Comparing a string with the empty string (Java)

Question

I have a question about comparing a string with the empty string in Java. Is there a difference, if I compare a string with the empty string with == or eq

Answer 1

Short answer

s1 == ""         // No!
s1.equals("")    // Ok
s1.isEmpty()     // Ok: fast (from Java 1.6) 
"".equals(s1)    // Ok: null safe

I would assure s1 is not null and use isEmpty().

Note: empty string "" is not a special String, but counts as any other "value".

A little longer answer

References to String objects depend on the way they are created:

String objects created using the operator new always refer to separate objects, even if they store the same sequence of characters so:

String s1 = new String("");
String s2 = new String("");
s1 == s2 // false

String objects created using the operator = followed by a value enclosed whitin double quotes (= "value") are stored in a pool of String objects: before creating a new object in the pool, an object with the same value is searched in the pool and referenced if found.

String s1 = ""; // "" added to the pool
String s2 = ""; // found "" in the pool, s2 will reference the same object of s1
s1 == s2        // true

The same is true for strings created enclosing a value whitin double quotes ("value"), so:

String s1 = "";  
s1 == "";        //true

String equals method checks for both, that's why it is safe to write:

s1.equals("");

This expression may throw a NullPointerException if s1 == null, so, if you don't check for null before, it is safer to write:

"".equals(s1);

Please read also How do I compare strings in Java?

Hope it may help not so experienced users, who may find other answers a bit too complicated. :)