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difference between null != something and something != null

Is there a difference between null != something and something != null in Java. And if there is a difference then which one should I use and why??

There's no difference between null != something and something != null. You must be thinking about the person.getName().equals("john") and the "john".equals(person.getName()) difference: the first one will throw a NullPointerException if getName() returns null, while the second won't. But this is not applicable for the example of your question.

its probably comming from the so-called joda-conditions where you write "bla" == myVariable instead of myVariable == "bla" because it could happen to accidentially write myVariable = "bla" which returns "bla" in some languages but also assign "bla" to myVariable

I just want to point out that the "Yoda condition" rationale for doing this (in languages like C & C++) does not apply in this (Java) case.
Java does not allow general expressions to be used as statements, so both
something == null;
and
null == something;
would be compilation errors.
The types of something == null and something = null are different; boolean and some reference type respectively. In this case, it means that both:
if (something = null) {
...
}
and
if (null = something) {
...
}
would be compilation errors.
In fact, I can't think of a realistic example where null == something would be compilation error and something == null would not. Hence, it doesn't achieve anything in terms of mistake-proofing.

There is no difference, but some people use it for ease of readability in their code.

Point of view of performance there will be no difference, both sides of the operator are executed any way. But for a more readable code second one seems more readable
obj.getSomething().getAnotherThing().doSomething() != null
null != obj.getSomething().getAnotherThing().doSomething()
But if you are going to just compare a variable or parameter this is more readable
something != null
Of course this depends on sense of reader.

In java if we compare any, always we have to place variables at left hand side and values are placed at right hand side...

They are both the same there is no difference.

How to prevent null check before equals

I find stuff like this rather annoying and ugly in equals methods:
if (field == null)
{
if (other.field != null)
return false;
}
else if ( ! field.equals(other.field))
return false;
In C# I could've done this:
if( ! Object.Equals(field, other.field))
return false;
Is there something similar in Java, or what is the preferred way to do this kind if thing?

Java 7 offers java.util.Objects.equals.

Use commons-lang:
org.apache.commons.lang.ObjectUtils.equals(Object object1, Object object2)
Source code:
public static boolean equals(Object object1, Object object2) {
if (object1 == object2) {
return true;
}
if ((object1 == null) || (object2 == null)) {
return false;
}
return object1.equals(object2);
}
From Apache
http://commons.apache.org/lang/
That's about equivalent to what you do in C#

Guava equal which does this :
public static boolean equal(#Nullable Object a, #Nullable Object b) {
return a == b || (a != null && a.equals(b));
}
or null object pattern
Guava also has the somewhat related comparison chain and a load of other goodies.

I would write it this way:
return field != null && other.field != null && field.equals(other.field);
which is not as elegant as the C# code line, but much shorter then the if tree you posted.

I accept all answers technically. Practically I will not use any of them in code I have under control because all provided solutions are working around the core problem: null-values. KEEP YOUR CORE MODEL FREE FROM NULL VALUES, and the question is obsolete in this case.
At system borders like third party libraries one has to deal with null values sometimes. They should converted into meaningful values for the core model. There the given solutions are helpful.
Even if Oracle recommends the equals-Methods to be null-safe, think about that: Once you accept null values your model it is getting fragile. The equals-method will not be the last method where you will check for null. You have to manage null-checks in your method call hierarchy. Methods may not be reusable out of the box anymore. Soon, every parameter will be checked for null.
I saw both sides:
On one side code full of null checks, methods that trust not a single parameter anymore and developers that are afraid to forget a null check.
On the other side code with full expressive statements that make clear assertions to have full functioning objects that can be used without fear of NullPointerExceptions.

As part of the Project Coin, there was a proposal for adding a series of null-safe operators to Java. Sadly, they didn't make it into Java 7, maybe they'll appear in Java 8. Here is the general idea of how they would work

Actually everyone follows there own way to do this and also i would like to introduce groovy here.
There is one way
field == null ? false : true; // So basically it will return true when it is not null.
In groovy there is null safe operator for objects. Lets take an example for class
A {
String name = "test1"
String surName = "test2"
public String returnName() {
return name + surName
}
}
A a = null
a?.name
// Mentioned operator ? will actually check whether a is null or not. then it will invoke name.
Note: i didn't applied semi colon in code as this is not require in groovy.

String.valueOf() will solve some of those problems if the toString is implemented for your classes. It will spit out the toString() answer or "null" if the pointer is null.

Use == operator when you are checking for object references, if both the references refers same object it will return true. Otherwise if you are looking for object content then go with .equals method of objects.
So null means it doesn't have any memory location given in heap. So it can be simply checked with '==' operator.

Object null-ness check in Java

Which one is recommended and to be used to check the Object null-ness?
null != Object
or
Object != null
and other way
null == Object
or
Object == null
...and is there any difference between them?

(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.

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.

String equality in Java

I have seen both of these when checking the equality of two Java String's:
// Method A
String string1;
// ...
if("MyString".equals(string1)) {
// ...
}
and
// Method B
String string1;
// ...
if(string1.equals("MyString")) {
// ...
}
My question is: which one is better and more widely used?

If you are sure that string1 can never be null then option 2 is readable and preferred. Otherwise option 1. Intention of option 1 is to avoid potential null pointer.

Method A won't throw a null pointer exception. There is no better of the two. It depends on whether on not you want it to throw a npe (and you might want that in your overall design).

Method B will fail with NullPointerException on null string1, whereas Method A will never throw this. Some authorities mandate this "defensive" programming. They have influenced me to do it, though it still does not come naturally!
It's also possible to write
if (string1 != null && string1.equals("MyString")) ...
though tools such as FindBugs flags this as a possible error, assuming that you should have made sure that string1 was already non-null. (Can you rely on the order of evaluation?).
So there are different schools of thought.

Method a does not throw NullPointerException and hence very convenient. It is widely used also.

Exceptions are for exceptional processing and have more overhead than checking for error conditions and handling them with regular logic. If you have been programming for a decade a NPE is a cringe matter which usually indicates careless code. Avoid them by using "constant".equals(variable) and people who read your code and use it will be happier.

The second one is more widely used. Neither is better.
It's the same idea as
if (1 == x)
but without a specific reason. but for a different reason. (Null pointer as noted by others).

Is it a bad idea if equals(null) throws NullPointerException instead?

The contract of equals with regards to null, is as follows:
For any non-null reference value x, x.equals(null) should return false.
This is rather peculiar, because if o1 != null and o2 == null, then we have:
o1.equals(o2) // returns false
o2.equals(o1) // throws NullPointerException
The fact that o2.equals(o1) throws NullPointerException is a good thing, because it alerts us of programmer error. And yet, that error would not be catched if for various reasons we just switched it around to o1.equals(o2), which would just "silently fail" instead.
So the questions are:
Why is it a good idea that o1.equals(o2) should return false instead of throwing NullPointerException?
Would it be a bad idea if wherever possible we rewrite the contract so that anyObject.equals(null) always throw NullPointerException instead?
On comparison with Comparable
In contrast, this is what the Comparable contract says:
Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.
If NullPointerException is appropriate for compareTo, why isn't it for equals?
Related questions
Comparable and Comparator contract with regards to null
A purely semantical argument
These are the actual words in the Object.equals(Object obj) documentation:
Indicates whether some other object is "equal to" this one.
And what is an object?
JLS 4.3.1 Objects
An object is a class instance or an array.
The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.
My argument from this angle is really simple.
equals tests whether some other object is "equal to" this
null reference gives no other object for the test
Therefore, equals(null) should throw NullPointerException

To the question of whether this asymmetry is inconsistent, I think not, and I refer you to this ancient Zen kōan:
Ask any man if he's as good as the next man and each will say yes.
Ask any man if he's as good as nobody and each will say no.
Ask nobody if it's as good as any man and you'll never get a reply.
At that moment, the compiler reached enlightenment.

An exception really should be an exceptional situation. A null pointer might not be a programmer error.
You quoted the existing contract. If you decide to go against convention, after all this time, when every Java developer expects equals to return false, you'll be doing something unexpected and unwelcome that will make your class a pariah.
I could't disagree more. I would not rewrite equals to throw an exception all the time. I'd replace any class that did that if I were its client.

Think of how .equals is related to == and .compareTo is related to the comparison operators >, <, >=, <=.
If you're going to argue that using .equals to compare an object to null should throw a NPE, then you'd have to say that this code should throw one as well:
Object o1 = new Object();
Object o2 = null;
boolean b = (o1 == o2); // should throw NPE here!
The difference between o1.equals(o2) and o2.equals(o1) is that in the first case you're comparing something to null, similar to o1 == o2, while in the second case, the equals method is never actually executed so there's no comparison happening at all.
Regarding the .compareTo contract, comparing a non-null object with a null object is like trying do this:
int j = 0;
if(j > null) {
...
}
Obviously this won't compile. You can use auto-unboxing to make it compile, but you get a NPE when you do the comparison, which is consistent with the .compareTo contract:
Integer i = null;
int j = 0;
if(j > i) { // NPE
...
}

Not that this is neccessarily an answer to your question, it is just an example of when I find it useful that the behaviour is how it is now.
private static final String CONSTANT_STRING = "Some value";
String text = getText(); // Whatever getText() might be, possibly returning null.
As it stands I can do.
if (CONSTANT_STRING.equals(text)) {
// do something.
}
And I have no chance of getting a NullPointerException. If it were changed as you suggested, I would be back to having to do:
if (text != null && text.equals(CONSTANT_STRING)) {
// do something.
}
Is this a good enough reason for the behaviour to be as it is?? I don't know, but it is a useful side-effect.

If you take object oriented concepts into account, and consider the whole sender and receiver roles, I'd say that behaviour is convenient. See in the first case you're asking an object if he is equal to nobody. He SHOULD say "NO, I'm not".
In the second case though, you don't have a reference to anyone So you aren't really asking anyone. THIS should throw an exception, the first case shouldn't.
I think it's only asymmetric if you kind of forget about object orientation and treat the expression as a mathematical equality. However, in this paradigm both ends play different roles, so it is to be expected that order matters.
As one final point. A null pointer exception should be raised when there's an error in your code. However, Asking an object if he is nobody, shouldn't be considered a programming flaw. I think it's perfectly ok to ask an object if he isn't null. What if you don't control the source that provides you with the object? and this source sends you null. Would you check if the object is null and only afterwards see if they are equals? Wouldn't it be more intuitive to just compare the two and whatever the second object is the comparison will be carried out without exceptions?
In all honesty, I would be pissed if an equals method within its body returns a null pointer exception on purpose. Equals is meant to be used against any sort of object, so it shouldn't be so picky on what it receives. If an equals method returned npe, the last thing on my mind would be that it did that on purpose. Specially considering it's an unchecked exception. IF you did raise an npe a guy would have to remember to always check for null before calling your method, or even worse, surround the call to equals in a try/catch block (God I hate try/catch blocks) But oh well...

Personally, I'd rather it perform as it does.
The NullPointerException identifies that the problem is in the object against which the equals operation is being performed.
If the NullPointerException was used as you suggest and you tried the (sort of pointless) operation of...
o1.equals(o1) where o1= null...
Is the NullPointerException thrown because your comparison function is screwed or because o1 is null but you didn't realise?
An extreme example, I know, but with current behaviour I feel you can tell easily where the problem lies.

In the first case o1.equals(o2) returns false because o1 is not equal to o2, which is perfectly fine. In the second case, it throws NullPointerException because o2 is null. One cannot call any method on a null. It may be a limitation of programming languages in general, but we have to live with it.
It is also not a good idea to throw NullPointerException you are violating the contract for the equals method and making things more complex than it has to be.

There are many common situations where null is not in any way exceptional, e.g. it may simply represent the (non-exceptional) case where a key has no value, or otherwise stand for “nothing”. Hence, doing x.equals(y) with an unknown y is also quite common, and having to always check for null first would be just wasted effort.
As for why null.equals(y) is different, it is a programming error to call any instance method on a null reference in Java, and therefore worthy of an exception. The ordering of x and y in x.equals(y) should be chosen such that x is known to not be null. I would argue that in almost all cases this reordering can be done based on what is known about the objects beforehand (e.g., from their origin, or by checking against null for other method calls).
Meanwhile if both objects are of unknown “nullness”, then other code almost certainly requires checking at least one of them, or not much can be done with the object without risking the NullPointerException.
And since this is the way it is specified, it is a programming error to break the contract and raise an exception for a null argument to equals. And if you consider the alternative of requiring an exception to be thrown, then every implementation of equals would have to make a special case of it, and every call to equals with any potentially null object would have to check before calling.
It could have been specified differently (i.e., the precondition of equals would require the argument to be non-null), so this is not to say that your argumentation is invalid, but the current specification makes for a simpler and more practical programming language.

Note that the contract is "for any non-null reference x". So the implementation will look like:
if (x != null) {
if (x.equals(null)) {
return false;
}
}
x need not be null to be deemed equal to null because the following definition of equals is possible:
public boolean equals(Object obj) {
// ...
// If someMember is 0 this object is considered as equal to null.
if (this.someMember == 0 and obj == null) {
return true;
}
return false;
}

I think it's about convenience and more importantly consistency - allowing nulls to be part of the comparison avoids having to do a null check and implement the semantics of that each time equals is called. null references are legal in many collection types, so it makes sense they can appear as the right side of the comparison.
Using instance methods for equality, comparison etc., necessarily makes the arrangement asymmetric - a little hassle for the huge gain of polymorphism. When I don't need polymorphism, I sometimes create a symmetric static method with two arguments, MyObject.equals(MyObjecta, MyObject b). This method then checks whether one or both arguments are null references. If I specifically want to exclude null references, then I create an additional method e.g. equalsStrict() or similar, that does a null check before delegating to the other method.

You should return false if the parameter is null.
To show that this is the standard, see 'Objects.equals(Object, Object) from java.util, that performs an assymetric null check on the first parameter only (on which equals(Object) will be called). From the OpenJDK SE 11 source code (SE 1.7 contains exactly the same):
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
This also handles two null values as equal.

This is a tricky question. For backward compatability you can't do so.
Imagine the following scenario
void m (Object o) {
if (one.equals (o)) {}
else if (two.equals (o)) {}
else {}
}
Now with equals returning false else clause will get executed, but not when throwing an exception.
Also null is not really equal to say "2" so it makes perfect sense to return false. Then it is probably better to insist null.equals("b") to return also false :))
But this requirement does make a strange and non symmetric equals relation.