thanks.

From: Chan Mei Ling

[Campbell Ritchie]

You're welcome

I don't know whether you have seen this recent thread, which looks at a similar problem from a different angle.

[Premila Devi Dayalan]

Dear Sir,

NOT NULL Example :
"not null" means that the refernce is pointing to some object and stores "a memory address about how to get to the object in the memory"

	public void add(Asset asset) {

		if (asset.getAssetAlternative() != null) {
			//if (!StringUtils.isEmpty(asset.getAssetAlternative().getAlternativeNumber().toString())) {	
				AssetAlternativeNumber assetAlternativeNumber = asset.getAssetAlternative();

				if (localConfig.getDefaultAlternativeType().toUpperCase().equals("POLLUX")) {
					assetAlternativeNumber.setType(AlternativeNumberType.POLLUX);
				}else if (localConfig.getDefaultAlternativeType().toUpperCase().equals("OLDSERIAL")) {
					assetAlternativeNumber.setType(AlternativeNumberType.OLD_SERIAL);
				}else if (localConfig.getDefaultAlternativeType().toUpperCase().equals("STATEMENT")) {
					assetAlternativeNumber.setType(AlternativeNumberType.STATEMENT);
				}

				assetAlternativeNumber.setAsset(asset);
				asset.addAlternativeSerialNumber(assetAlternativeNumber);

			//}
		}

and

Null Example :
a reference having a value null means that it doesn't point to any object.it is simply declaring a refernce variable

		if (asset.getAssetModel().getId() == null) {
			assetModelService.add(asset.getAssetModel());
		} else {
			asset.setAssetModel(assetModelDao.findById(asset.getAssetModel().getId()));
		}

But here,it is "pointing to some object as well"

Chan Mei Ling

[Campbell Ritchie]

Premila Devi Dayalan wrote:But here,it is "pointing to some object as well"

No, it isn't. A reference either points to an object or it points to null. It can't point to both simultaneously. Your == null test looks whether that reference points to an object or not (== null), and the if-then-else block uses that result to decide which option to take.

And yes, it is called if-then-else, even though Java uses {} instead of the "then."

[Premila Devi Dayalan]

Dear Sir,

if (asset.getAssetModel().getId() == null) {
assetModelService.add(asset.getAssetModel());
} else {
asset.setAssetModel(assetModelDao.findById(asset.getAssetModel().getId()));
}

So,at these situation,it will check wherever,the (asset.getAssetModel().getId() == null),and **where it will check the **NULL Conditions,is it in the database,refering to **asset.getAssetModel().getId() method,or etc ? How it says/Check "Null"?

Chan Mei Ling

[Campbell Ritchie]

What has "Null" got to do with it? You are looking for null, not "Null", nor NULL. Whatever is returned from that method call might be a primitive (in which case you would suffer a compiler error) or an object reference or null. No other possibilities. You are checking whether the reference pointed to by that method call and the reference (probably 0) pointed to by null are the same. That checking, the way you have written it, is done by the JVM looking at heap memory.

Beware of looking in databases, where one uses three-value logic and NULL may be interpreted "don't know", so 123 < NULL, 123 > NULL and 123 = NULL always return NULL. SQL is different from Java, and NULL in SQL may mean something completely different from null in Java. You may need to use the isNull() method when retrieving anything from a database to see whether it was NULL i nSQL.

[Manish Singh]

Code tags for code, please Manish Singh, not quote tags.

Sure. Thanks for pointing

[Campbell Ritchie]

Manish Singh wrote:Sure. Thanks for pointing

[Premila Devi Dayalan]

Dear Sir,

Null fields in a database (bad design practice).Specifiying a field as NOT NULL means when you insert a new record you will always have a value to fill that column. It allows the database system to optermise your database and as NOT NULL are used for primary keys for the database,but programmatically [JAVA] ~ Confused :

Example:

if (asset.getAssetModel().getId() == null) {
assetModelService.add(asset.getAssetModel());
} else {
asset.setAssetModel(assetModelDao.findById(asset.getAssetModel().getId()));
}

Please correct me,if wrong.

Programmatically,at here we are checking the asset.getAssetModel().getId() == null whether the reference pointed to by that method call and the reference (probably 0) pointed to by null are the same. That checking, the way you have written it, is done by the JVM looking at heap memory.

and basically at my code, asset.getAssetModel().getId() is getting the asset_model_id from the database.At here,are it checking,whether asset_model_id is null or not null? Please advise

Thanks.

Chan Mei Ling

[Campbell Ritchie]

You seem to have quoted my post without quote tags. Please use the quote tags so people can distinguish what I wrote from what you wrote.

Yes, the first == null test looks to see whether whatever that method returns is null or is somewhere on the heap. [In Java6 some objects can be created and deleted from the stack, but that doesn't make any difference to the principle.] If it is null you choose the first option in your if-then-else, otherwise you choose the second option (after the else). Since you didn't write "== null" or "!= null" in the else block, no, there is no checking whether anything is null.

When I said "beware" I didn't mean that NULL values in a database were bad; there are instances where they must be NULL, for example when my middle name is entered in a database. What I meant was that you must remember that NULL in SQL and null in Java™ mean different things, although I take your point that database design is often better if you can write NOT NULL.

[Premila Devi Dayalan]

thank a lots.

[Campbell Ritchie]

You're welcome