Object==Null or Null==Object

object==null or null==object?

This is probably a habit learned from C, to avoid this sort of typo (single = instead of a double ==):

if (object = null) {

The convention of putting the constant on the left side of == isn't really useful in Java since Java requires that the expression in an if evaluate to a boolean value, so unless the constant is a boolean, you'd get a compilation error either way you put the arguments. (and if it is a boolean, you shouldn't be using == anyway...)

Difference between null==object and object==null

^{(Similar question: Which is more effective: if (null == variable) or if (variable == null)?)}

Difference between null==object and object==null

There is no semantical difference.

object.getItems() == null and null == object.getItems() are equivalent.

Perhaps you're mixing it up with the fact that

nonNullObj.equals(obj)

and

obj.equals(nonNullObj)

can make a difference (since the second alternative could result in a NPE in case the callee is null).

java.util.Objects.isNull vs object == null

should use object == null over Objects.isNull() in a if statement?

If you look at the source code of IsNull method,

 /* Returns true if the provided reference is null otherwise returns false.*/

 public static boolean isNull(Object obj) {
     return obj == null;
 }

It is the same. There is no difference. So you can use it safely.

(obj == null) vs (null == obj)?

You can't accidently assign null to obj by typing obj = null instead. However, that's a reminiscence from C times, in java, it is not possible, as the = expression returns the right hand side of the assignment. As null is not a boolean, the compiler will complain.

I would try to explain it to my boss once, demonstrate it. If he still doesn't agree with you, just do it. It's a petty thing to fight about with your boss.

What is the difference between (Object)null and null in Java?

The first invocation will call the String.valueOf(Object) method, as you have explicitly typecasted null to Object reference. Conversely, the second one will invoke the overloaded String.valueOf(char[]) method, as char[] is more specific than Object for a null argument.

There are other overloaded versions of this method that accept primitive parameters, but those are not a valid match for a null argument.

From JLS §15.12.2:

There may be more than one such method, in which case the most
specific one is chosen. The descriptor (signature plus return type) of
the most specific method is one used at run time to perform the method
dispatch.

A method is applicable if it is either applicable by subtyping
(§15.12.2.2), applicable by method invocation conversion (§15.12.2.3),
or it is an applicable variable arity method (§15.12.2.4).

[...]

If several applicable methods have been identified during one of the
three phases of applicability testing, then the most specific one is
chosen, as specified in section §15.12.2.5.

Now check the source code of both the methods:

// This won't throw NPE for `obj == null`
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

// This will throw `NPE` for `data == null`
public static String valueOf(char data[]) {
    return new String(data);
}

Why does Object == null work?

Why does Object == null work?

This doesn't really mean anything. Objects aren't values in Java. You can't write that. All you can write is someObjectReference == null.

So when we are comparing objects

We aren't. See above. We are comparing references.

we use equals() methods, or something similar in an if statement for example. If we have the following code

String a = "foo";
String b = "foo";
return a==b

we would get false returned to us because a and b refer to different objects.

No we wouldn't, and no they don't. It will return true. Try it. String literals are pooled by Java. There is only one "foo" object.

On the other hand,

String a = null;
return a == null

we would get true. Why is that?

Because the value of the reference a is null, and so is the value of expression on the RHS of the == operator. Equal values => result of == is true. Note that a is a reference, not an object.

How to check if object is null or not except == null

The easiest way to check is entity == null. There is no shorter way to do that.

Note that there is a method for this in the standard lib:

Objects.isNull(Object obj)

And another one which is the opposite of the above one:

Objects.nonNull(Object obj)

And there is yet another one which can be used to force a value to be not null, it throws a NullPointerException otherwise:

T Objects.requireNonNull(T obj);

Note: The Objects class was added in Java 7, but the isNull() and nonNull() methods were added only in Java 8.

Why is null an object and what's the difference between null and undefined?

(name is undefined)

You: What is name? ^(*)

JavaScript: name? What's a name? I don't know what you're talking about. You haven't ever mentioned any name before. Are you seeing some other scripting language on the (client-)side?

name = null;

You: What is name?

JavaScript: I don't know.

In short; undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope; null is where the thing is known to exist, but it's not known what the value is.

One thing to remember is that null is not, conceptually, the same as false or "" or such, even if they equate after type casting, i.e.

name = false;

You: What is name?

JavaScript: Boolean false.

name = '';

You: What is name?

JavaScript: Empty string

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^{*: name in this context is meant as a variable which has never been defined. It could be any undefined variable, however, name is a property of just about any HTML form element. It goes way, way back and was instituted well before id. It is useful because ids must be unique but names do not have to be.}

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