Differencebetween Null and Empty

Difference between null and empty string

String s1 = ""; means that the empty String is assigned to s1.
In this case, s1.length() is the same as "".length(), which will yield 0 as expected.

String s2 = null; means that (null) or "no value at all" is assigned to s2. So this one, s2.length() is the same as null.length(), which will yield a NullPointerException as you can't call methods on null variables (pointers, sort of) in Java.

Also, a point, the statement

String s1;

Actually has the same effect as:

String s1 = null;

Whereas

String s1 = "";

Is, as said, a different thing.

Difference between null and empty () Java String

"" is an actual string, albeit an empty one.

null, however, means that the String variable points to nothing.

a==b returns false because "" and null do not occupy the same space in memory--in other words, their variables don't point to the same objects.

a.equals(b) returns false because "" does not equal null, obviously.

The difference is though that since "" is an actual string, you can still invoke methods or functions on it like

a.length()

a.substring(0, 1)

and so on.

If the String equals null, like b, Java would throw a NullPointerException if you tried invoking, say:

b.length()

If the difference you are wondering about is == versus equals, it's this:

== compares references, like if I went

String a = new String("");
String b = new String("");
System.out.println(a==b);

That would output false because I allocated two different objects, and a and b point to different objects.

However, a.equals(b) in this case would return true, because equals for Strings will return true if and only if the argument String is not null and represents the same sequence of characters.

Be warned, though, that Java does have a special case for Strings.

String a = "abc";
String b = "abc";
System.out.println(a==b);

You would think that the output would be false, since it should allocate two different Strings. Actually, Java will intern literal Strings (ones that are initialized like a and b in our example). So be careful, because that can give some false positives on how == works.

Difference between NULL and Blank Value in Mysql

NULL is an absence of a value. An empty string is a value, but is just empty. NULL is special to a database.
NULL has no bounds, it can be used for string, integer, date, etc. fields in a database.
NULL isn't allocated any memory, the string with NULL value is just a pointer which is pointing to nowhere in memory. however, Empty IS allocated to a memory location, although the value stored in the memory is "".

What is difference between null and empty list?

In Java, collections won't magically spring into existence just by adding something to them. You have to initialize pcList by creating an empty collection first:

List<Map<String, Object>> pcList = new ArrayList<>();

An empty collection isn't the same as null. An empty collection is actually a collection, but there aren't any elements in it yet. null means no collection exists at all.

Note that an object can't be of type List, because that's an interface; therefore, you have to tell Java what kind of List you really want (such as an ArrayList, as I've shown above, or a LinkedList, or some other class that implements List).

What is the difference between null and empty?

A variable is NULL if it has no value, and points to nowhere in memory.

empty() is more a literal meaning of empty, e.g. the string "" is empty, but is not NULL.

The following things are considered to
be empty:

"" (an empty string)

0 (0 as an integer)

0.0 (0 as a float)

"0" (0 as a string)

NULL

FALSE

array() (an empty array)

var $var; (a variable declared, but without a value in a class)

Source.

Example

$a is NULL.

$a = '' is empty, but not NULL.

Update

If $a='' is empty but not NULL, when do I use the empty() function and when do I use the isset() function.

isset() will return FALSE is the variable is pointing to NULL.

Use empty() when you understand what is empty (look at the list above).

Also when you say it points nowhere in memory, what does that mean exactly?

It means that $str = '' will be in memory as a string with length of 0.

If it were $str = NULL, it would not occupy any memory.

Empty string vs NULL

Some differences between them:

NULL can be assigned to any type, as opposed to empty string which won't be compatible with date/numerical fields.
NULL is an UNKNOWN value, it doesn't have a value as opposed to an empty string, which is a value, but empty one.
As far as I'm aware of, NULL shouldn't capture memory as opposed to an empty string which does.
null = null will result in null as opposed to ''='' which will result in TRUE.

What is the difference between null and undefined in JavaScript?

undefined means a variable has been declared but has not yet been assigned a value :

var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefined