The difference between a Java empty string and a null string

I've never figured out the difference between a string and a null string, and today the code has been nullpointerexception, and I haven't figured it out.

String s = null;
s = xxx.getstring ();
if (!s.isempty () && s! = null) {
System.out.println ("OK");
}

I haven't read it for a long while (refusing to taunt). So I looked at someone else's blog and wrote a note.

1. Differences

Null indicates that the reference to a string object is null, string a=null; A string variable A is declared, and the reference to variable a is empty. So a doesn't point to any memory space, nor does it open up any space in the heap.

An empty string indicates that the value of a string object reference is null, string a = "" declares a string variable a, and the value of variable A is null. And a points to the memory space of the empty string.

2. Determine how the string is empty

(1) if (s! = null && s.equals (""));

(2) if (s! = null && s.length () = = 0);

public int Length () {        return value.length;    }

(3) if (s! = null && s.isempty ());

Java SE 1.6 Adds the method, the underlying implementation is similar to 2.

Method of implementation:

public Boolean isEmpty () {        return value.length = = 0;    }

(4) if (s! = NULL && s = = "");

Note: Do not write the same as I do, two judgment logic is reversed, there may be a nullpointerexception exception.

The difference between a Java empty string and a null string