Difference between null and ""

String str1 = null; str reference is empty
String str2 = ""; str apply an empty String

That is, no space is allocated for null. "" space is allocated. Therefore, str1 is not an instantiated object, and str2 has been instantiated.

Note that "" is an object because null is not an object. Therefore, the comparison must be if (str1 = null) {...} And if (str2.equals ("")){}

Objects are compared with equals, and null objects are compared with equal signs. Therefore, if str1 = null; the following statement is incorrect:
If (str1.equals ("") | str1 = null) {// if str1 has no value, then ....
//....
}

The correct syntax is if (str1 = null | str1.equals ("") {// first judge whether it is an object. if yes, then judge whether it is a null string.
//...
}