Object null-ness check in Java

Which one is recommended and to be used to check the Object null-ness?

null != Object

开发者_运维百科

or

Object != null

and other way

null == Object

or

Object == null

...and is there any difference between them?

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(In)equality is commutative, so there is no difference.

Historically the former stems from C to avoid accidentally assigning a value in a conditional statement, however that mostly applies to ==, not !=. Also Java requires the condition in a conditional statement to have a boolean value, so the only place where it could go wrong nowadays would be

if (a == false) ...

if you accidentally omit one of the =. A rare case, I guess (though probably not so much, given what students frequently write in their first two terms). Joonas also points out another (more obscure) case in the comments.

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It's always more readable to use

Object != null

because that reads as "the object is not null", which is literally what the condition is. The only case where you want to swap the two is to avoid accidentally using

Object = null

which will return true even though it is not the desired behavior, when you wanted to say

Object == null

but in reality not only do modern tools catch these kinds of mistakes, but wide use of the reverse can actually be an impediment to anyone who has to read the code.