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

Given two strings:

String s1 = "abc";
String s2 = "abc";

-or -

String s1 = new String("abc");
String s2 = new String("abc");

The == operator performed on two Objects checks for object identity (it returns true if the two operators return to the same object instance.) The actual behavior of == applied to java.lang.Strings does not always appear to be consistent because of String interning.

In Java, Strings are interned (at least partly at the discretion of the JVM.) At any point in time, s1 and s2 may or may not have been interned to be the same object reference (supposing they have the same value.) Thus s1 == s2 may or may not return true, based solely on whether s1 and s2 have both been interned.

Making s1 and s2 equal to empty Strings has no effect on this - they still may or may not have been interned.

In short, == may or may not return true if s1 and s2 have the same contents. s1.equals(s2) is guaranteed to return true if s1 and s2 have the same contents.

Answer 2

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. :)