What is String Interning in Java, when I should use it, and why?
8 Answers
Basically doing String.intern() on a series of strings will ensure that all strings having same contents share same memory. So if you have list of names where 'john' appears 1000 times, by interning you ensure only one 'john' is actually allocated memory.
This can be useful to reduce memory requirements of your program. But be aware that the cache is maintained by JVM in permanent memory pool which is usually limited in size compared to heap so you should not use intern if you don't have too many duplicate values.
More on memory constraints of using intern()
On one hand, it is true that you can remove String duplicates by internalizing them. The problem is that the internalized strings go to the Permanent Generation, which is an area of the JVM that is reserved for non-user objects, like Classes, Methods and other internal JVM objects. The size of this area is limited, and is usually much smaller than the heap. Calling intern() on a String has the effect of moving it out from the heap into the permanent generation, and you risk running out of PermGen space.
-- From:
From JDK 7 (I mean in HotSpot), something has changed.
In JDK 7, interned strings are no longer allocated in the permanent generation of the Java heap, but are instead allocated in the main part of the Java heap (known as the young and old generations), along with the other objects created by the application. This change will result in more data residing in the main Java heap, and less data in the permanent generation, and thus may require heap sizes to be adjusted. Most applications will see only relatively small differences in heap usage due to this change, but larger applications that load many classes or make heavy use of the String.intern() method will see more significant differences.
-- From Java SE 7 Features and Enhancements
Update: Interned strings are stored in main heap from Java 7 onwards.
There are some "catchy interview" questions, such as why you get equals! if you execute the below piece of code.
String s1 = "testString";
String s2 = "testString";
if(s1 == s2) System.out.println("equals!");
If you want to compare Strings you should use equals(). The above will print equals because the testString is already interned for you by the compiler. You can intern the strings yourself using intern method as is shown in previous answers....
JLS
JLS 7 3.10.5 defines it and gives a practical example:
Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.28) - are "interned" so as to share unique instances, using the method String.intern.
Example 3.10.5-1. String Literals
The program consisting of the compilation unit (§7.3):
package testPackage; class Test { public static void main(String[] args) { String hello = "Hello", lo = "lo"; System.out.print((hello == "Hello") + " "); System.out.print((Other.hello == hello) + " "); System.out.print((other.Other.hello == hello) + " "); System.out.print((hello == ("Hel"+"lo")) + " "); System.out.print((hello == ("Hel"+lo)) + " "); System.out.println(hello == ("Hel"+lo).intern()); } } class Other { static String hello = "Hello"; }and the compilation unit:
package other; public class Other { public static String hello = "Hello"; }produces the output:
true true true true false true
